Exporting PME

[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]

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.

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]

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.

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

 

[wonkey donkey]

If using PME the garage must be fitted with an earth spike as it is illegal to fit another PEN.

 

[helpfulsparks]

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.

Technical at the IEE agree. I'm wiring an outbuilding in Cornwall at the moment and I tried to use an earth rod. It took 2 hours to install just a single 1m rod as I kept hitting granite. Once the rod was finally down, Ze came back at near 1000 Ohms so I really wanted an alternitve. I have an IEE wiring matters from 2005, it covers supplies to outbuildings and gives examples using both methods mentioned here, as the artical is pre-17th, I contacted the IEE just to be sure, they confirmed that it is still acceptable to export the PME, as long as the building has no extraneous conductive parts.

 

[wayne]

thank you for the update, very helpful(sorry)

 

[super spark]

Just had this discussion on another forum, took me a long time to find the evidence to back me up but here it is, hope this helps;

Subcategory - 1. BS7671 - Voltimum UK - Electrical Installation Products and Contracting



Subcategory - 1. BS7671

*
Local supply authorities*appear to have different requirements when taking supplies externally to*garages/outbuildings.
Here in West Sussex we are*told that we should take a Phase and Neutral supply only to a detached building and then install a local earth rod for the**external supply.
Some time ago an article*appeared in "The IEE Wiring Matters" by John Ware.*This stated that even with a PME supply to the house it could be achieved without an earth rod. The article appeared in Autumn 2005 issue 16.
I would appreciate your fact and regulatory*based comments.


*
BS 7671, and indeed, the article you refer to, give the requirements for electrical installations generally. Should local conditions, or in this case, the electricity supplier, require that further measures should be taken, their advice should always be followed.**
30-03-2006



*Back


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