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Selection of a circuit-breaker

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The choice of a range of circuit-breakers is determined by: the electrical characteristics of the installation, the environment, the loads and a need for remote control, together with the type of telecommunications system envisaged

Choice of a circuit-breaker

The choice of a CB is made in terms of:

  • Electrical characteristics of the installation for which the CB is intended
  • Its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc.
  • Short-circuit current breaking and making requirements
  • Operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection
  • Installation regulations; in particular: protection of persons
  • Load characteristics, such as motors, fluorescent lighting, LED ligthing, LV/LV transformers

The following notes relate to the choice LV circuit-breaker for use in distribution systems.

Choice of rated current in terms of ambient temperature

The rated current of a circuit-breaker is defined for operation at a given ambient temperature, in general:

  • 30°C for domestic-type CBs
  • 40°C for industrial-type CBs

Performance of these CBs in a different ambient temperature depends mainly on the technology of their tripping units (see Fig. H40).

Fig. H40Ambient temperature

Uncompensated thermal magnetic tripping units

Circuit-breakers with uncompensated thermal tripping units have a trip current level that depends on the surrounding temperature

Circuit-breakers with uncompensated thermal tripping elements have a tripping-current level that depends on the surrounding temperature. If the CB is installed in an enclosure, or in a hot location (boiler room, etc.), the current required to trip the CB on overload will be sensibly reduced. When the temperature in which the CB is located exceeds its reference temperature, it will therefore be “derated”. For this reason, CB manufacturers provide tables which indicate factors to apply at temperatures different to the CB reference temperature. It may be noted from typical examples of such tables (see Fig. H41) that a lower temperature than the reference value produces an up-rating of the CB. Moreover, small modular-type CBs mounted in juxtaposition, as shown typically in Figure H27, are usually mounted in a small closed metal case. In this situation, mutual heating, when passing normal load currents, generally requires them to be derated by a factor of 0.8.


What rating (In) should be selected for a iC60 N?

  • Protecting a circuit, the maximum load current of which is estimated to be 34 A
  • Installed side-by-side with other CBs in a closed distribution box
  • In an ambient temperature of 50 °C

A iC60N circuit-breaker rated at 40 A would be derated to 35.6 A in ambient air at 50°C (see Fig. H41). To allow for mutual heating in the enclosed space, however, the 0.8 factor noted above must be employed, so that, 35.6 x 0.8 = 28.5 A, which is not suitable for the 34 A load.

A 50 A circuit-breaker would therefore be selected, giving a (derated) current rating of 44 x 0.8 = 35.2 A.

Compensated thermal-magnetic tripping units

These tripping units include a bi-metal compensating strip which allows the overload trip-current setting (Ir or Irth) to be adjusted, within a specified range, irrespective of the ambient temperature.

For example:

  • In certain countries, the TT system is standard on LV distribution systems, and domestic (and similar) installations are protected at the service position by a circuit-breaker provided by the supply authority. This CB, besides affording protection against indirect-contact hazard, will trip on overload; in this case, if the consumer exceeds the current level stated in his supply contract with the power authority. The circuit-breaker (≤ 60 A) is compensated for a temperature range of - 5 °C to + 40 °C.
  • LV circuit-breakers at ratings ≤ 630 A are commonly equipped with compensated tripping units for this range (- 5 °C to + 40 °C)

Examples of tables for the determination of derating/uprating factors to apply to CBs with uncompensated thermal tripping units, according to temperature

Rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 60 °C
1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 0.85
2 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 1.74
3 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 2.37
4 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 3.24
6 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 5.30
10 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 7.80
16 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 13.5
20 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 16.8
25 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 20.7
32 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 27.5
40 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 33.2
50 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 40.5
63 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 49.2

Fig. H41aiC60a, iC60H: curve C. iC60N: curves B and C (reference temperature: 30 °C)

Rating Temperature (°C)
(A) 10 15 20 25 30 35 40 45 50 55 60 65 70
16 18.4 18.7 18 18 17 16.6 16 15.6 15.2 14.8 14.5 14 13.8
25 28.8 28 27.5 25 26.3 25.6 25 24.5 24 23.5 23 22 21
32 36.8 36 35.2 34.4 33.6 32.8 32 31.3 30.5 30 29.5 29 28.5
40 46 45 44 43 42 41 40 39 38 37 36 35 34
50 57.5 56 55 54 52.5 51 50 49 48 47 46 45 44
63 72 71 69 68 66 65 63 61.5 60 58 57 55 54
80 92 90 88 86 84 82 80 78 76 74 72 70 68
100 115 113 110 108 105 103 100 97.5 95 92.5 90 87.5 85
125 144 141 138 134 131 128 125 122 119 116 113 109 106
160 184 180 176 172 168 164 160 156 152 148 144 140 136
200 230 225 220 215 210 205 200 195 190 185 180 175 170
250 288 281 277 269 263 256 250 244 238 231 225 219 213

Fig. H41bCompact NSX100-250 equipped with TM-D or TM-G trip units

Electronic trip units

Electronic tripping units are highly stable in changing temperature levels

An important advantage with electronic tripping units is their stable performance in changing temperature conditions. However, the switchgear itself often imposes operational limits in elevated temperatures, so that manufacturers generally provide an operating chart relating the maximum values of permissible trip-current levels to the ambient temperature (see Fig. H42).

Moreover, electronic trip units can provide information that can be used for a better management of the electrical distribution, including energy efficiency and power quality.

Masterpact NW20 version 40°C 45°C 50°C 55°C 60°C
H1/H2/H3 Withdrawable with horizontal plugs In (A) 2,000 2,000 2,000 1,980 1,890
Maximum adjustment Ir 1 1 1 0.99 0.95
L1 Withdrawable with on-edge plugs In (A) 2,000 200 1,900 1,850 1,800
Maximum adjustment Ir 1 1 0.95 0.93 0.90
DB422416 EN.svg

Fig. H42Derating of Masterpact NW20 circuit-breaker, according to the temperature

Selection of an instantaneous, or short-time-delay, tripping threshold

Figure H43 below summarizes the main characteristics of the instantaneous or short-time delay trip units.

Type Tripping unit Applications
DB422417.svg Low setting

type B

  • Sources producing low short-circuit- current levels (standby generators)
  • Long lengths of line or cable
DB422418.svg Standard setting

type C

  • Protection of circuits: general case
DB422419.svg High setting

type D or K

  • Protection of circuits having high initial transient current levels (e.g. motors, transformers, resistive loads)
DB422420.svg 12 In

type MA

  • Protection of motors in association with discontactors (contactors with overload protection)

Fig. H43Different tripping units, instantaneous or short-time-delayed

Selection of a circuit-breaker according to the short-circuit breaking capacity requirements

The installation of a LV circuit-breaker requires that its short-circuit breaking capacity (or that of the CB together with an associated device) be equal to or exceeds the calculated prospective short-circuit current at its point of installation

The installation of a circuit-breaker in a LV installation must fulfil one of the two following conditions:

  • Either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or exceeds the prospective short-circuit current calculated for its point of installation, or
  • If this is not the case, be associated with another device which is located upstream, and which has the required short-circuit breaking capacity

In the second case, the characteristics of the two devices must be co-ordinated such that the energy permitted to pass through the upstream device must not exceed that which the downstream device and all associated cables, wires and other components can withstand, without being damaged in any way. This technique is profitably employed in:

  • Associations of fuses and circuit-breakers
  • Associations of current-limiting circuit-breakers and standard circuit-breakers.

The technique is known as “cascading” (see Coordination between circuit-breakers)

The selection of main and principal circuit-breakers

The circuit-breaker at the output of the smallest transformer must have a short-circuit capacity adequate for a fault current which is higher than that through any of the other transformer LV circuit-breakers

A single transformer

If the transformer is located in a consumer’s substation, certain national standards require a LV circuit-breaker in which the open contacts are clearly visible such as Compact NSX withdrawable circuit-breaker.


(see Fig. H44)

What type of circuit-breaker is suitable for the main circuit-breaker of an installation supplied through a 250 kVA MV/LV (400 V) 3-phase transformer in a consumer’s substation?

In transformer = 360 A

Isc (3-phase) = 9 kA

A Compact NSX400N with an adjustable tripping-unit range of 160 A - 400 A and a short-circuit breaking capacity (Icu) of 50 kA would be a suitable choice for this duty.

Fig. H44Example of a transformer in a consumer’s substation

Several transformers in parallel

(see Fig. H45)

  • The circuit-breakers CBP outgoing from the LV distribution board must each be capable of breaking the total fault current from all transformers connected to the busbars, viz: Isc1 + Isc2 + Isc3
  • The circuit-breakers CBM, each controlling the output of a transformer, must be capable of dealing with a maximum short-circuit current of (for example) Isc2 + Isc3 only, for a short-circuit located on the upstream side of CBM1.

From these considerations, it will be seen that the circuit-breaker of the smallest transformer will be subjected to the highest level of fault current in these circumstances, while the circuit-breaker of the largest transformer will pass the lowest level of short-circuit current

  • The ratings of CBMs must be chosen according to the kVA ratings of the associated transformers

Fig. H45Transformers in parallel

Note: The essential conditions for the successful operation of 3-phase transformers in parallel may be summarized as follows:

1. the phase shift of the voltages, primary to secondary, must be the same in all units to be paralleled.

2. the open-circuit voltage ratios, primary to secondary, must be the same in all units.

3. the short-circuit impedance voltage (Zsc%) must be the same for all units.

For example, a 750 kVA transformer with a Zsc = 6% will share the load correctly with a 1,000 kVA transformer having a Zsc of 6%, i.e. the transformers will be loaded automatically in proportion to their kVA ratings. For transformers having a ratio of kVA ratings exceeding 2, parallel operation is not recommended.

Figure H46 indicates, for the most usual arrangement (2 or 3 transformers of equal kVA ratings) the maximum short-circuit currents to which main and principal CBs (CBM and CBP respectively, in Figure H45) are subjected. It is based on the following hypotheses:

  • The short-circuit 3-phase power on the MV side of the transformer is 500 MVA
  • The transformers are standard 20/0.4 kV distribution-type units rated as listed
  • The cables from each transformer to its LV circuit-breaker comprise 5 metres of single core conductors
  • Between each incoming-circuit CBM and each outgoing-circuit CBP there is 1 metre of busbar
  • The switchgear is installed in a floormounted enclosed switchboard, in an ambient-air temperature of 30 °C

Moreover, this table shows selected circuit-breakers of M-G manufacture recommended for main and principal circuit-breakers in each case.

Number and kVA ratings of 20/0.4 kV transformers Minimum S.C breaking capacity of main CBs
(Icu) kA
Main circuit-breakers (CBM)
total discrimination with out going circuit-breakers (CBP)
Minimum S.C breaking capacity of principal CBs
(Icu) kA
Rated current In of principal circuit-breaker
(CPB) 250A
2 x 400 14 NW08N1/NS800N 27 NSX250F
3 x 400 28 NW08N1/NS800N 42 NSX250N
2 x 630 22 NW10N1/NS1000N 42 NSX250N
3 x 630 44 NW10N1/NS1000N 67 NSX250S
2 x 800 19 NW12N1/NS1250N 38 NSX250N
3 x 800 38 NW12N1/NS1250N 56 NSX250H
2 x 1,000 23 NW16N1/NS1600N 47 NSX250N
3 x 1,000 47 NW16N1/NS1600N 70 NSX250H
2 x 1,250 29 NW20N1/NS2000N 59 NSX250H
3 x 1,250 59 NW20N1/NS2000N 88 NSX250S
2 x 1,600 38 NW25N1/NS2500N 75 NSX250S
3 x 1,600 75 NW25N1/NS2500N 113 NSX250L
2 x 2,000 47 NW32N1/NS3200N 94 NSX250S
3 x 2,000 94 NW32N1/NS3200N 141 NSX250L

Fig. H46Maximum values of short-circuit current to be interrupted by main and principal circuit-breakers (CBM and CBP respectively), for several transformers in parallel


(see Figure H47)

  • Circuit-breaker selection for CBM duty:
For a 800 kVA transformer In = 1155 A; Icu (minimum) = 38 kA (from Figure H46), the CBM indicated in the table is a Compact NS1250N (Icu = 50 kA)
  • Circuit-breaker selection for CBP duty:
The s.c. breaking capacity (Icu) required for these circuit-breakers is given in the Figure H46 as 56 kA.
A recommended choice for the three outgoing circuits 1, 2 and 3 would be current-limiting circuit-breakers types NSX400 L, NSX250 L and NSX100 L. The Icu rating in each case = 150 kA.
These circuit-breakers provide the advantages of:
  • Absolute discrimination with the upstream (CBM) breakers
  • Exploitation of the “cascading” technique, with its associated savings for all downstream components

Fig. H47Transformers in parallel

Choice of outgoing-circuit CBs and final-circuit CBs

Short-circuit fault-current levels at any point in an installation may be obtained from tables

Use of table G40

From this table, the value of 3-phase short-circuit current can be determined rapidly for any point in the installation, knowing:

  • The value of short-circuit current at a point upstream of that intended for the CB concerned
  • The length, c.s.a., and the composition of the conductors between the two points

A circuit-breaker rated for a short-circuit breaking capacity exceeding the tabulated value may then be selected.

Detailed calculation of the short-circuit current level

In order to calculate more precisely the short-circuit current, notably, when the short-circuit current-breaking capacity of a CB is slightly less than that derived from the table, it is necessary to use the method indicated in Short-circuit current .

Two-pole circuit-breakers (for phase and neutral) with one protected pole only

These CBs are generally provided with an overcurrent protective device on the phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme, however, the following conditions must be respected:

  • Condition (B) of table G67 for the protection of the neutral conductor against overcurrent in the case of a double fault
  • Short-circuit current-breaking rating: A 2-pole phase-neutral CB must, by convention, be capable of breaking on one pole (at the phase-to-phase voltage) the current of a double fault equal to 15% of the 3-phase short-circuit current at the point of its installation, if that current is ≤ 10 kA; or 25% of the 3-phase short-circuit current if it exceeds 10 kA
  • Protection against indirect contact: this protection is provided according to the rules for IT schemes

Insufficient short-circuit current breaking rating

In low-voltage distribution systems it sometimes happens, especially in heavy-duty networks, that the Isc calculated exceeds the Icu rating of the CBs available for installation, or system changes upstream result in lower level CB ratings being exceeded

  • Solution 1: Check whether or not appropriate CBs upstream of the CBs affected are of the current-limiting type, allowing the principle of cascading (described in Coordination between circuit-breakers ) to be applied
  • Solution 2: Install a range of CBs having a higher rating. This solution is economically interesting only where one or two CBs are affected
  • Solution 3: Associate current-limiting fuses (gG or aM) with the CBs concerned, on the upstream side. This arrangement must, however, respect the following rules:
    • The fuse rating must be appropriate
    • No fuse in the neutral conductor, except in certain IT installations where a double fault produces a current in the neutral which exceeds the short-circuit breaking rating of the CB. In this case, the blowing of the neutral fuse must cause the CB to trip on all phases.