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Jargon Busted – MCBs

The electrical world can be confusing and we’ve all come across some terms that don’t make much sense. That’s why in each issue of Wired we unravel a piece of industry gobbledygook and tell you what it means – In plain old English.

What does MCB stand for?

MCB stands for Miniature Circuit Breaker – in other words, a circuit breaker that is rated lower than 100A. They are found in both domestic and industrial circuits.

What is a circuit breaker?

As its name suggests, a circuit breaker is a switch that interrupts the flow of an electrical current, thus breaking the circuit. Operating automatically, its aim is to prevent an electrical circuit being damaged when it gets overloaded or short circuits – protecting wiring and equipment on the circuit.

What are the characteristics of an MCB?

As well as having a rated current of no more than 100A, an MCB has a thermal or thermal-magnetic operation and usually has trip characteristics that are not adjustable.

An MCB will usually have two tripping mechanisms:

For overload protection – a delayed thermal tripping mechanism

For short circuit protection – a magnetic tripping mechanism

All MCBs in the UK must conform to BS 7671 (The IET Wiring Regulations). Co- published by the Institute of Engineering and Technology (IET) and the British Standards Institution (BSI), this sets the standards for electrical installation and is due for a major update in January 2015.

What are the three main types of MCB?

Type B – this trips between three and five times its full load current and is suitable for domestic and light commercial use

Type C – this trips between five and 10 times its full load current and is suitable for commercial and industrial use, for example to handle a site with motors and florescent lighting.

Type D – this trips between 10 and 20 times its full load current and is suitable for more specialist industrial use, for example with powerful machinery.

It’s important to choose the right MCB so that it doesn’t trip unnecessarily, and to have it fitted by a professional electrical engineer.

Why are MCBs preferable to fuses?

When a fuse blows, it has to be replaced. However, when an MCB trips, it can easily be reset – either automatically or manually by flicking a switch or button.

Not to be confused with...

MCCBs – Moulded Case Circuit Breakers.

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