What does an rcd do (and why they are so important)?

Electric shock and fire are two outcomes nobody wants – anywhere.
Most people don’t realise a fuse or circuit breaker offer no protection from a person coming in to contact with a live wire. RCDs (residual current devices) offer a good level of protection from someone receiving a fatal shock or fire from current going somewhere it shouldn’t.
Mains electricity is located wherever people, livestock, machines and equipment requires it to be. Most agree electrical appliances and installations are taken for granted and not in the way something so potentially dangerous should be.

An RCD is a piece of mechanical equipment that protects people, animals and buildings by removing the mains power automatically when current leaks from a circuit or appliance.

A common misconception is that “everything is fused” when it comes to electrical safety. Fuses were designed to protect cables from becoming overheated, becoming damaged and posing a fire risk. If you have ever had to change a fuse in a plug top it is because:

A fault with the piece of equipment caused it to blow
Something happened causing the appliance to draw a much larger amount of current than it would normally – such as something wedged under your lawnmower causing the motor to stall for example.

Fuses and minature circuit breakers inside consumer units work in exactly the same way; they are designed to protect the cables, nothing more.
Humans become sensitive to mains voltage at milliamps and that is why an rcd will trip somewhere before the 30ma threshold to protect human life. The smallest fuse you can install in a plug top is 3000ma (3 amp). The smallest sized circuit breaker inside a consumer unit is 6000ma (6 amp)

How rcds operate
An rcd works on the principle that what leaves should return. So when you turn your kettle on, power is drawn through the flex, through the switch, element and then returns back to the source. If the kettle developed a fault (usually the element starting to break down) some of the current would not return because it is leaking through the faulty element. So, in this scenerio on an installation without an rcd – there could be an electrical charge present on the casing posing a risk to anyone who uses it.

Residual current devices don’t know the amount of earth leakage when they trip; they just know there is an inbalance within the threshold they have been manufactured to and remove the mains very quickly.

Electricians have calibrated test equipment that can check an rcd is working correctly and not causing unwanted disconnections.

Home and business onwers can carry out a basic test of their rcds by pressing the test button manufacturers incorporate to cause an inbalance.

Different types of rcd

An rccb is an rcd that incorporates switching.

RCBOs offer earth leakage protection with built in overload protection (just like a fuse and minature circuit breaker provides). This type of devide offers the best protection from earth faults for two reasons:

1. They protect individual circuits and not a collection of them which happens with dual rcd consumer units. With the increasing number of electronic items in use the residual current increases the chances of disconnections dispite not one fault, but an accumualtion of appliances.
2. In the event of a fault it is clear what could be causing the problem because items (or the wiring to them) will not work.

FCURCD is a connection unit that incorporates a fuse and rcd. It’s common to see these for powering a device where the main consumer unit is not protected, or someone has taken the view that two rcds are better than one.

A Socket RCD (or SRCD) is a socket outlet that incorporates earth leakage protection. In the event of earth current leaking then it could be the device inside the socket that operates, or the one from where the supply is being taken from (assuming the distribution board is equipped with one).

Generic rcds
Disconnects the power instantly. If two rcds are present then one could trip, or both. The problem with this if the one trips inside a meter cupboard area – staff may not have access leaving them without power until someone with access (and permission) to reset the main rcd arrives.

Time delayed residual devices
These devices provide a small time frame from when the fault is detected to when it operates. The idea is to allow an rcd closer to the source of the problem to operate first.

Electrocution
Scientists state continuous current of 40ma can cause an interruption of the cardiac cycle when a person comes in to contact with mains voltage.
RCDs fitted inside domestic and commercial premises are designed to disconnect the power within 40 milliseconds to help prevent the risk of cardiac arrest.
The outcome of a shock will depend on which parts are in contact with the mains. It is for this reason portable appliance testing hand held equipment is important. Holding a kettle or drill correctly means having a firm grip with s piece of equipment that has live parts enclosed.
It is generally understood that a shock of 30ma would be unpleasant although unlikely to cause permanent harm to a human. It is for this reason rcds are designed to remove the power just before this level should occur.

Electric shock through the human body
0.5ma – 5ma
No dangerous effects although may cause victim to drop a faulty appliance with the leaking current

5ma – 10ma
Muslce reaction could cause hand or fingers to lock in position

10ma – 40ma
Pain increased. Electrical current of 20ma and above is believed to cause altered breathing.

40ma – 250ma
Cardiac cycle disturbance likely to occur at this level. Higher current flow means greater risk of burns and cardiac arrest.

Rcds and RCBOs increase the chance of survival, because they are designed to operate before the amount or current (and the time it is present) from becoming fatal.

How rcds help prevent fire
The fire service attend thousands of fires – many a result of faulty apppliances and wiring. Research has discovered earth faults (that is current leaking somewhere it shouldn’t be) as low as 100ma can cause ignition and fire. An rcd working correctly will limit the amount of current and the time it is allowed to travel therefore reducing the risk of fire.
Appliances in order of highest fire risk:

Washing machines
Hot blankets
Cooking devices
Televisions
Dishwashers
Fridge freezers

Special locations
The wiring regulations sets out that some areas of a building pose a bigger risk when electricity is in use – like a room containing a bath or shower. This is because a wet body provides less resistance for curent to flow increasing the risk of shock. RCDS have been required for work inside a bath or shower room for a considerable amount of time for this very reason.

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