What does an rcd do?

RCDs help reduce the risk of fatal shock and fire. They do this by monitoring an imbalance in live conductors. Circuit breakers offers no protection from touching a live component.

A healthy circuit. Current flows and returns with no leak.

Mains power is inside and outside buildings and used by many. Electrical appliances and fixed wiring can develop faults which pose a risk.  Some faults aren’t obvious unless something stops working.

An RCD is a mechanical device. It protects people, animals and buildings by removing power.

A common myth is a fuse will help save us. Fuses protect cables from becoming overheated and posing a fire risk. That’s it. They are the weak link in a circuit or appliance. If you have replaced a fuse it is because the appliance drew a larger amount of current than necessary. In a plug top fuse it could be something wedged under your lawnmower causing the motor to stall for example.
Fuses and minature circuit breakers inside consumer units work the same way. They are designed to protect the cables, nothing else.

Humans become sensitive to mains voltage at milliamps. An rcd will trip below the 30ma threshold to protect human life. The smallest fuse you can install in a plug top is 3 amp (3000ma) . The smallest sized circuit breaker inside a consumer unit is 6 amp (6000ma).

How rcds operate

An rcd works on the principle that what leaves should return. Turn your kettle on, power travels through the flex, through the switch, element and returns back to the source. If the kettle developed a fault (the element started to fail); some of the current would be missing. Due to the leak through the faulty element some current would be missing. In an installation without an rcd this could mean an electrical charge poses a risks to its users. The greater the current leaking, the greater the risk to anyone operating it.

An unhealthy circuit where current is leaking. The rcd detects the imbalance.

Residual current devices don’t know the amount of earth leakage upon tripping. They know an in balance is present remove the mains power reducing the risk of harm.

Electricians have calibrated test equipment to confirm an rcd is working and tripping correctly. Rcds need to operate to prevent harm to humans, livestock and buildings.

The makers of rcds provide a test button. This enables users to test the mechanical device. It causes an imbalance and the device should disconnect.

The different types of rcd.

An rccb is an rcd that incorporates switching.

RCBOs offer protection with built in overload protection. This type of devide offers the best protection from earth faults for two reasons:.

  1. They protect a circuit instead of a collection of them like in dual rcd consumer units.
    The increasing number of electronic items in use increases the residual current and tripping.
  2. In fault situations it’s clear what circuit is causing the problem because items (or the wiring to them) won’t work.

FCURCD is a connection unit that incorporates a fuse and rcd. These power devices if the main consumer unit doesn’t contain an rcd. Some designers have the view that two rcds are better than one for protection.

A Socket RCD (or SRCD) is a socket outlet that incorporates earth leakage protection. In the event earth current leaks; the device inside the socket may operate. If rcd protection is at the distribution board that could operate in the first instance.

Time delayed rcds
Provide a small time frame from detection to tripping time. The idea is to allow an rcd closer to the source of the problem to operate first.

Electrical related injury

Scientists state continuous current of 40ma can cause an interruption of the cardiac cycle. This could happen if a person came in to contact with mains voltage.

RCDs are designed to trip 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 means having a firm grip with a piece of equipment that has live parts enclosed.

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 below this level.

Electric shock through the human body – and how rcds help prevent harm

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 occurance. Higher current flow means greater risk of burns and cardiac arrest.

Rcds and RCBOs increase the chance of survival. They are designed to operate prior to 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. Earth faults (current leaking) as low as 100ma can cause ignition and fire. An rcd will limit the amount of current allowed to travel therefore reducing the risk of fire.

Appliances in order of highest fire risk:

Washing machines.
Hot blankets.
Cooking devices.
Fridge freezers.

Special locations

The wiring regulations state rooms that contain a bath or shower pose a bigger risk to electrical injury. A wet body provides a lower resistance for current to flow increasing the risk of shock. RCDS have been a requirement inside a bathroom to help reduce the risk of shock.