The current wiki entry on RCDs consists of just two links which are meant to be to IET articles plus a brief explanation by bernardgreen as to why RCDs trip with N-E faults. The two links merely take one to the IET home page, so maybe whoever put them there can mend that?
Following recent mention in a discussion here of the things which DIYers, the general public and electricians do, and do not, understand about RCDs, I am reminded of the many ‘common misconceptions about RCDs’ which I have often mentioned. I therefore thought that it might perhaps be useful if someone added such a ‘common misconceptions’ section to the wiki entry on RCDs. With that in mind, I've had a stab at a draft of what I think it might say, as follows, and I’d be grateful for any thoughts/comments – particularly in relation to (5), which some people may disagree with. The first two are misconceptions commonly held by the general public. The remainder are more technical, and therefore apply to misconceptions held by some DIYers and electricians:
Kind Regards, John.
Following recent mention in a discussion here of the things which DIYers, the general public and electricians do, and do not, understand about RCDs, I am reminded of the many ‘common misconceptions about RCDs’ which I have often mentioned. I therefore thought that it might perhaps be useful if someone added such a ‘common misconceptions’ section to the wiki entry on RCDs. With that in mind, I've had a stab at a draft of what I think it might say, as follows, and I’d be grateful for any thoughts/comments – particularly in relation to (5), which some people may disagree with. The first two are misconceptions commonly held by the general public. The remainder are more technical, and therefore apply to misconceptions held by some DIYers and electricians:
- Common Misconceptions about RCDs
1...Above all, RCDs do NOT by any means guarantee the avoidance of serious injury or death if someone comes in contact with a live conductor. Indeed, an RCD offers NO protection to a person who comes in contact with both live and neutral conductors. If they come in contact with a live conductor and earth (e.g. ‘earthed’ metalwork), an RCD affords some protection, but still no guarantee of immunity from serious injury or even death. Accordingly, the existence of an RCD should never be allowed to lead to complacency or relaxation of fastidious attention to safety considerations when using or working on anything electrical.
2...An RCD will NOT trip if a two-core cable, such as that used to power many garden tools (mowers, strimmers, hedge cutters etc.) is cut. It will, however, afford some, but not total, protection to someone who (foolishly) picks up the live end of the cable after it is cut whilst they are standing on soil or otherwise in contact with earth.
3...A, say, 30mA RCD does NOT limit to 30mA the current which can flow through a person in contact with live and earth. The current which flows through the person depends on circumstances, and may be much higher than 30mA. What the RCD does do is limit the duration of flow of a current of 30mA or greater can flow to a sufficient extent that serious injury/death will often (but not always) be avoided.
4... Similarly, a correctly-installed RCD does NOT limit the voltage difference which can exist between the installation’s ‘earth’ system (CPCs) and true earth during fault conditions (‘touch voltage’) to 50V. Again, all it does is limit the duration of any ‘touch voltage’ greater than 50V (in practice, usually also the duration of lower voltages).
5...Contrary to what some people believe, a ‘TT’ electrical installations does NOT necessarily have to include a 100mA RCD close to its origin. Because of the high ‘earth fault loop impedance’ (EFLI) in TT installations, over-current protective devices such as fuses or miniature circuit breakers (MCBs) will usually not offer any protection against live-earth faults, so that such protection has to rely on RCDs. In the days when some or all of the circuits within an installation did not have their own RCD protection, the practice therefore evolved of having a 100mA RCD close to the origin of the installation. However, if (as is not uncommon these days), all final circuits in the installation have their own 30mA RCD or RCBO protection, then an additional RCD at the origin of the installation is probably not required. Without such an RCD, lack of protection only exists in relation to the tails going to the consumer unit, and the chances of a live-earth fault occurring there is extremely small provided that these tails are (as generally required by DNOs) short. If an additional RCD at the origin of an installation which contains some 30mA RCDs/RCBOs is utilised (e.g. because some of the final circuits are not RCD/RCBO-protected, or for any other reason), it should not only be a 100mA one but should also be of a ‘time-delayed’ type (‘Type S’) to ensure ‘discrimination’ between it and the 30mA RCDs/RCBOs (i.e. so it will not trip if a 30mA RCD/RCBO trips first), and must be installed in an insulated enclosure. Note that, in relation to any circuits protected by single-pole RCBOs in the consumer unit, this discrimination will only exist in relation to live-earth faults, not neutral-earth ones.
Kind Regards, John.