Q Should the electric hook-up point be earthed?
My motorhome is fitted with a Sargent reverse polarity tester with the facility to switch the positive and neutral when required. It has proved invaluable when travelling in both Spain and Morocco.
Occasionally, it shows that the polarity is reversed and I have had to switch it. So, thanks to Sargent for such a useful gadget.
My problem, however, is that it also detects if there is no earth connection and an audible alarm goes off. Whilst this is irritating, it raises the serious question as to whether it is safe to use an electric hook-up where there is no earth.
Am I wrong to be nervous about connecting 230V to my metal ’van without an earth? Also, is there an issue with connecting the earth to the bodywork, which also provides the neutral for my 12V car battery system?
This problem I have had on several occasions both in the UK and abroad. Your advice would be most welcome.
Dr Bevis Miller
A Mains electric is an AC supply, so the two wires are live and neutral. The neutral is also bonded to earth at the local transformer and frequently near each dwelling. Positive is associated with a DC supply of the type you get from a battery, this alongside negative.
Your motorhome will be fitted with a double-pole mains master switch called an RCD (residual current device) and this compares the current flowing in the live with that flowing in the neutral. If these are both exactly the same, then we know there are no earth faults as every bit of current going in is accounted for by the current going out.
If the RCD identifies a difference between the incoming current and outgoing current of more than 0.03A (30mA) then the RCD will switch off and disconnect the mains supply. If the live and neutral supplies to the motorhome are transposed, the RCD will still work as it should, you remain protected in this respect.
However, if you start taking mains-powered devices apart inside the motorhome then those that you may have believed were switched off will still have live connected. The lesson here is never do any work on the mains side of the electrical installation unless the hook-up lead is disconnected.
A lack of earth is far more serious and dangerous. If the motorhome has a fault between its live and the ’van metalwork, but you had no earth connection to the hook-up point, then there is nowhere for any fault current to flow, so the RCD will remain closed.
However, if someone were to get in or out of the ’van there would be a path to earth via that person’s feet and hands and, at best, a shock would ensue. If the current were sufficient, the RCD would open, disconnecting the supply. If there is no earth then it is unsafe, so you should try a different hook-up point.
Yes, the motorhome metalwork is connected to both the battery negatives and the earth connection via your hook-up lead. This is normal.
Clive Mott