Mines safety bulletin no. 42 | 04 July 2003 | Version 1
Residual current devices (RCD) (earth leakage circuit breakers - ELCB)
The use of 30 milliampere RCDs has been commonplace in mines and quarries in Queensland for many years; RCDs used to be called ELCBs – earth leakage circuit breakers. They have now become mandatory throughout Queensland in industrial, commercial and domestic premises. The Electrical Safety Office, part of the Department of Industrial Relations, has promoted their use in Queensland with the campaign slogan of 'In the blink of an eye'.
Safety switches monitor the leakage of electrical current to earth through faulty appliances or incorrect wiring and disconnect the power quickly.
A safety switch can cut off power in 30 milliseconds - a 30th of a heart beat.
If a fault occurs in an appliance or wiring causing current to flow to earth - perhaps through a person - the switch's earth leakage sensor responds to the imbalance of current and cuts the power.
However, the passage of 30mA through a person, even for only 30 milliseconds, may be painful. Unfortunately, for the RCD to work when a person is involved, this has to happen! If the person affected is on a ladder or in a similar elevated situation this shock may cause a fall, which could lead to further injuries!
Over the years the mining and quarrying industry has become familiar with, and in some cases, over-dependent, on these devices. This is apparent from the reports from mines and quarries of minor electric shocks; the number reported seems to be on the increase! It appears as if people are relying purely on the RCD to protect them rather than ensuring that the equipment is being used correctly. One incident occurred recently where a contractor using a portable device which had no "Test or Tag" label stated emphatically that it was not necessary as the outlet he was using was protected by a 30mA RCD! There have been several reported incidents where portable appliances and/or their power leads have been subject to excessive moisture and the operator felt a tingle. Fortunately the RCD worked and no long-term ill effects occurred.
An RCD is an electro-mechanical device that is inherently extremely reliable, however, they can fail and in this case the tingle may become more serious!
An RCD is there to isolate faulty electrical items; the primary barrier to prevent inadvertent contact with dangerous electrical conductors is the integrity of the insulation.
The users of portable electrical appliances and/or electrical leads must ensure that they are not subject to mechanical abuse or the ingress of water. Testing and tagging should weed out damaged items but will not eliminate misuse particularly if moisture is present. The user has the primary responsibility and obligation to visually check and use portable appliances in a safe manner.
NB: An RCD will not provide protection against a live to neutral fault!
For further information on RCDs and their use, contact your local Inspector of Mines (Electrical). See the list of contact details for Queensland Inspectors of Mines.
Also see the electrical safety page at the WorkCover Queensland website.
Issued by Queensland Department of Natural Resources and Mines