There has been much recent discussion concerning RCBOs and the separate earth connection used by some of these devices. David Pitt, Product Marketing Manager - Power Distribution Components, Eaton, claims that a lot of information appearing has been misleading and, indeed, sometimes wrong:
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| An Eaton RCBO. |
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One big change brought about by the implementation of the
17th Edition Wiring Regulations was a big increase in
RCBO use. However, says David Pitt, "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, he claims, is not true. While the standard specifies that an RCBO that does have a separate earth connection must behave in a certain way, nowhere does it state or imply that a separate earth connection is mandatory. Indeed, RCBOs are currently available that have no separate earth connection, yet they achieve full compliance with the standard, supported with declaration of conformity documents.
The complaint is also often made that, without a separate earth connection, an RCBO cannot detect an earth fault if the neutral becomes disconnected. This, says Pitt, is true, but is not worthy of consideration, partly because losing the neutral connection is very rare event in itself, but the probability of losing the neutral simultaneous with having an earth fault is 'vanishingly small'.
Many installers, however, persist in specifying RCBOs having a separate earth connection, even though - as a result - consumer units and distribution boards take longer to wire, which is costly. But David Pitt says that there's worse problem - many installers don't seem to know what to do with the earth connection, and some even cut off the earth wires, which is very bad practice because the units have been tested on the basis of them being earthed.
Finally, there's the testing issue. Most manufacturers of RCBOs with earth connections insist that these are removed during testing to avoid possible RCBO damage - this is time consuming, and it is not unknown for installers to forget to reconnect the earth wires, leaving the RCBO operating under conditions different from those for which it was designed. Additionally, in effect the installation will not - as it should - be in exactly the same state when tested as when in service.
Concludes Pitt: "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."
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