1c081-19

Licensed copy: Tarhafra Con

CIRCUIT-BREAKERS C C81-19Miniature, conforming to BS 3871

© Electrical Safety Council

CIRCUIT-BREAKERS - Miniature, conforming to BS 3871

Contents

1. Introduction

2. Instantaneous tripping current (‘Type’ of circuit-breaker)

3. Short-circuit capacity

4. Suitability for use for isolation and switching

ANNEX

AA: Table 41B2 from BS 7671: 1992 (As amended) giving information on

circuit-breakers conforming to BS 3871.

1. Introduction

BS 3871: Part 1 was withdrawn in 1994, and production of miniature circuit-breakers to thatstandard ceased some years ago. Yet such circuit-breakers will be encountered for yearsto come by electrical contractors carrying out alterations, additions and periodic inspectionand testing of existing installations.

This topic gives information on the more significant differences between the characteristicsof miniature circuit-breakers conforming to BS 3871: Part 1 and those of circuit-breakersconforming to BS EN 60898, which supersedes BS 3871: Part 1.

General information on circuit-breakers conforming to BS EN 60898 is given inTopic C81-13. Two other topics relating to circuit-breakers which may be of interest are:

• C81-5 CIRCUIT-BREAKERS: Ambient temperature and grouping, implications of, and

• C81-25 CIRCUIT-BREAKERS: Selection, to suit load characteristics.

2. Instantaneous tripping current (‘Type’ of circuit-breaker)

It was not until 1981, sixteen years after BS 3871: Part 1 was first published, that it firstbegan to classify miniature circuit-breakers into 'Types', according to their instantaneoustripping current. First came Types 1, 2, 3 and 4, introduced by the 1981 amendment ofBS 3871: Part 1. Then came Types B, C and D, introduced by the 1991 amendment,having the same instantaneous tripping characteristics as Types B, C and D circuit-breakersconforming to BS EN 60898.

BS 3871: Part 1 permits instantaneous tripping to begin to occur at a value of fault currentwithin a range of values, expressed as multiples of rated current (I n ) for each of the aboveseven Types of miniature circuit-breaker. The standard ranges are shown in Table 1although manufacturers were free to design miniature circuit-breakers to operateinstantaneously within smaller ranges of current, falling within the standard ranges.

sulting Ens. Co.,Tue Jun 8 05:18:13 2010

ELECTRICAL SAFETY COUNCIL ESSENTIAL GUIDE April 2010 1 of 7




Generally, in order to meet the requirements of BS 7671 for automatic disconnection tooccur within a prescribed time limit (such as 0.4 s or 5 s, as appropriate) under earth faultconditions, the prospective earth fault current must be not less than the upper value of therange of instantaneous tripping currents for the miniature circuit-breaker concerned.

For example, on the basis of the standard ranges of current referred to in Table 1, the earthfault current for a 32 A Type 1 miniature circuit-breaker is required to be not less than 128 A(that is, 4 I n ). Correspondingly, earth fault currents of not less than 224 A and 320 A,respectively, are required for instantaneous tripping of 32 A Type 2 and Type 3 miniaturecircuit-breakers. Type 4 miniature circuit-breakers should be treated as a special case, asreferred to later in this item.

Table 1

The higher the prospective fault current required to cause instantaneous tripping, the lowerthe limiting earth fault loop impedance (Z s ) needs to be. This can be seen from thelimiting values of earth fault loop impedance given in Table 41B2 of BS 7671: 1992*, forminiature circuit-breakers, reproduced in Annex AA of this topic, and in the other similartables of BS 7671: 1992* such as Tables 471A and 604B2.

Due to the particular time/current characteristics of a circuit-breaker, values of earth faultloop impedance even marginally exceeding the limiting values given in the above tablesmay result in greatly increased disconnection times which would not be acceptable for faultprotection (protective measure Automatic Disconnection of Supply (ADS)).

As it cannot be predicted where in the range a circuit-breaker will trip, the limiting valuesof earth fault loop impedance in Table 41B2 and similar tables of BS 7671: 1992*, forminiature circuit-breakers, are based on the upper range values of current indicated byTable 1. The limiting values are derived by the method given in the introduction toAppendix 3 of BS 7671:1992 (Time/current characteristics of overcurrent protectivedevices). That method is discussed in Topic F18-9.

Circuit-breaker type Standard range

1

2

3

4

B

C

D

above

above

above

above

above

above

above

2.7 I n

4 I n

7 I n

10 I n

3 I n

5 I n

10 I n

up to and including

up to and including

up to and including

up to and including

up to and including

up to and including

up to and including

4 I n

7 I n

10 I n

50 I n

5 I n

10 I n

20 I n

* It has been necessary to make reference to BS 7671: 1992 (now superseded) because the tables of earthfault loop impedance in BS 7671: 2008 do not include data for miniature circuit-breakers conforming toBS 3871.






Example of circuit-breakers to BS 3871

Illustration courtesy of Electrium Ltd.

Limiting values of earth fault loop impedance for Type 4 miniature circuit-breakers were notgiven in the tables of earth fault loop impedance in BS 7671: 1992*. Normally, where sucha limiting value is required, this must either be obtained from the manufacturer of theminiature circuit-breaker, or derived by the method given in Appendix 3 of BS 7671 using avalue of instantaneous tripping current obtained from the manufacturer. Although thecurrent necessary to cause instantaneous tripping of a Type 4 miniature circuit-breaker ispermitted to be as high as 50 I n (see Table 1), the actual fault current required may besomewhat less than that value. This is because manufacturers may stipulate other upperrange values of instantaneous tripping current (such as 20 I n ).

Manufacturers of BS 3871 miniature circuit-breakers used to publish, in relation to theirown circuit-breakers only, limiting values of earth fault loop impedance which are higherthan the corresponding limiting values given in Table 41B2 of BS 7671: 1992*. This isbecause, as indicated earlier in this item, manufacturers were free to designcircuit-breakers to operate instantaneously within ranges of current smaller than, but fallingwithin, the standard ranges given in Table 1.

48.8 13.5

56.5

86

181 1.5 2 3 4 5

3 56 8 10 15 20 30

0.01

0.02

0.040.06

0.1

0.2

0.40.6

1

2

46

10

20

401

2

46

10

20

4060

120

Tri

pp

ing

tim

e

Min

ute

sS

eco

nd

s

1.13 1.45

7

Multiple of rated current

ac

dc

NSB MCBs

British standardsRated voltage (single pole)Tripping characteristicsShort circuit ratingReference calibration temperatureterminal capacity - outgoing cable

BS 3871: Part1240 voltsType 2M6 (6kA)40°C0.75 to 25mm2

Cat. No. Rating AmpsNSB06 6NSB10 10NSB16 16NSB20 20NSB32 32NSB40 40






Furthermore, for Type 4 and Type D miniature circuit-breakers, manufacturers used topublish limiting values of earth fault loop impedance to facilitate tripping in a time of up to5 s (that is in addition to limiting values of earth fault loop impedance based oninstantaneous tripping current). This is because the characteristics of a Type 4 and Type Dminiature circuit-breaker, unlike those of the other circuit-breaker types, can be such as toallow tripping other than instantaneously without the risk of a greatly increaseddisconnection time.

It is important to ensure that earth fault loop impedances meet the requirements ofBS 7671 and that this must not be overlooked when an existing overcurrent protectivedevice is replaced. This is particularly important where the Product Standard for thedevice permits a manufacturer’s design to be within standard ranges (as is the case withinstantaneous tripping current in BS 3871). Whenever such a circuit-breaker is replacedby other than an identical unit, even though the replacement has the same rated current, itis necessary to verify that the earth fault loop impedance of the circuit is within the earthfault loop impedance limit of the replacement circuit-breaker.

3. Short-circuit capacity

Unlike a circuit-breaker conforming to BS EN 60898, which has two defined short-circuitbreaking capacities (ultimate and service), a miniature circuit-breaker conforming toBS 3871: Part 1 has only one short-circuit capacity, designated as its ‘category ofduty’. This is marked on the device.

The preferred categories of duty according to BS 3871: Part 1 are listed in Table 2 below,together with the corresponding maximum values of prospective fault current which applyunder specified conditions of type-test. BS 3871: Part 1 requires that after type-testing,a miniature circuit-breaker will trip within three minutes at a current of twice its ratedcurrent.

Table 2

A miniature circuit-breaker must not be installed at a point where the prospectiveshort-circuit current or prospective earth fault current exceeds the appropriate value givenin Table 2, unless suitable back-up protection (such as an appropriately rated HBC fuse) isprovided in accordance with Regulation 434.5.1 of BS 7671.

Category of duty Maximum prospective fault current

(A)

M1 1000

M1.5 1500

M2 2000

M3 3000

M4 4000

M6 6000

M9 9000






Selection of a device for back-up protection should be made in consultation with themanufacturer of the miniature circuit-breaker. On request, a manufacturer should be ableto state the type and characteristics of fuses to be used in conjunction with a miniaturecircuit-breaker, and the maximum short-circuit current for which the combination is suitableat the stated rated operational voltage.

4. Suitability for use for isolation and switching

A miniature circuit-breaker conforming to BS 3871: Part 1 may not be suitable for selectionas a device for isolation or for switching off for mechanical maintenance. This is becauseBS 3871: Part 1 does not require such a device to have externally visible contact gaps orequivalent means of reliably establishing whether or not its contacts are in the openposition, as is required by Regulations 537.2.2.2 and 537.3.2.2 of BS 7671.

An installation designer intending to select a miniature circuit-breaker conforming toBS 3871: Part 1 for isolation or for switching off for mechanical maintenance should consultthe manufacturer to determine whether the device will meet the requirements ofRegulations 537.2.2.2 and 537.3.2.2.






(e) Ty

Ratin(amp

I n

Z s (o /I n

(f) Ty

Ratin(amp

I n

Z s (o 6/I

n

(g) Ty

Ratin(amp

I n

Z s (o /I n

(h) Ty

Ratin(amp

I n

Z s (o /I n

(i) Ty

Ratin(amp

I n

Z s (o /I n

ANNEX AA: Maximum earth fault loop impedance (ZS ) for circuit-breakers to BS 3871 with U 0 of 230 V, for instantaneous operation giving compliance with disconnection times of 0.2 s, 0.4 s and 5 s.

Note: The circuit loop impedances given in the table should not be exceeded when the conductors are at theirnormal operating temperature. If the conductors are at a different temperature when tested, the readingshould be adjusted accordingly.

Note: The protective conductor impedances given in the table should not be exceeded when the conductors areat their normal operating temperature. If the conductors are at a different temperature when tested, thereading should be adjusted accordingly.

pe 1 miniature circuit-breakers to BS 3871

g eres)

5 6 10 15 16 20 25 30 32 40 45 50 63

hms) 11.5 5.75 3.59 2.3 1.80 1.28 0.91 57.5

9.58 3.83 2.88 1.92 1.44 1.15

pe 2 miniature circuit-breakers to BS 3871

g eres)

5 6 10 15 16 20 25 30 32 40 45 50 63

hms) 6.57 3.29 2.05 1.31 1.03 0.73 0.52 32.8

5.48 2.19 1.64 1.10 0.82 0.66

pe B circuit-breakers to BS EN 60898

g eres)

6 10 16 20 25 32 40 45 50 63

hms) 4.60 2.87 1.84 1.44 1.02 0.73 46

7.67 2.30 1.15 0.92

pe 3 miniature circuit-breakers to BS 3871 and Type C circuit-breakers to BS EN 60898

g eres)

5 6 10 15 16 20 25 30 32 40 45 50 63

hms) 4.60 2.30 1.44 0.92 0.72 0.51 0.36 23

3.83 1.53 1.15 0.77 0.57 0.46

pe D circuit-breakers to BS EN 60898

g eres)

5 6 10 15 16 20 25 30 32 40 45 50 63

hms) 2.30 1.15 0.72 0.46 0.36 0.26 0.18 11.5

1.92 0.77 0.57 0.38 0.29 0.23






Topics referred to in this text:

C81-5 CIRCUIT-BREAKERS: Ambient temperature and grouping, implications ofC81-13 CIRCUIT-BREAKERS: For household and similar installations (BS EN 60898)C81-25 CIRCUIT-BREAKERS: Selection, to suit load characteristicsF18-9 FAULT PROTECTION: Earth fault loop impedance, maximum values of

Topics not referred to in this text, which are related and may be of interest:

D61-1 DISCRIMINATION: GeneralF18-13 FAULT PROTECTION: Protective devices for

BS 7671 (Requirements for electrical installations)Some of the most important requirements are found in:

Protection, isolation, switching, control and monitoring Chapter 53



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