BS7671 17th ed draft 1 Part 2 - Legh Richardson 17th ed... · January 08 Legh Richardson ☺ 1...

January 08 Legh Richardson 1

BS7671 2008: C&G 2382 Outcomes 4

Outcome 4 Protection for safetyThe candidate will be able to:

4.1 Identify the differences between basic and fault protection

4.2 State means of protection against electrical shock by

a basic protection

b fault protection

c both basic and fault protection (excluding IT)

d additional protection

4.3 Describe how the requirements for shock protection are affected by

a value of the external loop Impedance (Ze)

b compliance with Zs = Ze + R1 + R2

c compliance with tables 41.1, 41.2, 41.3, 41.4, 41.5 and 41.6

Methods of reducing the likelihood of electric shock

1. Limit the current flow to a person2. Limit the current flow through a person3. Limit the duration of shock

Protection Against Shock


1. Limit the current flow to a person (Basic Protection)Insulation (416)Class II equipment (412)Barriers and Enclosures (416.2)Non conducting location (418.1) restricted to trained personnel

Obstacles and placing out of reach (417)Electrical Separation (413, 418.3) also (411.6)

2. Limit the current flow through a person Basic and Fault protection)SELV, PELV, FELV, RLV (414,411.8)Earth free protective earth bonding zone (418.2)

3. Limit the duration of shock (Fault Protection)Automatic Disconnection of Supply (RCDs, OPDs not shock protection) (411)RCDs in TT systems (411.5)


Protection for Safety Part 4

Protection from Electric Shock 411.2

416 – 417 Basic Protection (Direct Contact)

1. Basic Insulation of Live Parts

2. Barriers or Enclosures - 416.2

3. Obstacles, Placing out of Reach – 417.2, 417.3

4. Class II Equipment - 412

418 Basic Protection where supervised by a skilled or instructed person

1. Non-conducting Location (418.1)(No earth contact at sockets)

2. Earth Free local equipotential bonding (Faradays Cage)

3. Obstacles 4. Placing out of Reach5. Electrical separation from more than one item of

electrical equipment (common neutrals in IT systems)

Protection for Safety Part 4

Protection from Electric Shock Part 4

411. Fault Protection (Indirect Contact)1. Automatic Disconnection (ADS)2. SELV, PELV, FELVThe use of:1. RCDs2. Class II equipmentAre additional measures added to ADS and ELV

Protection from Electric Shock Part 4

Additional Protection 411.3.330mA RCDs to be used on Socket outlets

Where:1. Socket outlets <20A

2. Mobile equipment < 32A

Exceptions permitted to 1. above:1. Under the supervision of a competent person

(Industrial, commercial installation)

2. Identified and labelled for a specific item of equipment (freezers, fire alarm)



Protection from Electric Shock

Why RCD protection is provided for ordinary persons(Electricity is the same in the USA as in Europe!)


Protection from Electric Shock



Certain Conditions apply for Automatic protection (ADS) 411.4.4

1. Under fault conditions the supply must be disconnected from a circuit within the time stated – Table 41.1

2. Have an Earth Fault Loop Impedance below or equal to Uo/Ia– Tables 41.2, and 41.3

IaUZs O≤


BS7671 2008: Protection from Electric Shock

Fault Protection – A low Earth Fault path controlling the duration of the fault

Supply

Circuit Fuse

Load

Fault

Alternative Earth-Fault PathSuppliers Earth

Earth-Fault Path

Origin of Installation

1. Stop the voltage potentials on exposed and extraneous conductive parts rising beyond the safety voltage - touch voltage – in fault conditions – 415.2

Where R is the resistance between two simultaneously touchable exposed/extraneous metalwork

1. Use an RCD for additional protection to detect earth fault currents at 50V ≥ Zs.I∆N - 415.1.1(Where I∆N ≤ 30mA, t ≤ 40mS at 5I∆N, )

aa IV120R:SystemsDC

IV50R:systemsAC ≤≤



BS7671 2008: Protection

Fault Protection using SELV or/and local additional supplementary bonding by controlling the current flowing through the body


Protection against Shock


Fault and short circuit protection

Table 41.3 Max Zs for 0.4-5sec disconnection at 230V

For the above devices :Type B Type C Type D

46/In 23/In 11.5/In

BS7671 2008 Update


Prove that the following tables provides instantaneous disconnection for a BS EN Type B 32A MCB using tables 41.3, App 3,

conforms to:

16th Edition 17th edition

Most manufacturer’s data show 3-5 * In for instantaneous disconnection

Type B Type C Type D

46/In 23/In 11.5/In

Type B Type C Type D

48/In 24/In 12/In

)3Appendix(.dist.instfor.Amps.Min

.distinstformultiplier

)2.41table(ZsMax

A1605A32

In.546

230In.548

240

44.1324644.1

160230Zs5.1

32485.1

160240Zs

=×

==

Ω=Ω=≤Ω=Ω=≤

IaUZs O≤


MCB instantaneous tripping currents

BS EN60898 and BS EN 61009 Instantaneous tripping currents

Type Amperes

1 * > 2.7 IN * ≤ 4.0 IN

2* > 4.0 IN * ≤ 7.0 IN

3* > 7.0 IN * ≤ 10.0 IN

4* > 10 IN * ≤ 50.0 IN

B > 3 IN ≤ 5.0 IN

C > 5 IN ≤ 10.0 IN

D > 10 IN ≤ 20.0 IN


Basic and Fault Protection

411.3.3 Additional Protection for ADS in accordance with 415

Two methods are used:415.1 Protection by Residual Current Devices1. 415.1.1 max value = 30mA, At 5 I∆N < 40ms2. 415.1.2 Not recognized as a sole means of protection415.2 Supplementary Bonding1. 415.2.1 Supplementary Bonding to localised and generalized zones

To include local cross-bonding to all extraneous and exposed conductive parts with structural elements and metallic service pipes

2. 415.2.2 Where doubt exists to the effectiveness of supplementary bonding then it should be tested against R ≤ 50 / Ia for AC systems and R ≤ 120V / Ia for DC systems

BS7671 2008 Update

The use of Supplementary bonding (415.2.2)Where doubt exists regarding the effectiveness of extraneous conductive

parts having the same potential then:

Where protection is given by a RCD then:

Where protection is given by a 20A type B MCB then:

As long as the impedance of the circuit (exposed conductive part) is < 0.5Ωthen the touch voltage on any simultaneously touchable metalwork for the time the fault is in operation will not rise above 50V

aa IV120R:SystemsDC

IV50R:systemsAC ≤≤

Ω=×

= − k67.11030

50R 3

A1004.3Fig3AppendixorA1003.2

230Ia === Ω== 5.010050'2'R



Protection provided by a Residual Current Device TN systems. Following condition applies:Regulation 411.5.2:

Zs = Earth fault loop impedance in Ohms; Ian = rated residual operating current in Amps

Protection in TT systems.The use of over current protective devices are not excluded although it is preferred to use a RCD with a disconnection time of not greater than 1 sec 411.3.2.4

Regulation 411.5.3RA = sum of all the resistances of earth electrode and protective conductors connected to the exposed conductive parts;Ia =the current causing automatic operation of the protective device with

1 sec

V50IZs n ≤× ∆

VIaRa 50≤×



Allowances from table 41.1411.3.2.3 TN systems disconnection time < 5sec for a distribution circuits411.3.2.4 TT systems disconnection time < 1sec for a distribution circuitsNot covered by 411.3.2.2 – (>32A circuits)411.3.2.6 where disconnection times cannot be met then supplementary bonding must be applied


BS7671 2008: Protection

Fault protection using an RCD to BS EN 61008/9 by controlling the duration of current flowing through the body

You do not need an earth with RCDs but you do need an imbalance of current between the Line and Neutral



411.5 TT earthing systems

Where disconnection times cannot be met, Zs is too high, then the Use of RCD protection is used for Earth Fault currents

Basic and Fault protection in TT systems

411.5.1 Where RCDs are used in series, each RCD can be earthed via an earth electrode

411.5.2 The method of achieving the disconnection times as stated in 411.3.2 and table 41.1 can be achieved by the use of:

1. RCD2. OPD

The former being preferred


Basic and Fault protection

411.5 TT earthing systems


Protection against shock and Fire

The TT earthing system in conjunction with RCD fault protection


Protection against shock and Fire

The use of and terminology of IT systems are more prevalent in the 17th edition (411.6.1)What is an IT system? High impedance to earth or no connection

Basic and fault protection in IT systems

IT systems – Insulation Monitoring Devices IMDs411.6.3

IMD

Main Switch.

Gear

Dist.BoardRCBOs

IMD

IMD

IMD

Final Circuits

Limiting Impedance

Supply Source

Supply Electrode

Single fault condition must behave like a TN system and disconnect within the times stated in 41.1


Basic and fault protection in IT systems

IT system of protection

Vpoc(V2) = VS × (R2 / R1 + R2 + R3)

Where R3 is fixed by the distributor around 50,000 Ohms R2 is the resistance of the earth path across a human body (1.0 kOhms) and R1 is the resistance of the phase conductor (1.0 Ohm)

The Max voltage across body to the source =

(Voltage drop sits outside EQBZ)

R1

R2

Resistance of the Phase conductors and Transformer windings

Point of first fault

Resistance of Earth. Voltage across body in contact with exposed conductive part Resistance Fault path

V2

V1

V3 R3

V5.4000,5010001

1000230 =++

×


Basic and Fault protection

Methods of protection IT systemsHigh impedance to earth means that the majority of the volt drop under fault conditions is outside of the equipotential bonding zone of the installation(Earth to phase monitoring device < 50kΩ causes an alarm to sound IEC 60364)

suitable protective devices 411.6.31. Insulation Monitoring Devices (IMD)2. Residual Current Monitoring Devices (RCM’s)3. Insulation Fault Location Systems4. Overcurrent Protective Devices5. Residual Current Devices (RCD’s)

PELV and IT systems

IT systems and earth monitoring equipment

The earth conductor is not separated with PELV but monitored



FELV Systems 411.7Designed where SELV and PELV are not fulfilled (414)Basic Protection 411.7.21. Insulation2. Barriers and/or enclosuresFault Protection 411.7.3Exposed conductive parts shall be connected to the primary earthing system to provide Automatic disconnection in the event of a faultMethods of reducing voltage for a FELV system do not include1. Autotransformers2. Semiconductor devices3. Potentiometers



Reduced Low Voltage Systems (RLV) – 411.8(Voltage range < 110V ac 55V to earth single phase, 63.5V ac to earth three phase)

Basic ProtectionInsulationBarriers and Enclosures

Fault ProtectionAutomatic Disconnection < 5secs by OPD or RCDFor RCDs: 50V ≥ I∆N x Zs ( I∆N x Zs ≤ 50V)See Table 41.6



Zs requirements for reduced voltage systems

Protection against shock

Examples:1/ What is the maximum Zs for a BS EN 61009 OPD protecting a 230V ac 20A Type B A3 radial final circuit?2/ What is the maximum Zs for a 110V AC final circuit protected by a 32A Type C BS EN 60898 OPD?3/ What is the maximum Zs for a BS 88 type 2 100A HRC fuse protecting a 400V sub-main cable to a distribution board?


Requirements for Basic and Fault protection for Electrical Equipment 412.2.1

“Where the protective measure of double or reinforced insulation is used for the complete installation or part of the installation, electrical equipment shall comply with the following:”

1. equipment type tested and marked to the relevant standard 412.2.1.1

2. equipment with basic insulation only shall have supplementary insulation applied in the process of being erected 412.2.1.2

3. equipment having no insulation around live parts shall have reinforced insulation applied in the process of being erected 412.2.1.3412.2.2 relates directly to enclosures which is applicable to (ii) and (iii) above


Symbols used for indicating protection by insulation412.2.1.1

412.2.1.2 and 412.2.1.3

412.2.2.3 Insulating enclosures must be rated at IP2X or IPXXB

And shall only be removed by the use of a tool or a key



414.1 Separated Extra Low Voltage (SELV)≤ 50V ac phase to earth and 120V DC Positive and negative terminalsSupplied to Double Insulated Appliances and EquipmentSupplied from: 414.3

1. Double wound isolating transformer2. Generator3. Battery4. Certain types of electronic SELV equivalent Equipment 414.2 The Primary Earthing functions separated from the secondary earthing – No secondary earth path back to sourceNominal ELV voltage ≤ 50V ac and 120V DC



Conditions for SELV systems – 414.4.2(i) insulated to the highest voltage present(ii) enclosed in a separate sheath or conduit additional to the basic insulation(iii) conductors of higher voltages to be separated by earthed metallic screen or sheath(iv) where installed in a multi-core cage or grouped together with higher voltage conductors the SELV conductors are insulated to the highest voltage present(v) Separation via earthed metallic screen(vi) Physical separation

414.4.5 SELV/PELV circuits >25V ac or 60V DC or equipment immersed underwater require additional protection by

InsulationBarriers or/and enclosures


BS7671 2008 C&G 2382 Outcomes 4 continued

4.4 Describe means of protection against fire, burns and harmful thermal effects and identify precautions where particular risks of danger of fire exists

4.5 Identify the difference between overcurrent and fault current

4.6 Identify the differences between overload currents, earth fault currents, short circuit currents and shock currents

4.7 Describe methods of overcurrent protection and the need for co-ordination with conductors and equipment.

4.8 State the requirements for protection against a voltage disturbances

I. overvoltage II. undervoltage

b electromagnetic disturbances.


Protection against Thermal effects

Chapter 42 now combines together chapters 42 and 48 from the 16th edition420.1 - Scope:

I. Heat generated by electrical equipmentII. Ignition, combustion or degradation of materialsIII.Propagated fire and smoke to other fire compartmentsIV. Protection of safety services being cut off by the

failure of electrical equipment


421.2 protection against hot surface temperaturesI. Mounted on a surface of low thermal conductanceII. Be screened by low thermal materialsIII.Positioned to allow dissipation of heat

421.3 protection against arcs and sparks

I. Totally enclosed in arc resistant materialII. Screened by arc resistant materialIII.Mounted to allow safe extinguishing of sparksIV. In compliance with its standard




422.3 and 422.4 Lamps and luminares must be positioned away from combustible structures and materials

1. < 100W = 0.5m2. > 100W < 300W = 0.8m3. > 300W < 500W = 1.0m

422.3.9 where MIMS, busbar, powertrack, are not used then:

1. TT,TN systems should be protected by <300mA RCD2. Where overheating and fire are high use a <30mA RCD3. IT systems use IMD



423.1 Protection against burns


Problem Currents

Examples:1. Fault2. Fault current (434)3. Overload currents (433) 4. Overcurrents (435)5. Short Circuit Current (434) 6. Earth Fault currents (435)7. Shock Currents8. Prospective Fault Current9. Protective device’s Operating current


Overcurrent Protective Devices

Types of Protective Devices (In) – 432.1, 433.3, 434.3,BS 3036 Rewireable fuse links

BS1361 Cartridge Fuse links

BS 88 pt 2 & pt6 HRC or HBC Fuses

BS 3871 MCBs - Miniature Circuit Breakers (old type)No longer included in BS7671

BS EN 60898 New type MCBsBS-EN 60947-2 MCCBs 10kA+BS EN 61009 RCBOs Residual Current Breaker with overload protection


Overcurrent protective devices

Fusing Factor – 533.1.1.2 ii

Typical Fusing Factors for:BS88 Pt2 & pt6 - 1.25 - 1.75BS3036 - ≈2.0BS1361/2 - 1.3 - 1.5BS3871 / BS EN 60898 - 1.1 - 1.4

Appendix 3 Time current characteristics

)(sin)(sinsin

InCurrentgFuRatedICurrentgFuActualFactorgFu 2=


Coordination of Protective Devices

Coordination of Protective Devices and Current Carrying Capacities of Conductors for overload and short circuit – 433.1.1, 435.1

Overload conditions

Note: It is the tabulated current after taking external factors into consideration

Compliance with 433.1.1 (i), (ii), (iii)HBC BS88pt2.1, pt6BS1361 Cartridge FuseCircuit Breaker to BS EN 60898 and BS EN 60947-2RCBO to BS EN 61009-1

Z

ZNB

NB

IIbIII

IIa

×≤

≤≤∴

≤

4512 .)(

)(

ZtNb IIII ≤≤≤



To Comply with the regs using BS3036 – 433.1.3In order to satisfy the terms in 433.1.1 (ii) then :

To satisfy the fact that rewirables have a fusing factor up to 2 timesFusing Current = Fusing Factor * Fuse Rating

= 2 * In

The use of Semi-enclosed or re-wireable fuses to BS3036 is not recommended for untrained persons – 533.1.1

ZN II ..7250≤

ZZ

ZN

ZN

II

IIandIIandII

......

...

72502451

45124512 22

≤×∴≤

≤

≤=


Coordination of Protective devices

Energy Let through (434)All electromechanical devices have a maximum breaking capacityMaximum amount of energy that the component will allow through without exploding or disintegrating General Equation

Note: The Resistance of the conductor can be regarded as negligible (but not zero) for the short time period and very high fault currents

Seconds

Joules

tEPwheretIE == 2



Short Circuit ProtectionRegulation 434.5.2 States that:The regulation is satisfied if the time for disconnection is equal to or less than:

The fault current must be cleared before the time given in the above equation Note: for short circuits between live conductors and for earth fault currents

2

22

IkSt .

=



434.5.2 and 543.1.1 states that for the time that the earth fault exists the cpc must be able to dissipate the heat generated without damage to the other cables

543.1.1434.5.2

So as long as :

Then the thermal characteristics of the cable are protected from the energy let through of the protective deviceThe size of the cpc usually works out to be much smaller than

anticipated, regulation 543.1.4 is applicable and then this must be sized up according to table 54.7

KtIS .2

=

tIKS .. 222 ≥

2

22

ISkt .

=


Applying Circuit Design 2

Operating Characteristics of Protective DevicesEnergy Let Through (Heat) 434.5.2

Energy (I2t) let through by fuse

t

I (r.m.s.)

I (r.m.s.)Energy (I2t) let through by one cycle of AC current

Fuse duration

Fuse Link Cut-off

t2t10t

t1 Pre-arcing I2t melting area t2 Total I2t area


Energy let-through for MEM 10kA MCBs


MCB Breaking Capacities

MCB to BS3871Obsolete from 2001 (blue)

Circuit Breakers BS EN 60898BS EN 61009

Category of Duty

Prospective current (A)

ICN (once only) ICS (repeatable)

kA kAM1 1000 1.5 1.5

M1.5 1500 3.0 3.0

M3 3000 6.0 6.0

M4.5 4500 10 7.5

M6 6000 15 7.5

M9 9000 20 10.0

25 12.5

Energy let-through video

GEC BS 88 Fuse protection


Operating Characteristics of Protective DevicesElectromagnetic Energy

Applying Circuit Design 2

Electromagnetic stress proportional to (I2) peak current

Electromagnetic stress(I2) let through by one cycle of AC current

Fuse Link Cut-off

t

I2 (peak)

Fuse duration


Discrimination of OPDs

435.2 Discrimination of Protective Devices

I2 characteristics of HBC fusesRated Current plotted against Amperes squared

103A

104A

105A

106A

30A 40A 50A 60A 80A 100A

Pre-arcing (I2t)

Total (I2t)

Ferraz Shawmut BS88 cartridge fuses

I2t Characteristics

Ampere Fuse rating

Discrimination achieved when downstream fuse when downstream fuse is is ½½ the size of the the size of the upstream OPDupstream OPD


Protection against Fault Currents

Discrimination of Protective Devices - 434.1 to 434.5.3

Discrimination of devices must take place to reduce danger and inconvenienceIf fuse ‘Z’ blows then fuse ‘Y’ should be of such size that it can withstand the energy let through without disconnecting

100A

80A?

40A

30A

30A

50A

300A

60A

X

Y

Z


Coordination of OPDs

The example, shows the superimposed characteristics of a 5 A semi-enclosed fuse and a 10 A miniature circuit breaker which we shall assume are connected in series.

If a fault current of 50 A flows, the fuse will operate in 0.56 s whilst the circuit breaker would take 24 s to open. Clearly the fuse will operate first and the devices have discriminated. However, if the fault current is 180 A, the circuit breaker will open in 0.016 s, well before the fuse would operate, which would take 0.12 s. In this case, there has been no discrimination.


Coordination of OPDs

Graph A shows discrimination from instantaneous disconnection of the 16A MCB from the 32A fuse at 180A

Graph B shows poor discrimination only starting at 100A in an overload condition but none for short circuit


Protection against overcurrents

Positioning of Device – 433.2 and 434Where a conductors diameter reduces along the line of a cable run a method of protection is required for that part of the cable which has a reduced cross sectional area 434.1.1

Examples of reduced cable conductors are:Fused spur on a ring final circuitInstallation method changed (overhead to underground)Type of cable has changed (PVC in conduit to MIMS)Ambient change in temperature (Boiler house to outside)

Rules for termination between reduction in current carrying capacity and Protective Device 434.2.1

1. Not Exceed 3m in length 2. Be erected to minimise risk of fault current3. Be erected to minimise fire and danger to persons


Protection against Overcurrent

Omission of protective devices for safety reasons 433.3.3

Used where unexpected disconnection would cause a dangerous situation

1. The exciter circuit of a rotating machine2. The supply circuit of a lifting magnet3. The secondary circuit of a current transformer4. A circuit supplying a fire extinguishing device5. A circuit supplying a safety circuit (fire or gas alarm)6. A circuit supplying medical equipment in IT systems


Voltage and Electromagnetic disturbances

441 Overvoltages due to HV and LV faults

Scope (See note)1. HV faults to earth at the substation 442.22. Loss of supply neutral on LV systems 442.33. Line to Neutral Short Circuit in LV systems 442.54. Accidental earthing of a line conductor to earth in IT systems 442.4

Stress Voltages created by HV currents circulating around exposed conductive parts producing an electromagnetic effect thus producing a secondary fault voltage (stress voltage U1 and U2)

442.1 rules for designers and installers of substations1. Quality of system earth2. Maximum level of earth fault current3. Resistance of earthing arrangements

HV fault conditions affecting LV systems




443 Overvoltage requirements


Problem Voltages

445 Undervoltages445.1.1 Suitable precautions shall be taken to provide protection when the voltage dips or is reduced

445.1.5 No automatic restarting of rotating machinery(Note: see external classifications appendix 5)

BS7671 17th ed draft 1 Part 2 - Legh Richardson 17th ed... · January 08 Legh Richardson ☺ 1...

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Transcript of BS7671 17th ed draft 1 Part 2 - Legh Richardson 17th ed... · January 08 Legh Richardson ☺ 1...