A better understanding of miniature circuit breakers (MCBs)

Before miniature circuit breakers, fuse distribution boards provided cable protection for electrical circuits. They worked well and continue in use for some applications. The biggest drawback was that in the event of an overcurrent or short circuit, the fuses needed replacing. Furthermore, it took time to find the electrical engineer and meant a wide range of fuses we needed in stock.

For current ratings up to 125A, the miniature circuit breaker (MCB) offers better safety and control for the electric system. Miniature circuit breakers conforming to BS-EN 60898 are suitable for operation by uninstructed (non-technical) people. They are suitable for installation in offices, commercial and industrial premises.

MCBs are automatic electro-mechanical switches that protect an electrical circuit from damage by switching themselves off in the event of a fault. Having found and removed the cause of the excess current the operator can reset the mcb.

Inside miniature circuit breakers

Inside an MCB, a bimetallic strip forms part of the conductive path and the current flowing causes it to bend. In the event of an overcurrent this deflection increases to release a mechanical latch linked to the operating mechanism and the breaker trips. The time it takes to trip depends on the size of the overcurrent and protects the cable as well as avoiding nuisance tripping.

If a short-circuit occurs, it is important to remove the power without delay to avoid electrical damage to the system. For this the magnetic effect of the fault current unlatches the trip mechanism allowing the mcb to open in milli-seconds.

Moreover, the trip mechanism is independent of the handle position (trip-free). This means that if an mcb has a padlocked, or the handle wedged closed, the internal mechanism will still trip.

Trip types for AC applications (BS EN 60898-1)

For general distribution applications, MCBs with trip characteristics B, C and D are available for consumer and commercial applications. For more information see the BEAMA Guide to Low Voltage Circuit-breaker Standards.

Selection depends on the application. For example:

1 – Type B: Instantaneous trip range 3-5 times rated current or applications such as resistive loads for heaters, showers cookers, socket outlets, battery systems and photovoltaics

2- Type C: Instantaneous trip range 5-10 times rated current for applications such as inductive loads for motors, general lighting, power supplies, rail battery systems, welding equipment and photovoltaics

3 – Type D: Instantaneous trip range 10-20 times rated current for applications such as high inductive loads for transformers, motors, discharge lighting circuits and computers

Other special purpose MCBs Types K and Z are also available for a range of applications, including DC, but not covered here.

Rated Short-Circuit Capacity (Icn)

Also known as the interrupting capacity, it means the prospective fault current at the incoming terminals of the circuit-breaker should not exceed Icn. The manufacturer must declare the short-circuit capacity of their circuit-breaker. The recognised short-circuit capacities range from 1.5kA up to 25 kA.

LED lighting inrush current

When switched on, LED lighting can cause an initial peak transient current hundreds of times higher than the normal current. The simultaneous switching of multiple LED lights compounds this inrush. It is due to the charging of capacitors in the power supplies of the LED lighting or because of the initial low magnetic flux in the transformer in the power supply. The peak inrush current magnitude and inrush current pulse duration are important when selecting circuit-breakers to avoid unintentional operation.

Electrical trade organisation BEAMA provides free advice to help installers using MCBs with LED lighting.

Accessories

Further improvements to miniature circuit breakers include extensive ranges of accessories to improve their usability. The most popular include auxiliary switches, residual current protection (RCBO), communication modules, signal contacts, shunt trips, under voltage releases, busbars systems and enclosures.

Caveat emptor (let the buyer beware)

Finally, official manufacturers’ UK distributors of electrical equipment go to great lengths to ensure they sell original branded products. Miniature circuit breakers are critical safety elements of any building or application. Copy products are available from the Internet and grey importers and often almost indistinguishable from the genuine article. What is on the label, may not be what you are buying.