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Protection for electrical systems E3

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by

Mark Hyde

on 1 November 2018

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Transcript of Protection for electrical systems E3

When dealing with electricity, there are many devices that we use to properly protect ourselves and the electrical circuit from damage such as-
Miniature circuit breaker
Residual circuit breaker
Rewirable fuses
RCBO circuit breaker
Cartridge fuses

Protecting electrical systems
Protection for electrical systems
A circuit breaker in a consumer unit can be known as a miniature circuit breaker (MCB). This is an automatically operated switch designed to protect an electrical circuit from damage caused by an overload or by short circuit. You can also turn it off manually to isolate individual circuits. When doing so it is advised that you lock the MCB until you have finished working on that circuit.
Miniature circuit breaker
An RCD is more sensitive than an MCB, and when installed in a consumer unit its job is to protect the 'group' of circuits. It's larger than an MCB and has a reset/test button on it. An RCD can be a lifesaver- it works by constantly monitoring the electric current flowing along the circuit. It is so sensitive that it can detect electricity flowing down an unintended path- such as a person. If this happens the RCD will turn off the power within a fraction of a second (40 milliseconds) and prevent the person from receiving the shock.
Residual circuit breaker
When working on old consumer units you will notice that protection is limited. These units use rewirable fuses.
Rewirable fuses are designed so that the wire inside the fuse would melt if the current passing through it was too high. The fuses were generally 5 amp, 15 amp, and 30 amp. The consumer could replace the wire if it had blown and this led to many electrical fires. People did not understand the danger of these fuses and to avoid the inconvenience of it tripping used to put in larger wires or even a nail to prevent the fuse from cutting out or tripping.
Rewirable fuses
To comply with the new 17th Edition Electrical Regulations, residual current breakers with overload protection (RCBO's) must be installed. RCBO's combine the functions of an MCB and an RCD in one unit. They are used to protect particular circuits, instead of having a single RCD for a whole building. These are used more in commercial buildings rather than domestic ones.
RCBO circuit breaker
Most commonly found in appliance plugs, these small resistors act as protection against surges in current. Like rewirable fuses a cartridge fuse has a wire inside that is designed to melt at a particular current but instead of the consumer being able to replace the wire they would need to change the whole cartridge.
Cartridge fuses
1. Why are cartridge fuses safer than rewirable fuses?
2. If you have turned off an MCB to work on the circuit, why should you lock the MCB until you have finished the work?
3. What is the most modern form of circuit breaker?
Re Cap-
For the benefit of safety, an RCD should have a label next to it that reads-
Test Quarterly Press Button
Side note-

The meter tails (the incoming electrical mains) are protected from overload by the supplier by means of a BS 88 cut-out fuse.
Side note-

Any junction box in an electrical installation that is installed in an area with restricted access should have connections made with crimped or compression terminals, not screw terminals.
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