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ALLEN-BRADLEY

Safety Guidelines for the Application,

Installation and Maintenance of Solid StateControl

Table of Contents

Foreword...........................................................................................1NEMA Standard Text .......................................................................1

Section 1 Definitions .......................................................................1

Section 2 General.............................................................................22.1 Ambient Temperature.............................................................2

2.2 Electrical Noise ......................................................................2

2.3 Off-State Current....................................................................32.4 Polarity ...................................................................................3

2.5 Rate of Rise-Voltage or Current (DV/DT or DI/DT).................42.6 Surge Current.........................................................................4

2.7 Transient Overvoltage ............................................................5Section 3 Application Guidelines ...................................................5

3.1 General Application Precautions............................................5

3.2 Circuit Isolation Requirements ...............................................83.3 Special Application Considerations........................................8

3.4 Planning Electrical Noise Rejection .....................................10

3.5 Countering the Effects of Off-State Current .........................123.6 Avoiding Adverse Environmental Conditions........................13

3.7 The Need for Education Knowledge Leads to Safety........14Section 4 Installation Guidelines..................................................15

4.1 Installation and Wiring Practice............................................154.2 Enclosures (Cooling and Ventilating) ...................................16

4.3 Special Handling of Electrostatic Sensitive Devices.............164.4 Compatibility of Devices with Applied Voltages and

Frequencies.........................................................................16

4.5 Testing Precautions ..............................................................164.6 Startup Procedures ..............................................................17

Section 5 Preventive Maintenance and Repair Guidelines........18

5.1 General ................................................................................18

5.2 Preventive Maintenance .......................................................185.3 Repair...................................................................................19

5.4 Safety Recommendations for Maintenance Personnel.........20

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NEMA Standard Text

SCOPE This Standards Publication is intendedto provide general guidelines for the application,installation, and maintenance of solid state controlin the form of individual devices or packagedassemblies incorporating solid state components.The emphasis of the guidelines is personnelsafety. Applicable NEMA standards and product

related instructions should be carefully followed.

Section 1Definitions

Electrical Noise Unwanted electrical energy thathas the possibility of producing undesirable effectsin the control, its circuits and system. Electricalnoise includes Electromagnetic Interference (EMI)and Radio Frequency Interference (RFI).

Electrical Noise Immunity The extent to whichthe control is protected from a stated electricalnoise.

Electromagnetic Interference (EMI) Electro-magnetic disturbance that manifests itself inperformance degradation, malfunction, or failure ofelectronic equipment. (IEC)

Off-State Current The current that flows in a solidstate device in the off-state condition.

Off-State Condition The conditions of a solidstate device when no control signal is applied.

On-State Condition The condition of a solid statedevice when conducting.

Radio Frequency Interference (RFI) RFI is usedinterchangeably with EMI. EMI is a later definitionthat includes the entire electromagnetic spectrum,whereas RFI is more restricted to the radio-frequency band, generally considered to bebetween 10k and 10G Hz. (IEC)

Surge Current A current exceeding the steadystate current for a short time duration, normallydescribed by its peak amplitude and time duration.

Transient Overvoltage The peak voltage inexcess of steady state voltage for a short timeduring the transient conditions (e.g., resulting fromthe operations of a switching device).

Foreword

This Allen-Bradley publication is formatted to harmonize with NEMA Standards Publication No. ICS1.1-1987, also titled Safety Guidelines for the Application, Installation and Maintenance of Solid StateControl. The text of the NEMA Standard has been reprinted verbatim, with NEMAs permission, in theleft column, captioned NEMA Standard Text. The right column, captioned Explanatory Information,contains Allen-Bradley comments. The comments provide supplementary information to the NEMAStandard to help the reader better understand the characteristics of industrial equipment employingsolid state technology. The Allen-Bradley Company is solely responsible for the explanatory comments,which are not part of the NEMA Standard.

NEMA Standards Publication No. ICS 1.1-1984, Rev. No. 1 October 1987, is available from theNational Electrical Manufacturers Association, 2101 L Street, N.W., Washington, D.C. 20037.

Explanatory Information

(Supplementary Comments Not part of NEMASTANDARDS PUBLICATION NO. ICS-1.1)

Scope

The scope of this Allen-Bradley Publication(SGI-1.1) is identical to the scope of NEMAStandards Publication No. ICS 1.1, quoted in the

left column.

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Section 2General

(Sections 2 through 5 are classified as AuthorizedEngineering Information 11-15-1984.)

Solid State and electro-mechanical controls canperform similar control functions, but there arecertain unique characteristics of solid statecontrols which must be understood.

In the application, installation and maintenance of

solid state control, special consideration should begiven to the characteristics described in 2.1through 2.7.

2.1 Ambient Temperature

Care should be taken not to exceed the ambienttemperature range specified by the manufacturer.

2.2 Electrical Noise

Performance of solid state controls can be affectedby electrical noise. In general, complete systemsare designed with a degree of noise immunity.Noise immunity can be determined through testssuch as described in 3.4.2. Manufacturerrecommended installation practices for reducingthe effect of noise should be followed.

General CommentsSolid state devices provide many advantages suchas high speed, small size, and the ability to handleextremely complex functions. However, they differfrom electromechanical devices in the basicoperating characteristics and sensitivity toenvironmental influences. In addition, solid statedevices exhibit different failure mechanisms when

overstressed.The comments which follow are intended toprovide additional information to help the readerbetter understand the operating characteristics,environmental limitations, and failure modes ofindustrial equipment that incorporates solid statetechnology. Those who select, install, use, andservice such equipment should apply thatknowledge to make appropriate decisions that willoptimize the performance and safety of theirapplications.

C.2.1 Ambient Temperature

Temperature of the air immediately surrounding anopen solid state device is the ambient temperaturewhich must be considered. When equipment isinstalled in an enclosure, the enclosure internal airtemperature is the ambient temperature whichmust be considered. Solid state componentmanufacturers usually publish the componentfailure rate for an ambient temperature of 40degrees Celsius. A useful rule of thumb is: Thefailure rate of solid state components doubles forevery ten degrees Celsius rise in temperature.This rule of exponential increases in failure rate isa strong incentive for the user to keep the ambienttemperature as low as possible.

Also see sections 3.6.1, and 3.6.2.

C.2.2 Electrical Noise

Solid state devices are generally more susceptibleto electrical noise interference than theirelectromechanical counterparts. The reasons arestraightforward. The operating mechanism forelectromechanical devices requires a deliberateinput of electrical energy that can be convertedinto a sustained mechanical force that is strongenough to close the hard contacts and maintainthe closure for the duration on the ON cycle. Most

random electrical noise signals lack the energycontent to produce that magnitude of mechanicalforce.

The operating mechanism for solid state devices istotally different. The deliberate electric energyinput is used to disturb the placement of theelectrically charged particles within the molecularstructure. This molecular displacement changesthe electrical characteristic from that of aninsulator to that of a conductor or vice versa.

(continued)

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2.3 Off-State Current

Solid state controls generally exhibit a smallamount of current flow when in the off-statecondition. Precautions must be exercised toensure proper circuit performance and personnelsafety. The value of this current is available fromthe manufacturer.

2.4 Polarity

Incorrect polarity of applied voltages may damagesolid state controls. The correct polarity of solidstate controls should be observed.

C.2.2 (continued)

The required energy level is very low. In addition, asustained signal is not required for componentssuch as SCRs, triacs, and logic gates becausethese types are self-latching. Most randomelectrical noise signals are of the momentary lowenergy type. Since it is difficult to separatedeliberate signals from random noise, the devicesare thereby more susceptible. This is cause forspecial concern regarding the electricalenvironment and possible need for noise rejectionmeasures.

All see sections 3.4, 3.4.1, 3.4.2, 3.4.3.

C.2.3 Off-State Current

Off-state current is also referred to as leakagecurrent in the literature. A solid state contact is asolid block of material which is switched from ONto OFF by a change internally from a conductor toan insulator. Since a perfect insulator does notexist, there is always some leakage currentpresent as long as voltage is applied to the device.The presence of leakage current indicates that

OFF does not mean OPEN. The reader is warnedthat simply turning a solid state device OFF doesnot remove the possibility of a shock hazard.

Solid state and electromechanical devices, usedas inputs to solid state controls, must becompatible with the solid state equipment withwhich they are used. Solid state devices haveinherent off-state current, as explained in thepreceding paragraph. Electromechanical devicesmay also permit a small amount of current to flowwhen the device is in the open position due topoor insulation characteristics, which may besubject to further deterioration with age and use.

An example is a switching device that employs acarbon brush in contact with an insulating segmentof the switch in the off-state, such that aconductive film may be deposited by the brush onthe insulating segment. Any input device that couldproduce an erroneous signal of sufficientmagnitude to cause a malfunction of the solid stateequipment, such as unintended turn ON or inabilityto turn OFF, should not be used with solid statecontrols.

Also see section 3.5.2.

C.2.4 Polarity

In some instances incorrect polarity can causedamage to controlled equipment or unintendedactuation of outputs. This could result in personalinjury due to an unexpected response of thecontrolled equipment or process.

Also see section 3.3.2.

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2.5 Rate of Rise-Voltage or Current(DV/DT or DI/DT)

Solid state controls can be affected by rapidchanges of voltage or current if the rate of rise(DV/DT and/or DI/DT) is greater than the maximumpermissible value specified by the manufacturer.

2.6 Surge Current

Current of a value greater than that specified bythe manufacturer can affect the solid state control.Current limiting means may be required.

C.2.5 Rate of Rise-Voltage or Current

(DV/DT or DI/DT)

The DV/DT rating specifies the maximum rate atwhich voltage may be applied to the powerterminals of a solid state device. Voltage applied ata rate exceeding the DV/DT rating can switch thedevice ON without an input signal being applied.Electrical noise with high frequency content is onesource of rapidly changing voltage.

Another common source of high DV/DT is aninductive load that is switched off faster than thestored energy can be dissipated. This fastswitching produces inductive kick voltages thatmight exceed the DV/DT limit.

The DI/DT rating specifies the maximum rate atwhich current flow may be increased whenswitching from OFF to ON. Currents that increasefaster than the DI/DT rating cause localized hotspots due to current crowding in a small area untilthe entire cross section can become conductive.This results in gradual degradation of the device.Subsequent operations generally result in over

dissipation and short circuit failures even undernormal load conditions. The most commonsituations for high DI/DT are low load impedance,or capacitance loads.

Manufacturers of solid state equipment usuallyinclude internal means to limit the rate of rise ofvoltage and current. Nonetheless, the user shouldbe aware that additional external means may benecessary to adjust to the specific conditions ofsome installations.

C.2.6 Surge Current

The manufacturer may specify allowable surgecurrent. Common practice is to specify the peaksinusoidal current that can be allowed for one-halfcycle at line frequency. The intent behind thispractice is to give the user information for selectingan appropriate fuse or other current limitingmeans.

Applications that require short term overcurrentcapability (e.g., motor starting) must observe themanufacturers restrictions on the number of timesthe device can be subjected to overcurrent in aspecified time interval. The user should be awarethat this specification may vary depending upon

whether the conditions call for hot starts or coldstarts. Hot start means that the solid statecomponent is at or near the normal operatingtemperature due to previous operation historywhen the overcurrent condition occurs. Cold startmeans that the solid state component is at orbelow 40 degrees Celsius when the overcurrentcondition occurs.

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2.7 Transient Overvoltage

Solid state controls may be affected by transientovervoltages which are in excess of thosespecified by the manufacturer. Voltage limitingmeans should be considered and may be required.

Section 3Application Guidelines

3.1 General Application Precautions

3.1.1 Circuit Considerations

The consequences of some malfunctions such asthose caused by shorted output devices,alteration, loss of memory, or failure of isolationwithin components or logic devices, require that

the user be concerned with the safety of personneland the protection of the electronics.

It is recommended that circuits which the userconsiders to be critical to personnel safety, such asend of travel circuits and emergency stopcircuits, should directly control their appropriatefunctions through an electromechanical deviceindependent of the solid state logic. Such circuitsshould initiate the stop function through de-energization rather than energization of the controldevice. This provides a means of circuit controlthat is independent of system failure.

C.2.7 Transient Overvoltage

Solid state devices are especially sensitive toexcessive voltage. When the peak voltage rating isexceeded, even for a fraction of a second,permanent damage can occur. The crystallinestructure of the device may be irretrievably alteredand the device may no longer be able to turn OFF.The external symptom of this situation is exactlythe same as that of an electromechanical device

with welded contacts.Minimum Holding Current

Another characteristic of concern is the minimumholding current requirement for triacs and SCRs.When the load current falls below the minimumvalue, typically 25 100 mA, the triac or SCRceases conduction and passes only off-statecurrent until again triggered. Thus, it may not bepossible for the circuit to turn-on or conduct fullload current for very light loads. In these instances,a load resistor called a bleeder resistor may beconnected to the output to provide the minimumload. In some equipment special circuitry is

provided to overcome this problem.

C.3.1.1 Circuit Considerations

The predominant failure mode of solid statedevices is in the ON condition. This failure modeand the other types of failures mentioned in theNEMA Standard are the reasons for the

precautions that are recommended for safety-critical circuits on systems that control potentiallyhazardous processes or machine operations.

Alternatively, if solid-state is used for circuitsdesignated as safety-critical, the circuits should bedesigned to provide safety equivalent to therecommended hard-wired electromechanicalcircuits. In such cases consideration should begiven to techniques such as: redundancy, feed-back loops, diagnostics, interlocking and read-onlymemory for critical parts of a program.

De-energization rather than energization of thecontrol device should be specified for STOP

circuits so broken wires or corroded contacts donot go undetected. E-stop push buttons or pullcords should be installed at appropriate locationson a machine to provide operators with a rapid andconvenient means for removing power fromdevices which control machine motion.

(continued)

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3.1.2 Power Up/Power Down Considerations

Consideration should be given to system designso that unsafe operation does not occur underthese conditions since solid state outputs mayoperate erratically for a short period of time after

applying or removing power.

C.3.1.1 (continued)

Alternate methods of stopping such as dynamicbraking and regenerative braking may be available.In order to accomplish the dynamic braking andregenerative braking function, the control circuit, orpart of it, in the equipment must remain energizedduring the stopping mode. However, operatorinterface components such as stop push buttonsor pull cords, must still be connected throughnormally closed contacts so broken wires orcorroded contacts do not go undetected. Also, ifdynamic or regenerative braking is used foremergency stopping, provision should be made tointerrupt the power circuit as soon as the machinecomes to a stop.

C.3.1.2 Power Up/Power Down Considerations

Response of a system during power up/powerdown can create hazards not encountered duringnormal operation. Erratic operation of solid stateoutputs due to the changing voltage of DC power

supplies during start up is one example. To avoidunpredictable outputs, many power suppliesincorporate a power turn-on time delay circuit. Thisallows power supply output voltage to reach itsspecified value before being applied to solid statelogic and output circuits. If this protection is notpart of the DC supplies for a system, a timingcircuit external to the power supply can be addedto delay the application of power to output devices.

Removing all power or losing all power from asystem simultaneously usually does not result in ahazard since the power for machine operation isalso being removed. However, when power other

than electrical power is being controlled, a powerinterlock circuit may be required to protect againstunexpected machine motion. Power interlocks withautomatic shutdown should be included if erratic orhazardous operation results due to loss of onepower supply in a system with multiple supplies.Automatic power supply sequencing should beemployed in systems that require the application orremoval of power in a specific sequence. If theSTOP or E-STOP sequence normally employsdynamic braking, alternative safeguards, such asautomatic mechanical braking upon loss of power,should be provided if coasting stops arehazardous.

If hazardous operation can result from unexpectedrestoration of power during a power outage or asystem shutdown, the system should include afeature which requires a deliberate operator actionbefore power is reapplied to the system.

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3.1.3 Redundancy and Monitoring

When solid state devices are being used to controloperations, which the user determines to becritical, it is strongly recommended thatredundancy and some form of checking beincluded in the system. Monitoring circuits shouldcheck that actual machine or process operation isidentical to controller commands; and in the eventof failure in the machine, process, or themonitoring system, the monitoring circuits shouldinitiate a safe shutdown sequence.

3.1.4 Overcurrent ProtectionTo protect triacs and transistors from shortedloads, a closely matched short circuit protectivedevice (SCPD) is often incorporated. TheseSCPDs should be replaced only with devicesrecommended by the manufacturer.

3.1.5 Overvoltage Protection

To protect triacs, SCRs and transistors fromovervoltages, it may be advisable to consider

incorporating peak voltage clamping devices suchas varistors, zener diodes, or snubber networks incircuits incorporating these devices.

C.3.1.3 Redundancy and Monitoring

The normal operating mechanism for solid statecomponents depends upon a deliberate electricalsignal input altering the internal molecularstructure of the semiconductor material.Unfortunately, spurious input signals may alsoalter the internal molecular structure without anymeans for external detection that this hashappened. Therefore, solid state devices aresubject to malfunction due to random causes thatare undetectable. Because of this, redundancyand monitoring are the most highly recommendedmeans for counteracting this situation.

When redundancy is used, dissimilar componentsnot susceptible to common cause failure should beused for the redundant elements if a commoncause could produce simultaneous failure of thoseelements in a dangerous mode.

A safe shutdown sequence can involve muchmore than disconnecting electrical power for somemachinery and processes. Examples includemachines with high inertia and hazardous access

points; processes that become unstable atshutdown unless a specific sequence is followed;etc. The control system for such applicationsshould be configured to deal with the particularhazard(s) through use of special features such asautomatic transfer of control functions toredundant devices in the event of failure of primarycontrols; alarm circuits and diagnostics to signaland identify failures that require repair in order tomaintain redundancy; emergency power sourceswith automatic transfer upon loss of primary powersource; or other appropriate features.

C.3.1.4 Overcurrent ProtectionEven a closely matched short circuit protectivedevice (SCPD) will generally protect a solid statedevice only against shorted loads, an accidentalshort to ground or a phase to phase short.Depending upon the application, additionalprotective measures may be needed to protect thesolid state devices against small to moderateovercurrents. Consult with the manufacturer ifnecessary.

C.3.1.5 Overvoltage Protection

See section 2.7.

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3.2 Circuit Isolation Requirements

3.2.1 Separating Voltages

Solid state logic uses low level voltage (e.g., lessthan 32 volts DC) circuits. In contrast, the inputsand outputs are often high level (e.g., 120 voltsAC) voltages. Proper design of the interfaceprotects against an unwanted interaction betweenthe low level and high level circuits; such aninteraction can result in a failure of the low voltage

circuitry. This is potentially dangerous. An inputand output circuitry incorporating effective isolationtechniques (which may include limiting impedanceor Class 2 supplied circuitry) should be selected.

3.2.2 Isolation Techniques

The most important function of isolationcomponents is to separate high level circuits fromlow level circuits in order to protect against thetransfer of a fault from one level to the other.

Isolation transformers, pulse transformers, reedrelays, or optical couplers are typical means totransmit low level logic signals to power devices in

the high level circuit. Isolation impedance meansalso are used to transmit logic signals to powerdevices.

3.3 Special ApplicationConsiderations

3.3.1 Converting Ladder Diagrams

Converting a ladder diagram originally designedfor electromechanical systems to one using solidstate control must account for the differences

between electromechanical and solid statedevices. Simply replacing each contact in theladder diagram with a corresponding solid statecontact will not always produce the desired logicfunctions or fault detection and response. Forexample, in electromechanical systems, a relayhaving a mechanically linked normally open (NO)and normally closed (NC) contact can be wired tocheck itself. Solid state components do not have amutually exclusive NO-NC arrangement. However,external circuitry can be employed to sample theinput and contact state and compare todetermine if the system is functioning properly.

C.3.2.1 Separating Voltages

For specifications of Class 2 circuitry, refer toArticle 725 of the National Electrical Code, NFPA70.

C.3.2.2. Isolation Techniques

In addition to utilizing the various componentsdiscussed in the left column, specific wiringtechniques should be applied to assure separationof power circuit wires from logic circuit wires. If atall possible, logic wires should be run in a conduitthat is segregated for that purpose only. Multipleconductors in a shielded cable is an appropriate

substitute for separate conduits. Another commonpractice is to run the logic signals through twistedpairs of wire. Regardless of the circumstances,wires carrying logic signals should never bewrapped in the same bundle with wires that carrypower signals.

C.3.3.1 Converting Ladder Diagrams

The example cited in this section of the NEMAStandard illustrates only one of a number ofreasons for special care in converting an

electromechanical (relay) ladder diagram to aprogrammable controller (PC) program. Someother basic considerations are:

A PC program is an instruction to the PCs

central processing unit to enable it to perform thelogic functions and sequences for a particular

application. Typically, the PCs logic levelcomponents are electrically isolated from the

actual input, sensor and actuator devices, ascontrasted with electromechanical controls which

usually include contacts and coils of the actual

plant floor devices in the control schematic.Therefore a PC program normally functions as

open-loop control, unless feedback loops from theplant floor devices to separate inputs of the PC are

provided and programmed to cause correctiveaction if inconsistencies are detected.

Programmers of PC systems should evaluate

functional and safety implications of all controlpaths and provide appropriate feedback

arrangements as needed.

(continued)

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3.3.2 Polarity and Phase Sequence

Input power and control signals should be applied

with polarity and phase sequence as specified bythe manufacturer. Solid state devices can bedamaged by the application of reverse polarity orincorrect phase sequence.

C.3.3.1 (continued)

In an electromechanical implementation of a

ladder diagram, power is available to every rung atall times, so that the logic of the various rungs is

executed continually and simultaneously, limited ofcourse by the operating delays inherent in the

electromechanical devices. By contrast, a typicalPC examines the status of input devices (I/O

scan), then executes the user program in

sequence (program scan), then changes outputsaccordingly in the next I/O scan. Therefore, the

sequential order of a PC program can be of moreimportance and significance than in its

electromechanical counterpart, particularly whenspecial instructions such as immediate inputs or

outputs are programmed as some PCs permit.

Also, differences in response characteristics ofcomponents, differences in system architecture,

and the scan time associates with a PC systemcan combine to change timing characteristics of a

circuit significantly. In particular, care must betaken in handling momentary or rapidly changing

inputs to a PC system which might be missedbetween scans. Simple transfer of a ladderdiagram without consideration of these

characteristics of PCs may produce unintendedand possibly hazardous results. Programmers

should consult the users manual in order tounderstand the characteristics of the particular PC

being used, and provide appropriate features in

the program to accommodate them.

Another concern is the operating mode of

devices connected to input terminals. Input signalsmust be arranged so loss of signal due to a broken

wire or corroded contact does not go undetectedand create a hazardous condition. In particular,stop functions should be initiated by opening a

normally closed external circuit rather than closinga normally open circuit even though the system is

capable of being programmed to accept either

type of input.

The considerations described in this section applyto the creation of new programs as well asconversion of existing ladder diagrams.

C.3.3.2 Polarity and Phase Sequence

Additionally, incorrect polarity or phase sequence

connection may cause erratic response by solidstate controls, with potential hazards to personnel.Frequently, such a system contains a detectioncircuit that illuminates an indicator when incorrectphase sequence is applied. Phase sequence maybe corrected by interchanging any two systeminput power leads. It is advisable to check rotationof motors whenever input power leads aredisconnected and reconnected in a system.

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3.4 Planning ElectricalNoise Rejection

The low energy levels of solid state controls maycause them to be vulnerable to electrical noise.This should be considered in the planning stages.

3.4.1 Assessing Electrical Environment

Sources of noise are those pieces of equipmentthat have large, fast changing voltages or currentswhen they are energized or de-energized, such asmotor starters, welding equipment, SCR type,adjustable speed devices and other inductivedevices. These devices, as well as the morecommon control relays and their associated wiring,all have the capability of inducing serious currentand voltage transients on their respective powerlines. It is these transients which nearby solid statecontrols must withstand and for which noiseimmunity should be provided.

An examination of the proposed installation site ofthe solid state control should identify equipmentthat could contaminate power lines. All power linesthat will be tapped by the proposed solid statecontrol should be examined for the presence,severity, and frequency of noise occurrences. Iffound, system plans should provide for the controlof such noise.

3.4.2 Selecting Devices to Provide

Noise Immunity

Installation planning is not complete withoutexamination of the noise immunity characteristicsof the system devices under consideration.Results of tests to determine relative immunity toelectrical noise may be requested from themanufacturer. Two such standardized tests are the

ANSI (C37.90a-1974) Surge Withstand CapabilityTest and the NEMA (ICS.1-1983) noise testreferred to as The Showering Arc Test. These areapplied where direct connection of solid statecontrol to other electromechanical control circuitsis intended. Circuits involving analog regulatingsystems or high speed logic are generally moresensitive to electrical noise; therefore, isolationand separation of these circuits is more critical.

Further information on electrical noise andevaluation of the severity of noise may be found inANSI/IEEE Publication No. 518-1982.

Where severe power line transients are anticipated

or noted, appropriate filters such as commerciallyavailable line filter, isolation transformers, orvoltage limiting varistors, should be considered.

All inductive components associated with thesystem should be examined for the need for noisesuppression.

C.3.4.1 Assessing Electrical Environment

Noise can also occur in the form ofelectromagnetic radiation, or due to improper

grounding practices. Section C.3.4.3 explainsthese forms of noise and precautionary measuresthat should be taken for protection against them.

In many instances a system may begin tomalfunction some time after it has been installedand is working properly. This may be due to recentinstallation of new equipment capable of inducingnoise into presently operating systems. Thus, it isnot sufficient to merely evaluate a system at thetime of installation. Periodic rechecks should bemade, especially as other equipment is moved,modified, or newly installed. When installing a solidstate system, it is wise to assume various noise

sources exist and install the system to guardagainst possible interference.

C.3.4.2 Selecting Devices to Provide

Noise Immunity

Inductive devices are capable of generating highvoltage transients when switched off. In addition topossibly causing damage to solid state devices byexceeding the semiconductor voltage rating, thehigh voltage transient can be coupled to otherportions of a system where it appears as noise.

Fortunately, it is fairly easy to limit the effects ofthis type of noise with some form of suppressiondevice. When necessary, in addition tosuppression devices often provided in solid stateequipment, an external suppressor should beconnected as close as possible to the source ofthe transient for maximum attenuation.

NOTE: A surge suppressor increases drop-outtime of an electromechanical device.

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3.4.3 Design of Wiring for

Maximum Protection

Once the installation site and power conductorshave been examined, the system wiring plans thatwill provide noise suppression should beconsidered.

Conducted noise enters solid state control at thepoints where the control is connected to inputlines, output lines, and power supply wires.

Input circuits are the circuits most vulnerable tonoise. Noise may be introduced capacitativelythrough wire to wire proximity, or magnetically fromnearby lines carrying large currents. In mostinstallations, signal lines and power lines shouldbe separate. Further, signal lines should beappropriately routed and shielded according tomanufacturers recommendations.

When planning system layout, care must be givento appropriate grounding practice. Because designdifferences may call for different grounding, thecontrol manufacturers recommendations shouldbe followed.

C.3.4.3 Design of Wiring for

Maximum Protection

Noise can also occur in the form ofelectromagnetic radiation. Examples include radiofrequency (RF) energy emanating from portabletransceivers (walkie-talkie) and fixed stationtransmitters of various types. Close coupling is notrequired; the various lines entering the system actas receiving antennas. Tests described in 3.4.2may not be sufficient to demonstrate noiseimmunity to radio frequency signals. Variations inbuilding construction and equipment installationmake it impossible for equipment manufacturers toperform meaningful tests of radio frequencysources. RF fields are affected by concentratingmasses of metal such as steel beams, piping,conduit, metal enclosures, and equipment used inproduction such as fork lift trucks and productsbeing transported on conveyors.

If the installation site will be subjected to this typeof noise, thorough testing should be performed toassure that the solid state system has sufficientnoise immunity for the expected levels of radiofrequency energy. Corrective measures should betaken if necessary. These include shielding of solidstate circuits and/or connected wiring and theestablishment of restrictions to provide safeoperating distances between the solid stateequipment and the RF sources.

Grounding practices in industry are frequentlymisunderstood and often ignored. Poor groundingcan lead to many problems in solid state systems.Intentionally grounding one circuit conductor ofany electrical supply system is widely acceptedand is generally required by electrical codes.However, the non-current carrying parts of asystem which enclose equipment and conductorsmust also be grounded. In addition to complyingwith various codes and standards, properequipment grounding achieves several desirableobjectives:

1. It reduces the potential difference betweenconductive surfaces to minimize electric shock

hazard exposure for personnel.

2. It provides a path for passage of fault current tooperate protective devices in the supply circuit.

3. It attenuates the electrical noise and transients

which can reach enclosed equipment and also

reduces the electrical noise which theequipment can contribute to its surroundings.

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3.5 Countering the Effects ofOff-State Current

3.5.1 Off-State Current

Solid state components, such as triacs, transistors,and thyristors, inherently have in the off-state asmall current flow called off-state current.

Off-state current may also be contributed bydevices used to protect these components, such

as RC snubbers.3.5.2 Off-State Current Precautions

Off-state currents in a device in the off-state maypresent a hazard of electrical shock and the deviceshould be disconnected from the power sourcebefore working on the circuit or load.

Precautions should be taken to prevent the off-state current of an output device which is in the off-state from energizing an input device.

C.3.5.1 Off-State Current

See section 2.3.

C.3.5.2 Off-State Current Precautions

The off-state current of a power switching devicesuch as a solid state motor controller can be lethal.Simply switching off power via a stop push buttonin a control circuit is not a sufficient precaution,since off-state current will continue to flow throughsolid state devices which remain connected to thesupply. Good practice requires disconnection of allpower from equipment before working on or nearexposed circuit parts. (See NFPA 70E, Part II.)

It should not be assumed that a shock hazarddoes not exist simply because a solid state circuitoperates at low voltage levels. Standing on a wetfloor or working in a damp location can lower apersons body impedance to the extent that off-state current from low voltage also presents anelectrical shock hazard.

If it is necessary to work on energized equipment,the guidelines detailed in section 5.2 for PreventiveMaintenance should be followed. In addition to thespecific procedures for personnel safety, care isneeded when making measurements in energizedsystems. First, there is a possibility of damage todelicate instruments due to off-state current.Second, the off-state current can lead to falseconclusions when using sensitive instruments tocheck for contact continuity.

When a device (solid state or electromechanical)that can produce a leakage current in the off-stateis used to provide the input to a solid state control,the precautions explained in section C.2.3 apply.

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3.6 Avoiding AdverseEnvironmental Conditions

3.6.1 Temperature

Solid state devices should only be operated withinthe temperature ranges specified by themanufacturer. Because such devices generateheat, care should be taken to see that the ambienttemperature at the device does not exceed thetemperature range specified by the manufacturer.

The main source of heat in a solid state system isthe energy dissipated in the power devices. Sincethe life of the equipment can be increased byreducing operating temperature, it is important toobserve the manufacturers maximum/minimumambient temperature guidelines, where ambientrefers to the temperature of the air providing thecooling. The solid state equipment must beallowed to stabilize to within the manufacturersrecommended operating temperature range beforeenergizing control functions.

When evaluating a system design, other sources

of heat in the enclosure which might raise theambient temperature should not be overlooked.For example, power supplies, transformers,radiated heat, sunlight, furnaces, incandescentlamps, and so forth should be evaluated.

In instances where a system will have to exist in avery hot ambient environment, special coolingmethods may have to be employed. Techniquesthat are employed include cooling fans (withadequate filtering), vortex coolers, heatexchanges, and air conditioned rooms.

Over-temperature sensors are recommended forsystems where special cooling is employed. Use of

air conditioning should include means forprevention of condensing moisture.

C.3.6.1 Temperature

Operation above the maximum rated temperaturewill usually result in many failures in a short time.Nuisance type malfunctions can also beencountered as a result of elevated ambienttemperature. These malfunctions, when theyoccur, are usually temporary and normal operationresumes when temperatures are lowered.

Some solid state devices temporarily cease tofunction when ambient temperature is below theirminimum rated operating temperature. Operationin cold environments should be avoided or heatersshould be installed in the equipment enclosures tobring the system up to the minimum specifiedoperating temperature before applying power tothe system.

Air circulating in a non-ventilated enclosure withequipment operating will be at a highertemperature than the room in which it is installed.

A temperature differential of 10 to 20 degreesCelsius can be expected in a typical industrialinstallation.

Also see section 2.1.

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3.6.2 Contaminants

Moisture, corrosive gases and liquids, andconductive dust can all have adverse effects on asystem that is not adequately protected againstatmospheric contaminants.

If these contaminants are allowed to collect onprinted circuit boards, bridging between theconductors may result in malfunction of the circuit.This could lead to noisy, erratic control operation,

or at worst, a permanent malfunction. A thickcoating of dust could also prevent adequatecooling on the board or heat sink, causingmalfunction. A dust coating on heat sinks reducestheir thermal efficiency.

Preventive measures include a speciallyconditioned room or a properly specified enclosurefor the system.

3.6.3 Shock and Vibration

Excessive shock or vibration may cause damageto solid state equipment. Special mountingprovisions may be required to minimize damage.

3.7 The Need for Education Knowledge Leads to Safety

Planning for an effective solid state circuit requiresenough knowledge to make basic decisions thatwill render the system safe as well as effective.

Everyone who works with a solid state controlshould be educated in its capabilities andlimitations. This includes in-plant installers,operators, service personnel, and systemdesigners.

C.3.6.2 Contaminants

Modules for solid state systems usually consist ofelectronic devices mounted on printed circuitboards with relatively close spacing betweenconductors. Moisture in the form of humidity is oneof the atmospheric contaminants which can causefailure. If moisture is allowed to condense on aprinted circuit board, the board metallizationscould electroplate across the conductor spacingswhen voltage is applied. In low impedance circuits,this conductive path would immediately burn open,then reform to be burned open again. This actioncan lead to erratic operation. In high impedancecircuits, a short circuit may appear resulting in apermanent malfunction. Specifications forequipment often include a relative humidityexposure limit, but appropriate precautions shouldbe taken to prevent condensation. Failures due tomoisture are often accelerated in the presence ofcorrosive gases or vapors. These increase theconductivity of the moisture layer allowingelectromigration to occur more rapidly and at lowerpotentials.

C.3.6.3 Shock and Vibration

Solid state systems usually have good resistanceto shock and vibration since they contain nomoving parts. However, at relatively high levels ofshock or vibration, circuit boards may disengagefrom mating connectors if not restrainedsufficiently. Circuit boards can crack, componentscan come out of sockets or component leads canbreak loose from a solder connection to the board.Mounting position is usually of little significance tosolid state devices except in instances where airflow is required for cooling.

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Section 4Installation Guidelines

4.1 Installation and Wiring Practice

4.1.1

Proper installation and field wiring practices are ofprime importance to the application of solid statecontrols. Proper wiring practice will minimize theinfluence of electrical noise, which may cause

malfunction of equipment.User and installers should familiarize themselveswith the follow installation and wiring instructions inaddition to requirements of all applicable codes,laws, and standards. The manufacturer of thedevice or component in question should beconsulted whenever conditions arise that are notcovered by the manufacturers instructions.

4.1.2

Electrical noise is a very important consideration inany installation of solid state control. While wiringpractices may vary from situation to situation, thefollowing are basic to minimizing electrical noise:

1. Sufficient physical separation should be

maintained between electrical noise sourcesand sensitive equipment to assure that the

noise will not cause malfunctioning or

unintended actuation of the control.

2. Physical separation should be maintained

between sensitive signal wires and electricalpower and control conductors. This separation

can be accomplished by conduits, wiring trays,

or as otherwise recommended by themanufacturer.

3. Twisted-pair wiring should be used in criticalsignal circuits and noise producing circuits to

minimize magnetic interference.

4. Shielded wire should be used to reduce themagnitude of the noise coupled into the low

level circuit by electrostatic or magneticcoupling.

5. Provisions of the 1984 National Electrical

Code with respect to grounding should befollowed. Additional grounding precautions may

be required to minimize electrical noise. These

precautions generally deal with ground loopcurrents arising from multiple ground paths.

The manufacturers recommendations shouldbe followed.

Available from National Fire Protection Association,Batterymarch Park, Quincy, MA 02269

C.4.1.2

A great deal of effort goes into the design of solidstate equipment to achieve a reasonable degree ofnoise immunity. Filters, shielding, and circuitdesign are all used. It is, however, impossible todesign equipment which is impervious to everyform of noise found in the industrial setting.

When installing a system using solid statetechnology it is wise to assume that electricalnoise exists and install the equipment inaccordance with the recommended guidelines tominimize problems.

Also see section 3.4.1.

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4.2 Enclosures(Cooling and Ventilating)

Suitable enclosures and control of the maximumoperating temperature, both of which areenvironmental variables, may be needed toprevent malfunctions of solid state control.

The manufacturers recommendations should befollowed for the selection of enclosures, ventilation,air filtering (if required), and ambient temperature.

These recommendations may vary frominstallation to installation, even within the samefacility.

4.3 Special Handling ofElectrostatic Sensitive Devices

Some devises may be damaged by electrostaticcharges. These devices are identified and shouldbe handled in the special manner specified by themanufacturer.

NOTE: Plastic wrapping material used to shipthese devices may be conductive and should notbe used as insulating material.

4.4 Compatibility of Devices withApplied Voltages and Frequencies

Prior to energization, users and installers should

verify that the applied voltage and frequency agreewith the rated voltage and frequency specified bythe manufacturer.

NOTE: Incorrect voltage or frequency may cause amalfunction of, or damage to, the control.

4.5 Testing Precautions

When testing solid state control, the proceduresand recommendations set forth by themanufacturer should be followed.

When applicable, instrumentation and testequipment should be electrically equivalent to thatrecommended by the manufacturer for the testprocedure. A low impedance voltage tester shouldnot be used.

High voltage insulation tests and dielectric testsshould never be used to test solid state devices. Ifhigh voltage insulation of field wiring is required,solid state devices should be disconnected.Ohmmeters should only be used when and asrecommended by the equipment manufacturer.

Testing equipment should be grounded; if it is not,special precautions should be taken.

C.4.2 Enclosures

(Cooling and Ventilating)

NEMA Standards Publication No. 250-1985,Rev. 2, May 1988, classifies enclosures by typenumber and specifies their design testrequirements.

Also see section 2.1 and 3.6.1.

C.4.3 Special Handling of

Electrostatic Sensitive Devices

Many problems due to electrostatic discharge(ESD) occur due to handling of modules duringinstallation or maintenance.

In addition to specific guidelines provided by anequipment supplier, the following generalguidelines can help reduce damage due to ESD.

1. Use a grounding bracelet if possible to minimize

charge build-up on personnel.

2. Handle a module by the edges without touching

components or printed circuit paths.

3. Store modules with ESD sensitive componentsin the conductive packaging used for shipping

the modules. Also, use conductive packagingwhen returning static sensitive modules for

repair.

C.4.5 Testing Precautions

Make-do test devices such as incandescent lampsor neon lamps should not be used for checkingvoltages in solid state systems. Incandescentlamps have low impedance; the low impedance ofthese devices can effectively change a voltagelevel from a logic 1 condition to a logic 0

condition when attempting to make ameasurement. Unexpected machine motion canresult if an output to a controlled device isenergized as a result. Neon lamps do not respondto voltages typically used in logic circuits (e.g., 32volts DC or less). Use of a neon lamp tester couldlead to false conclusions about the voltage levelpresent in a circuit.

(continued)

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4.6 Startup Procedures

Checks and tests prior to startup and startupprocedures recommended by the manufacturershould be followed.

C.4.5 (continued)

High input impedance meters are required toobtain accurate voltage measurements in highimpedance circuits. Unless otherwise specified bythe manufacturer, a meter with an input impedanceof ten megohms or greater is recommended formaking voltage measurements. The meter mustalso have sufficient sensitivity to measure logiclevel voltages; some meters do not respond to lowvoltages.

C.4.6 Startup Procedures

Startup procedures can provide important benefitsfor safety with new installation, or aftermodifications or repairs. A dry run undercontrolled conditions can verify proper installationand functioning of the control system before it isturned over to operating personnel.

Many programmable solid state systems have thecapability for simulating operation in a modeknown as test mode or dry run mode. Thesemodes allow a user to check a program andcorrect obvious programming errors with outputsdisabled. Unexpected machine motion andpossible damage to workpieces and equipment isthus avoided. These modes can also be used toverify proper system operation after a repair.

Many programmable systems provide capability forforce on and force off of inputs and outputs.Use of these functions can reduce troubleshootingand maintenance time by enabling personnel tobypass certain operations without physicallyoperating switches on a machine. Care must betaken when using force functions to avoidexposing personnel to hazardous machinemotions or process operations.

For controllers operating a machine tool or robot,running a part program at a fraction of theprogrammed operating speed with a workpiece ofsoft material is considered good practice. Thisallows an operator to observe possibleinterference of tooling with the part and makecorrections to the program. A workpiece of softmaterial such as wood, plastic, or machinable waxwill minimize risk of tool damage if there is a toolcrash with the workpiece.

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Section 5Preventive Maintenance and

Repair Guidelines

5.1 General

A well-planned and executed maintenanceprogram is essential to the satisfactory operationof solid state electrical equipment. The kind andfrequency of the maintenance operation will vary

with the kind and complexity of the equipment aswell as with the nature of the operating conditions.Maintenance recommendations of themanufacturer or appropriate product standardsshould be followed.

Useful reference publications for setting up amaintenance program are NFPA 70B-1983,Maintenance of Electrical Equipment, and NFPA70E-1983, Electrical Safety Requirements forEmployee Workplaces.

5.2 Preventive Maintenance

The following factors should be considered when

formulating a maintenance program:1. Maintenance must be performed by qualified

personnel familiar with the construction,

operation, and hazards involved with thecontrol.

2. Maintenance should be performed with thecontrol out of operation and disconnected from

all sources of power. If maintenance must be

performed while the control is energized, thesafety related practices of NFPA 70E should be

followed.

3. Care should be taken when servicing

electrostatic sensitive components. Themanufacturers recommendations for thesecomponents should be followed.

4. Ventilation passages should be kept open. Ifthe equipment depends upon auxiliary cooling,

e.g., air, water, or oil, periodic inspection (with

filter replacement when necessary) should bemade of these systems.

5. The means employed for grounding orinsulating the equipment from ground should be

checked to assure its integrity (see 4.5).

6. Accumulations of dust and dirt on all parts,

including on semiconductor heat sinks, shouldbe removed according to the manufacturersinstructions, if provided; otherwise, the

manufacturer should be consulted. Care mustbe taken to avoid damaging any delicate

components and to avoid displacing dust, dirt,

or debris in a way that permits it to enter orsettle into parts of the control equipment.

(continued)

C.5.2 Preventive Maintenance

Lithium batteries are frequently used for memory

backup in solid state equipment due to theirexcellent shelf life and high energy to weight ratio.Lithium is a highly reactive metal that can causeburns if there is contact with skin. The batteriesare sealed so there is seldom a problem of contactwith lithium as long as reasonable care isexercised when handling them. They should onlybe used in their intended application and notsubjected to rough handling. When batteries arereplaced in equipment, the batteries removedshould be disposed of in accordance withsuppliers instructions.

The Department of Transportation has certain

regulations that prohibit shipment of equipmentwith batteries installed if the batteries contain 0.5gram or greater of lithium. The batteries must beremoved from equipment and shipped separatelyin a container approved by the Department ofTransportation. Additional Department ofTransportation restrictions apply to the shipment oflithium batteries.

NEMA Standards Publication No. ICS 1.3 1986,Preventive Maintenance of Industrial Control andSystem Equipment, is recommended for personnelresponsible for maintenance of equipment.

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5.2 Preventive Maintenance (continued)

7. Enclosures should be inspected for evidence ofdeterioration. Accumulated dust and dirt should

be removed from the top of the enclosuresbefore opening doors or removing covers.

8. Certain hazardous materials removed as part

of maintenance or repair procedure (e.g.,polychlorinated biphenyls (PCB) found in some

liquid filled capacitors) must be disposed of as

described in Federal regulations.

5.3 Repair

If equipment condition indicates repair orreplacement, the manufacturers instructionmanual should be followed carefully. Diagnosticinformation within such a manual should be usedto identify the probable source of the problem, andto formulate a repair plan. The level of field repairrecommended by the manufacturer should befollowed.

When solid state equipment is repaired, it isimportant that any replacement part be in

accordance with the recommendations of theequipment manufacturer. Care should be taken toavoid the use of parts which are no longercompatible with other changes in the equipment.Also, replacement parts should be inspected fordeterioration due to shelf life and for signs ofrework or wear which may involve factors critical tosafety.

After repair, proper start-up procedures should befollowed. Special precautions should be taken toprotect personnel from hazards during start-up.

C.5.3 Repair

Follow manufacturers instructions exactly whenreplacing power semiconductors mounted onheatsinks since improper installation may becomethe source of further difficulties. Torquesemiconductors or bolts retaining semiconductorsto the value specified using a torque wrench. Toomuch pressure against a heatsink can damage asemiconductor while too little can restrict theamount of heat transferred from the semiconductorto the heatsink and result in operation at higher

temperature with decreased reliability.Exercise care when removing modules from asystem during maintenance. Failed modules arefrequently returned to the manufacturer for repair.Any physical damage sustained during removalmay result in more expensive repair or render themodule unrepairable if damage is too great.

Modules with electrostatic sensitive componentsshould be handled by the edges without touchingcomponents or printed circuit conductors. Usepackaging material supplied with the replacementmodule when shipping the module to themanufacturer for repair.

When the scope of repairs exceeds themanufacturers recommendations for field repair,the module(s) should be returned to themanufacturer for repair. Doing so will help toensure that only properly selected components areused, and that all necessary hardware andfirmware revisions are incorporated into the repair.Failure to make necessary updates may result insafety, compatibility or performance problemswhich may not become apparent for some timeafter the repaired module has been placed back inservice. When firmware is protected by copyrightlaw, updates can be provided legally only by the

manufacturer or licensee.Also see section 4.3.

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5.4 Safety Recommendations forMaintenance Personnel

All maintenance work should be done by qualifiedpersonnel familiar with the construction, operation,and hazards involved with the equipment. Theappropriate work practices of NFPA 70E should befollowed.

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