What Are rcds are and why we need them

RCDs come in all shapes and sizes. They can be fitted as part of the circuit protection inside consumer units protecting an entire electrical circuit, socket outlets or inline with electrical equipment. A residual current device is designed to reduce the risk of fatal shock and fire -often caused by a faulty appliance. They help with electrical safety by monitoring the live conductors inside mains circuits. When an imbalance is detected; the rcd switch will quickly remove the power from the faulty circuit. RCDs are a mechanical device designed to protect people, animals and buildings from an earth fault and it’s associated electrical-related injury.

The different types of rcd.

  • An rccb is an rcd that incorporates switching.
  • RCBOs offer residual protection (which is what an rcd does) and overload (what a fuse and circuit breaker protect). This type of device offers the best protection from earth faults for two reasons:
  1. They protect a circuit instead of a collection like dual rcd consumer units.
    The increasing number of electronic items in use increases the residual current and therefore more prone to nuisance trips.
  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 are usually installed to power devices when 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. They provide a small time frame from detection to disconnection time. The idea is to allow an rcd closer to the source of the problem to operate first.

How rcds operate (and what to do if they don’t)

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.

Residual current devices don’t know the amount of earth leakage. They know an in balance is present remove the mains power reducing the risk of harm. Rcds need to operate to prevent harm to humans, livestock and buildings.

Electricians have calibrated test equipment to confirm an rcd tripping isn’t too soon (often referred to as “nuisance tripping”) due to faulty wiring or equipment. A typical multi-function tester will confirm the rcd trips correctly as per manufacturers recommendations at x5 their rated current. A ramp test is where the tester will start leaking current intentionally and confirm how much is leaked when the trip (or disconnect) occurs.

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. So, if you aren’t sure if you are looking at an rcd or mcb – look for the test button. In the video below, you can see a faulty rcd assembly. If your rcd does not remove the power when the test button is pressed, then you need to contact your local electrician.

What did we have before rcd protection?

Before rcds were invented we only had fuses and circuit breakers inside our fuse boards / consumer units. Some still think a fuse will help prevent electrical injury. Fuses and circuit breakers were designed to protect cables from becoming overheated and posing a fire risk. That’s it. They are supposed to be 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. Or, a cable has been crushed causing a short circuit inside.
Fuses and minature circuit breakers inside consumer units work the same way. They are designed to protect the cables, nothing else.

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

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). So if you drilled a hole in a wall, hit a live wire and got electrocuted; the fuse or circuit breakers won’t operate because no overload is present.

Electrical related injuries rcds help prevent

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.

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
  • Televisions
  • Dishwashers
  • 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.