There has been a lot of discussion recently in the press and on-line about RCBOs and, in particular, about the separate earth connection that is used by some of these devices. Unfortunately, a lot of the information that has appeared has been misleading and, in some cases, it is simply wrong. Given that the recent introduction of the 17th Edition of the IEE Wiring Regulations is resulting in a big increase in the usage of RCBOs, it is important to clarify the situation.
One of the most misleading statements, but one that seems to be repeated time and again, is that in order to comply with BS EN 61009, RCBOs must have a separate earth connection. This is not true. Certainly, the standard specifies that an RCBO that does have a separate earth connection must behave in a certain way, but nowhere does it state or imply that a separate earth connection is mandatory.
In fact, RCBOs are currently available that have no separate earth connection, yet they achieve full compliance with the standard, supported with declaration of conformity documents.
Another often-made point is that, without a separate earth connection, an RCBO cannot detect an earth fault if the neutral becomes disconnected. This is true, but in reality it is not an issue worthy of consideration.
Losing the neutral connection is a rare event in itself, especially in the TN-C and TN-C-S systems that are almost invariably used today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is vanishingly small – certainly so small that there is no reason to take it into account when specifying protection devices.
In spite of this, there are those who claim that it’s still better to use an RCBO with a separate earth connection, “just to be on the safe side”. This is not necessarily a good judgement call. The extra connection means that consumer units and distribution boards take longer to wire, which pushes up costs. Even worse, many contractors don’t seem to know what to do with the earth connection, as a search of the on-line electrical forums will quickly reveal. In fact, there have been suggestions that some contractors have actually been cutting off the earth wires, which has to be very bad practice for an RCBO that’s been designed and tested on the basis that it will be earthed.
Finally, there’s the thorny issue of testing. Most if not all manufacturers of RCBOs with earth connections insist that these connections are removed during insulation testing to avoid possible damage to the RCBO, and during loop testing to eliminate the possibility of misleading test results. Removing and replacing the connections adds more time and cost, of course, but there are also other concerns.
First, there is always the possibility that the contractor will forget to reconnect the earth wires, leaving the RCBO operating under conditions different from those for which it was designed. Second, disconnecting the earth wires goes against one of the basic principles of installation testing, which is that installations should in exactly the same state when they are tested as they will be when they are in service.
Claiming that a separate earth connection is essential on RCBOs is not only wrong, it also makes life unnecessarily complicated for contractors, while trying to protect against a risk that’s theoretical rather than real. Protection products should always be selected with care, but they should be selected on the basis of performance and reliability rather than on the basis of some invented and imaginary criterion.
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