Post on 29-Oct-2014
Hazardous area Non Hazardous area
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1. Principles of Intrinsic Safety
2. Application of Intrinsic Safety
3. Installation
4. Inspection
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THE IGNITION TRIANGLE
Fuel
Note : Air has approximately 21% oxygen
IGNITION SOURCEOXygen
EXPLOSIONEXPLOSION
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Plant is classified according to :
* the probability that the Hazardous Atmosphere will be present
AREA CLASSIFICATION
* the nature of the Hazardous Atmosphere
GAS GROUPING
( in the UK these are Zone 0, 1 & 2 )
( IIA , IIB and IIC )
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Representative Gas for Sub-Division of Groups IIA , IIB and IIC
Group IIA Propane Energy level for Ignition 160J
Group IIB Ethylene Energy level for Ignition 95J
Group IIC Hydrogen Energy level for Ignition 20J
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Area Classification - Recap
Probability of Gas/Air
Mixture being
present
Zone 0,1, 2
X
Probability of source of
ignition
Matched to
Type ofProtection
d , e ,n, I
=
Acceptable Risk
Nothing is 100% safe
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A system, comprising apparatus and interconnecting wiring, in which any spark or thermal effect in any part of the system intended for use in hazardous areas is incapable of causing ignition.
Intrinsically Safe System
By Electrical Design
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Power supply Conventional Power Circuits
What happens when a fault occurs?
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12V Car Battery
conductor resistance 0.5
12V headlamp
I = V/R
I = 12/1
I = 12 A
P = V x I
P = 12 x12
P = 144W
Energy W = P x t
Energy W= 144 x 0.01
Energy W = 1.44 Joules
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Short circuit fault
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12V Car Battery
conductor resistance 5
12V headlamp
I = V/R
I = 12/10
I = 1. 2 A
P = V x I
P = 12 x1.2
P = 14. 4W
Energy W = P x t
Energy W = 14. 4 x 0.01
Energy W = 0.144 Joules
Energy W = 144mJ
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Short circuit fault
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12V supply
Current limiter of 20k
TransducerI = V/R
I = 12/20000
I = 0.00072 A
I = 0.72mA
P = V x I
P = 12 x 0.00072
P = 0.0072W
P = 7.2mW
Energy W = P x t
Energy W = 0.0072 x 0.01
Energy W = 0.000072 Joules
Energy W = 72J
Load
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How can the principles of Intrinsic Safety be achieved ?
Careful design of circuits which limit the amount of available electrical energy
Where can the principles of Intrinsic Safety be applied ?
Due to the small levels of available energy , this method of protection is onlysuitable for instrumentation or control circuits
How can the principles of Intrinsic Safety be maintained ?
By regular planned inspection and maintenance
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Approx . 140mA
140mA . Applying a safety factor Of 1.5 , gives approx 93mA
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For a 28V Zener with a300 ohmn resistorfor use in a IIC area
Apply a 10% safetyfactor for zenervoltage=30.8V
28V/0.093mA= 300 ohmsBarrier is suitable formaintaining theintegrity of the IS circuit in aIIC hazard
12 V
PSU240VRelay12 V 400
Hazardous area
Low power circuit - IS or non - IS ?
I = V/R I = 12/400
I = 0.03A I = 30mA
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What if the relay coil inductance created a spark at the contactscapable of causing ignition?
Low power circuit - Insertion of an energy limiting network
12 V
PSU240VRelay12 V 400
Hazardous area
Limitingnetwork
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Limit Voltage
i
Limit Current Limit stored electrical energy
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Ex iaIntrinsically safewith two faults
Zones 0, 1 and 2
Ex ibIntrinsically safewith one fault
Zones 1 and 2
"Faults" are those in components upon which the safety of the installation depends
Intrinsic Safety is the only protection concept which considers failure of the field wiring
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AND THEIR APPLICATIONAND THEIR APPLICATION
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800 700 600 500 400 300 200 100
Reverse voltage ( Volts )
0.6 0.7 0.8
Forward voltage ( Volts )
A Forward current
mA Reverse current
Forward bias conducting voltage of 0.6V
Negative
Positive
Forward Bias
Positive
Negative
Reverse Bias
SILICON DIODE CHARACTERISTICS
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Hazardous area connection
Non Hazardous area connection
Resistors are usually wire wound and are assumed to fail open circuit
Vz
Fuse
Any load connected in parallel with the Zener Diode will maintain the same potential
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Hazardous area connection
Non Hazardous area connection
Using 2 diodes in this example indicates that the device is safe with one fault
A device like this would have the category ?
Vz
Fuse
i b
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Hazardous area connection
Non Hazardous area connection
Using 3 diodes in this example indicates that the device is safe with 2 faults
A device like this would have the category ?
Vz
Fuse
ia
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Fuse rating is chosen to protect the diodes and is not directly related to the barrier short circuit current
CENELEC and other standards require the fuse to be encapsulated within the barrier so that the fuse is not replaceable
Fuse
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Fuse
Secondary replaceable fuse versions of barriers are available. These are useful where there is a possibility of faults occurring during commissioning which would otherwise blow the barriers' internal safety fuses.
One secondary replaceable fuse for each barrier channel is provided and is lower in value than the related safety fuse. Fuses are packaged in small mouldings which can be latched in a 'disconnect' position to break the safe and hazardous areas during commissioning, maintenance or fault finding, avoiding the need for additional disconnect terminals.
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Fuse
Vz
If the barrier is used the wrong way round, then the fuse will no longer protect the Zener.
Hazardousarea connection
Non Hazardous area connection
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Barriers are usually described in terms of their safety parameters: 28V 93mA 300 is a common barrier safety description
Where Vz = 28 V R = 300
28 V is the maximum OPEN Circuit voltage available at the hazardous area terminals and 93 mA is the maximum short circuit current available.
Fuse
Vz
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1
2
Recap Questions 1
Q1. A safety factor of 1.5 is applied to components designed to be used for Intrinsically safe devices . True / False.
Q2. An intrinsically safe system identified as ( ib ) will remain safe with 2 faults . True / False.
Q3. The fuse encapsulated within a zener barrier is designed in relation to the barrier short circuit current . True / False .
Q4. An Intrinsically safe instrument designed for operation in the field would be connected to terminals 1 and 2 of a zener barrier . True / False.
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Intrinsically Safe System
AssociatedElectricalApparatus
Intrinsically SafeElectricalApparatus
Field wiring
Intrinsically SafeElectricalApparatus
AssociatedApparatus
Intrinsically SafeElectrical Apparatus
Non-hazardous area Hazardous area
Intrinsically Safe Circuit
Note:
An intrinsically safe circuit consists of all the intrinsically safe apparatus in the loop PLUS the cable
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The Entity Concept
In the entity concept each item possesses its own certificate ofconformity that provides sufficient information to enable end users toassemble intrinsically safe systems.
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Safe area limitations
InstrumentElectronics
Powersupply
250 VRMS.max.
Uncertified instrument
RequirementsSuitably fused double wound mains transformer
Should not be fed from or contain voltages greater than 250V rms.
P
N
E
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Intrinsically safe systems
Certifiedhazardous
areaapparatus
Uncertifiedsafe areaapparatus
Hazardous area Non Hazardous area
Certified interface remote from the uncertified safe area apparatus
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Intrinsically safe systems - typical installation
Uncertified safe areaapparatus
Non Hazardous area
Certifiedinterface
Hazardous area
Junction BoxField Apparatus
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Intrinsically safe systems
SimpleApparatus
Uncertifiedsafe areaapparatus
Hazardous area Non Hazardous area
Certifiedinterface
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Hazardous area equipment in an intrinsically safe loop is either
Certified energy storing instruments etc.or
Simple apparatus non-energy storing devices
Simple apparatus: Devices in which, according to the manufacturer's specifications, none of the values 1.5V, 100mA, or 25 mW is exceeded, need not be certified or marked
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Piezodevice1.2 V
Piezoelectric device limited to 1.2 V outputDoes this still meet the simple apparatus criteria?
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Piezodevice1.2 V
Simple apparatus has to be inherently simple.
You cannot make apparatus simple by the addition of a limiting device, which could fail.
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Hazardous (Classified) Location Non-Hazardous Location
Monitoring
Instrumentation
110 V
Thermocouple
mVNormalOperation
Bad Day
Monitoring
Instrumentation
110 V
Thermocouple
110 V
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Intrinsically safe systems ( REMINDER )
Non-Certifiedhazardous area
Simple apparatus
Uncertifiedsafe areaapparatus
Non Hazardous area
Certifiedinterface
hazardous area
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Hazardous (Classified) Location Non-Hazardous Location
Monitoring
Instrumentation
110 V
Thermocouple
mV
Intrinsically safe interface device
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Hazardous Location Non-Hazardous Location
Simple apparatus•Thermocouples•Photocells•RTDs
Intrinsicallysafe apparatus•Transmitters•Solenoid Valves
Intrinsically safe
interface
Controlroomequipment
AssociatedApparatus
• Zener Barrier• OPTO Isolator
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Barriers are required to be connected to the main system electrical earth with a resistance of less than one Ohm
Main electrical system earth point is usually the neutral star point
The important resistance is between the barrier earth rail and the main earth busbar connection point
Minimum CSA of conductor is 4mm2 copper . However ,standards indicate that TWO copper conductors of 1.5mm2 can also be used.
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Shunt-diode safety barrier installation
Safearea
EquipmentDistributiontransformer
IS earth (<1 )
Barrier Fuse
For safety reasons the connection to the neutral star point must be of low resistance and secure
ONE 4mm2 conductor or TWO 1.5mm2 parallel conductors
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Shunt-diode safety barrier under safe area mains fault
Safearea
EquipmentDistributiontransformer
IS earth (<1 )
Fault
For safety reasons the connection to the neutral star point must be of low resistance and secure
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ApparatusIntrinsically Safe Hazardous Area apparatus falls into one of two categories
• Portable Apparatus
• Field Mounted Apparatus
Equipment of this nature is typically marked EEx ia IIC T4
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The Intrinsically Safe Continuityand Insulation Resistance Tester
METROHM
PORTABLE APPARATUS
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Apparatus
• Intrinsically Safe Non Hazardous Area Apparatus
Equipment of this nature is typically marked [EEx ia] IIC
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Certificate of Conformity
Example
Z barrier CE II 1 GD [EEx ia] IIC CE ll 3 G EExn A ll T4
Applies to APPARATUS certified by an EC approved test Authority to a CENELEC standard recognised by the EC Directives
Will Include the EEC Distinctive Community MarkApproved code will be EEx
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Shunt-diode safety barrier under hazardous area mains fault
Safearea
EquipmentDistributiontransformer
Fault
Raisedpotential
JB
Plant earth
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Recap Questions 2
Q1. A device marked EEx ib IIB T4 is suitable for mounting in a Zone 1 hazardous area where Ethylene gas is being processed . True / False.
Q2. The minimum size of earthing lead permitted for the connection from plant earth to the I.S. earth is a 6.00 mm2 copper conductor. True / False .
Q3. It is permitted for the resistance / impedance of the earthing lead to be greater than 1True / False.
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(L & C parameters)
Assume an interface of 28V 300 ohms resistanceMax’m source current = 28/300 = 93.33 A (IIC gas area)Apply safety factor of 1.5 Max’m source current = 140 maFrom ignition curves the maximum safe inductance for Group llC is found to be 4.0 mH
For Capacitive circuits apply safety factor of 1.5 to the zener barrier voltage of 28V = 42V For a IIC gas the maximum safe capacitance is 0.08 microFarads
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Segrataion of IS and non Is circuits in hazardous areas is essential to avoid the possibility of higher voltages invadingEExi circuits
Cable screens should be earthed at one point only (zener barrier)
Cable marking ---Marking of Is cables is not necessary if either theIs or non-Is cables are armoured, screened or metal sheathed
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Is and non Is circuits must not be run in the same multi -core cable
Multi core cables must be capable of withstanding a 750V dc IR test
Installation must comply with system documentation and the manufacturer's recommendations
Location of the interfaces should be permanently marked to show the correct type of replacement barrier in each position
Barriers are normally mounted in the safe area at the nearest convenient point to the hazardous/safe area boundary
Hazardous area mounting is permissible if the appropriate type of protection is provided for the barriers and cabling :
Zone 1 : Flameproof (Ex d) enclosureZone 2 : Type n enclosure
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Mechanical protection not necessary for safety purposes
Mechanical protection not necessary for safety purposes
The cable must:
Be capable of withstanding a 500 V insulation test to earth for 1 minute
Conform to the cable parameter requirements
Be clearly identified as carrying intrinsically safe circuits by the use of
Blue cable Blue tape
Be suitable for environmental conditions
The cable must:
Be capable of withstanding a 500 V insulation test to earth for 1 minute
Conform to the cable parameter requirements
Be clearly identified as carrying intrinsically safe circuits by the use of
Blue cable Blue tape
Be suitable for environmental conditions
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Multicore Cables are permitted in IS systems but IS and non-IS circuits must not be contained in the same multicore
Multicores
should be run where the risk of mechanical damage is slight should be fixed throughout their length each IS circuit to occupy adjacent cores no voltage should exceed 60V peak
Outer sheaths need not necessarily be blue for IS systems but easy identification of IS cables is required.
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X
R
R
NON - HAZARDOUS AREA HAZARDOUS AREA
CPC
CPC
Connected to plant earthvia gland at detector
Screen , insulated and leftto float in detector box
Screen connected to plant earth at detector
Connected to plant earthvia gland at enclosure
I.S. earth terminal
Break in screen when carrying earth leakage current
SPARK
Cable Screen EarthingRequirements
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0 0
0 0
0 0
HAZARDOUS AREA
NON-HAZARDOUS AREAIS Earth ( Min csa 4.00mm2 )
BARRIER ENCLOSURE
JOINT BOXScreen insulated and leftto float in enclosure
Screen connected to through terminal . Isolated from plant earth.
Screen connected to barrier busbar
CPC connected via gland body
TRANSDUCER
SCREEN EARTHING ARRANGEMENT
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ISCircuits
Separate cable trays.
Partition must be earthed.
IS and non-IS circuits can be in the same tray so long as at least one cable is armoured or has a metal sheath.
non-ISCircuits
ISCircuits
non-ISCircuits
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Earthing where fault currents are caused to flow via a dedicated return path (backed up by connection to physical ground) in such a way as to operate a protective device in an appropriately short time.
Bonding where voltage difference between parts of plant, handrails etc., are eliminated by physical cross-connection ( i.e. common potential )
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The length of the cable in the hazardous area is the only crucial factor when using simple apparatus and an approved safety barrier
The length of the cable in the hazardous area is the only crucial factor when using simple apparatus and an approved safety barrier
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Safety barrier
Hazardous area Non Hazardous area
Certified system
31/2 digitIndicator
MTL 633B
EEx ia IIC T4Ex94C2053
4 - 20 mA
Safety description
Input terminals: non-energy storing apparatus; can be connected without further certification into any IS loop
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Least expensive no requirement for lockable fused isolators, protected cable, special glands
Simple apparatus permits the use of normal industrial devices if they are non-energy storing
Fault Tolerant IS is the only technique which remains safe after faults develop in cables and fallible components
Live maintenance IS is the only technique which permits live working without gas clearance certificates
Unarmoured cables System is electrically, not mechanically protected
Safe for personnel extra low voltage and currents
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Recap Questions 3
Q1. Cable screens should be insulated and left to float in the enclosure of a transducer in the hazardous area . True / False .
Q2. Intrinsically safe circuit cables should be tested with a potential of 500V to determine if the insulation has been damaged . True / False .
Q3. 6mm is the minimum clearance between similar I.S. circuits . True / False .
Q4. 100mm is the minimum distance for segregation between I.S. and Non I.S circuits . True / False .
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Screens are to be connected to earth at one point only, usually the same point as the barriers
Barriers are the correct type and firmly fixed to earth bar
Record results by an effective and verifiable means
Inspection
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Apparatus must be installed in accordance with drawings and schedules, fully labeled and undamaged
Segregation of IS and non-IS cable should be adequately maintained
Creepage and clearance distances must be acceptable where cable cores enter terminal blocks
IS earth connection must be separate from plant earth and other electrical earths, except at one point - the main electrical system earth point
Inspection
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Where screens are used in IS cables then they should be earthed at one point only, usually at the same point as the interface devices are earthed.
In the field, screens should be isolated from earth and each other .
Cable armour or metal shields should be solidly earthed to the plant structure
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Electrical inspection
Insulation testing (hazardous area)
Resistance of core to core and core to screen, typically
Pass > 10MFail < 1 M
Insulation testing (hazardous area)
Resistance of core to core and core to screen, typically
Pass > 10MFail < 1 M
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Despite the fact that these are low current devices, they still generate 500V dc ( 500V rms ). If the cable has sufficient capacitance it has the potential to store enough energy to provide an incendive spark.
!Read the manual
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Don't bridge the interface
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V V Non-ISmeter
Simple test on barrier
Zener Diode Barrier
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Work Live
Short circuit wiring on one loop at a time
Open-circuit wiring on one loop
Claim simple apparatus for:wire links as switchesresisters as loads
Perform measurements through barriers
You must not You mustUse the correct tools to avoid possible damage to enclosures etc.
Use certified test equipment
Use barriers for measurements with uncertified equipment
You may Interfere with the process
without authorisation
Compromise the safety of the loop by :
bridging the interface interconnecting separate
loops incorrect substitution of
interfaces invade IS circuits with
unrestricted power
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Multiple Choice Questions on the topic of Intrinsic Safety
The standards indicate that a limitation is placed on the input voltage when supplying an instrument system via a zener barrier.The level stated in the standards is :
( a ) 55V ( b ) 110 V ( c ) 250V ( d ) 440V
When a screened cable is used to supply an intrinsically safe transducer , as part of an emergency shut - down system , itis recommended that the screen be terminated :
( a ) at one end only ( b ) at both ends
( c ) to the plant earth at both ends ( d ) isolated at both ends
A suitable instrument for measuring the resistive value of the copper protective conductor when used for intrinsic safety is :
(a ) an insulation tester ( b ) an ohmmeter
( c ) an audible continuity tester ( d ) a neon test screwdriver
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Multiple Choice Questions on the topic of Intrinsic Safety
The correct clearance distance between an IS circuit and earth , operating at a potential of 65V would be :
( a ) 1.5mm ( b ) 3mm ( c ) 4mm ( d ) 6mm
It is a recommendation that the unused cores of an instrument cable supplying intrinsically safe circuits , when terminatedin a control panel containing zener barriers be :
( a ) left to float in the enclosure until required ( b ) connected together and terminated to the plant earth
( c ) terminated in a connector and left to float until required ( d ) connected together and terminated to the intrinsically safe earth
An example of “ Simple Apparatus “ as defined in the standards would be :
(a ) a transducer producing an output of 120mA ( b ) a transducer producing an output of 2V
( c ) a light emitting diode ( d ) a capacitor capable of storing 30J
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Multiple Choice Questions on the topic of Intrinsic Safety
The minimum CSA of the protective conductor required for a circuit using a zener barrier to maintain the intrinsic safety wouldbe :
( a ) 1.5mm2 ( b ) 2.5mm2 ( c ) 4.00mm2 ( d ) 6.00mm2
Intrinsically safe and non - intrinsically safe circuits are permitted in the same enclosure provided that :
( a ) the cables have the same thickness of insulation ( b ) the cables are made by the same manufacturer
( c ) the cables are bound together in the same cable loom ( d ) the correct clearance is applied
Intrinsically safe components , circuits and systems are categorised as “ ia “ and “ i b “ . A device which is identified as thatwith the category “ ia “ indicates that it will remain safe with :
(a ) no faults ( b ) 1 fault
( c ) 2 faults ( d ) 3 faults
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PORTABLE APPARATUS
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Simple apparatus or any approved intrinsically safe apparatus with entity parameters
Vmax 30V
Imax 350mA
Ci + Ccable 0.02µF
Li + Lcable 0.07mH
Hazardous (Classified) Location Non-Hazardous Location
R&H
Model 728+
I.S. Ground
C (F)Group L (mH)
0.11IIC 4
0.33IIB 12
Vt It
29.2V 297mA
Entity Concept parameters
Notes The barrier must not be connected to any device which uses or generates in excess of 250Volts RMS or DC unless it has been determined that the voltage has been adequately isolated from the barrier
The barrier must be connected to a suitable ground electrode. The resistance of the ground electrode must be less than 1 Ohm.
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Hazardous area
24VApprovedbarrier
Barrier ParametersO/C voltage VbS/C current IbCapacitance CbInductance Lb
250 Approved apparatus
Max. Voltage VaMax. Current IaInternal Capacitance CaInternal Inductance La
Non Hazardous area
For this example Va , Ia Ca and La relate to the Apparatus . Likewise , Vb , Ib , Cb and Lb relate to the Barrier
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