Earthing Armour - Can I Earth to Iron Mounting Bracket?


Postby Tonyonly » Tue Aug 05, 2014 3:33 pm

I have a hook-up on my drive for my motorhome and have used armoured cable buried eighteen inches down for the supply. It terminates in a commando socket mounted on a piece of angle iron concreted into the ground. Does earthing work to iron and can I earth the armour to it? At the other end I have an existing 30mA RCD and can earth the armour to an existing earth rod but I'd rather play safe and earth both ends if possible.
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Postby ericmark » Tue Aug 05, 2014 8:51 pm

Earthing for caravan or car charging where the vehicle is outside should be TT but that assumes that the distance required for fire prevention is obeyed so where the vehicle breaks the fire regulations then also one has to give very serious consideration to earthing and the voltage gradient.

So where ALL rules are followed it is easy just use TT but where fire rules are broken you have to work it out from first principles you can't simply follow the rule book.

I park my caravan in my father-in-laws drive within a meter from the electric and gas meters and I considered with a metal body it was safer to use the TN supply than use TT supply even though that breaks the rules.

Clearly with the correct TT supply you must measure the earth loop impedance and also of course the tripping currents and times of the RCD.

OK I have a full test set and can measure all the required parameters but for most DIY people this is just not possible.

So to play safe it needs far more than what you have given.

1) Distance between any earthed items on the house and the caravan supply really should be more than 15 meters for fire regulations.
2) Earthing system of the house TN or TT and if TN is it TN-S or TN-C-S the latter causes a problem but is also the most common.
3) Earth loop impedance of system.
4) What milliamp does the RCD trip at looking for less than 30 mA.
5) How fast does it trip looking at less than 40 ms clearly needs a meter can't measure that time with stop watch.

The regulations give rules for caravan parks as to if this includes a house it does not say and one can debate the rules until the cows come home so it's more to do with common sense and ones knowledge of earthing systems.

Clearly every situation has to be considered on it's own merit there are no rules once one breaks the rules.

There has been a lot of talk over the supply to electric cars and earthing arrangements one of the big questions is should the metal body of the car be earthed or not. The same question arises with motor caravans years ago they had aluminium bodies but today plastic is used a lot and clearly this has a large impact on earthing arrangements.

This post could go on for years as to be frank I can quote regulations but with a caravan or motor home parked close to the house the only real way would be to treat same as a boat and use an isolation transformer and the cost of a 3kVA transformer is quite high.

So in the main we assess the risk and do what we think is right for that risk rather than follow the rule book. But clearly there is a risk and only you can decide if it's an acceptable risk.
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