harry98

Hi could someone please explain why you cant export a PME over 20 metres?

And what are the dangers of doing so?

Cheers

wayne

where does that come from? i ask out of genuine curiosity ?

harry98

I got told of someone that you cant export PME over 20 metres, so say if you had a garden shed 50 metres away you would have to put a TT earthing system in.

The question is though, how come?

Cheers

Jurassic Spark

You can export a PME supply to an outbuilding providing that the outbuilding in question has no structural steel metalwork. If it has, a TT supply feeding an RCD protected CU is required, all structural steel is to be bonded and an earth rod installed. The danger with PME is if the neutral conductor fails, extraneous metalwork is used as the return path for the supply.

Shakey

i agree,

By structural steel he is of course talking about any extraneous conductive path, which would typically be structural steel or an incoming water supply

If you are going under ground and using SWA, connect the armour at the supply end only

not sure about the RCD though, if you are coming ioff an RCD protected supply anyway you shouldnt put one in the garage (unless you are going to discriminate)

If coming straight of a NON RCD protected MCB then yes put one in the garage

Never heard of the distance before, I have always considered the presence of extraneos conductive paths as the deciding factor as to whether or not to export

Jurassic Spark

Hi Shakey. If the supply to an outbuilding is going to be TN-S, then I'd put the RCD at the supply end because you'd have a continuous earth from the house to the outbuilding via the armouring and maybe a spare cable conductor. If the supply is PME and you're not connecting the armouring at the outbuilding, the RCD wouldn't work as they'd be no return earth path for fault current. Provided that you can guarantee a disconnection time of 0.4s or less, the cable can be protected by an MCB.

Shakey

sorry, hate to say you are wrong, but you are wrong

If you have made the garage into a TT, and its less than 1667 ohms, the RCD in the house just sees an imbalabce between line and neutral, and trips off

The fact that the fault current flows to earth through the electrode in the garage is irrelelvent to the RCD

harry98

So if you have to TT an outbuilding that has structural steel why dont you have to TT any building that has structural steel?

Cheers

Had8Lives

Any steel structure should be bonded to the main earth, if the PME supply is within the Structural steel buliding then there's no need for an extra earth :-O

Shakey

like the man said, we are within the equipotential zone, we are talking about exporting outside the zone