Things About MCBs That Every Low Voltage Electrician Should Know

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.

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

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

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 × I_z 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.

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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.

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 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

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.

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.

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Circuit Breakers – How they Work, What’s Inside

Siemens Miniature Circuit Breaker 5SY

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