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Things About MCBs That Every Low Voltage Electrician Should Know

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MCB Operation and characteristics

An MCB is a thermo-magnetic device, meaning that it has two methods of circuit interruption. A thermal mechanism, usually a bi-metallic strip, provides protection against moderate overcurrent. The heating action of the current causes the bi-metallic strip to curve and break circuit contact. This method is complemented by a solenoid designed to respond to larger currents.

Things About MCBs That Every Low Voltage Electrician Should Know
Things About MCBs That Every Low Voltage Electrician Should Know

A diagram of an MCB is shown in Figure 1 below.

Figure 1 - Internal view of an MCB
Figure 1 – Internal view of an MCB

It should be apparent that the thermal trip has a slow response time and the solenoid trip has a rapid response time. When combined, these devices provide quite a sophisticated protection characteristic profile.

Table 1 //

BS EN 60898 device thermal characteristics

CurrentDesired Result
 1.13 In Must not trip within 1 h
 1.45 In ≤ 63A Must trip within 1 h
 1.45 x In, > 63A Must trip within 2 h
 2.55 In ≤ 32A Must trip between 1 and 60 s
 2.55 In, > 32A Must trip between 1 and 120 s

The two MCB’s characteristics are described:

  1. Thermal characteristic
  2. Magnetic characteristic

1. Thermal characteristic

The thermal, bi-metallic characteristic is summarized in Table 1. A further co-ordination of the requirement is that of Regulation 433.1.1 (iii) which is:

I2 ≤ 1.45 × Iz where I2 is the current that causes operation of the device.

By studying Table 1 above, it can be seen that this requirement is built into the product standard for BS EN 60898 devices and is effectively the calibration of the bi-metallic strip.

Go back to Characteristics ↑


2. Magnetic characteristic

The maximum rated current available for MCBs is 125A, and these BS EN 60898 devices are available with different magnetic sensitivities, denoted with a prefix B, C or D accordingly.

The different magnetic characteristics of BS EN 60898 circuit breakers are provided in Appendix 3 of BS 7671: 2008, but to illustrate the differences in the magnetic characteristics, Figure 2 shows a comparison of B, C and D types for devices of the same basic rating. A 32A circuit breaker with type C sensitivity is denoted C32, and it is a requirement of the equipment standard to apply this marking to the device.

The stated B, C or D sensitivities each have a minimum current that causes operation, and this is conventionally taken to be operation within 0.1 second. This is conventionally termed instantaneous operation or instantaneous tripping.

This minimum time convention is due to the mechanics of the circuit breaker, which will always require a certain minimum time, regardless of current for the trip mechanism to open.

Figure 2 - 32A MCB sensitivity characteristics comparison
Figure 2 – 32A MCB sensitivity characteristics comparison

Figure 2 shows that in order to achieve instantaneous tripping or tripping at 0.1 s, a 32A type B breaker requires 160A, a type C breaker 320A and a type D breaker 640A.


Table 2 //

Circuit breaker (BS EN 60898) selection for inrush current applications

TypeManufactured magnetic trip setting
(× In)
Typical applications
B3 to 5General domestic and resistive loads
C5 to 10Small motors (a few kW), small transformers fluorescent lighting and most inductive loads
D10 to 20DOL motors, large star delta motors, low- pressure sodium discharge lighting, larger transformers, welding machine supplies

Below these threshold currents the thermal mechanism is dominant, and has the same characteristic for all three devices. The magnetic characteristics determine the sensitivity type. Equipment connected or likely to be connected to the circuit must be assessed in terms of likely peak or inrush current.

Inrush current is the current that a load draws when the supply is switched on.

Values can range from being insignificant (a few times the normal current), 5 to 10 times normal current for iron core transformers (e.g. conventional ballast fluorescent luminaires) and up to 20 times normal current for much modern electronic equipment, including the power supplies found in user equipment.

While short-lived (often the peak current is a few milliseconds), this can cause circuit breakers to trip, but assessing the likelihood of a circuit breaker tripping is complicated. Table 2 above recommends circuit breaker types for typical inrush current applications.

Separately to inrush current, load peak current also needs to be considered. Peak current in respect of circuit breaker selection is a term used to describe a peak within the normal operation of a cyclic or time varying load.

If you have loads with significant cyclic peaks you need to confirm that the circuit breaker will not trip. This can be confirmed by studying the circuit breaker characteristic curve, but confirmation with the manufacturer may be necessary.

Go back to Characteristics ↑


Circuit Breakers – How they Work, What’s Inside


How does a Miniture Circuit Breaker (MCB) work


Reference // Guide to the Wiring Regulations – Darrell Locke IEng MIEE ACIBSE

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on