When should earth leakage protection be used? Paul Martin argues that residual current device protection should be considered for all circuits in dwellings.
plit-load consumer units incorporating residual current device (rcd) protection are now relatively common. Invariably the use of rcds opens up the debate about which circuits should receive 30 mA earth leakage protection.

To quote the regulations: "Every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault." And as any qualified electrician knows, the problem with a rcd protecting all the circuits is nuisance tripping, which could potentially leave the building without essential circuits such as lighting.

Typically, lighting circuits have no rcd protection, but other circuits such as the upstairs, kitchen and utility ring final circuits, as well as dedicated circuits for the immersion heater, cooker and fridge-freezer, may not be included. These decisions are left to the individual electrician's judgement and common sense. The primary consideration tends to be the likelihood of direct or indirect contact with live parts.

So far so good, but this whole argument can miss the point. RCDs are also used to help prevent electrical fires caused by arcing in high resistance earth faults. So is there a case for rcd protection on all circuits in the home?

RoSPA has recognised the hazards posed by unsafe electrical installations. A recent report states: "The risk of injury and property damage has increased in recent years because of the large number and variety of electrical systems in buildings and the demands placed on them. Hazards are posed by both contact with electricity…and injuries resulting from fires in buildings that have been ignited by overheated electrical components or arcing."

The report states that there are 12 500 fires each year deemed to have an electrical source of ignition. These fires cause 590 injuries and 25 fatalities. They are frequently due to the failure of a cable's insulation causing earth leakage. An rcd on a circuit can lead to unwanted tripping, but it can also help prevent a fire.

Putting all of the circuits on a single 30 mA rcd or two separate 30 mA rcds can lead to nuisance tripping and the loss of essential circuits, so that is not a realistic solution.

12 500 fires each year are deemed to have an electrical source of ignition...due to failure of a cable’s insulation

One alternative is to use a split-load board with rcd protection across several circuits, which is supplemented by individual residual circuit-breaker overload (rcbo) protection for those circuits not protected by the rcd switch. This would remove the problem of nuisance tripping to essential circuits when the earth fault is present on another circuit. At the same time, if there were an earth fault on an essential circuit it could be dangerous and a tripped rcbo should be investigated.

The safety case for using rcbos is certainly a strong one. Unfortunately economics can take over at this point and in competitive tendering situations such solutions are rarely proposed for dwellings.

Fortunately there is a more cost-effective answer that does not compromise safety. A split-load board can be used with two rcds. Circuits where there is a greater chance of direct or indirect contact should be protected by a 30 mA rcd.

For the other circuits, including lighting, a time-delayed 100 or 300 mA rcd can be used for additional protection. Too often electrical faults can go unnoticed, especially in domestic installations, and the results can be fatal. The residual current protection on these other circuits would give homeowners additional protection against the risk of domestic fires.

Such a solution would be slightly more expensive than using a conventional split-load board with only some circuits protected by a rcd, but what price safety?

Consider that for domestic installations the cabling only needs checking every ten years and in practice this may not even happen. It may take years for cable insulation to wear down, but the statistics suggest that it does happen. Equally, fixed appliances such as washing machines or fridge-freezers may traditionally be on non-rcd protected circuits. If moved the flex may become damaged and remain unseen.