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first of all if the readings are low enough is it allowed to change a TT system to a PME, does it contravy any regs at all? your thoughts please
 
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B

beaver74

first of all if the readings are low enough is it allowed to change a TT system to a PME, does it contravy any regs at all? your thoughts please
sorry not sure i get you the reading needs to be around .35 and i cant see a rod giving that also pme is an issue for the supply authority and they can only give it to you as it is dependant on the network you are supplied from
 
S

Spudnik

The max reading allowed for PME is .35.

If you loop test the incoming tails on the CU (after isolating the entire installation and disconnecting all earths) and its below the max then fill ya boots!

Bear in mind that main and poss supp bonding will need upgrading, as well the CU.

Being near the 'sticks' im doing this all the time, and never have a prob with the DNO.

No regs contravened as its not covered in there, only what needs to be done for that type of earthing arrangement.

Make sense?:D
 
J

Jurassic Spark

You have to check with the supply authority first to see if PME is available in that area. It's not just a question of you connecting N & E together at the customer's property and hoping for the best. ALL properties in the area need to be PME as well.

When I worked for the Y.E.B. we used to install replacement supplies in 16mm singles pvc/pvc cable clipped at high level around the houses. At every fourth house, we'd run a 16mm earth connected to the neutral, down the wall and into an earth spike to form the PME system.

The internal bonding needs to be upgraded to 10mm as well.
 
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J

Jurassic Spark

Sorry, wrote that down wrong. What I meant was all the supplies feeding properties in a particular area have to be PME. As I said, the neutral supply is earthed at intervals so if a neutral breaks anywhere down a supply cable's length, the earth takes over as the return feed for any load.
 
B

Bright-spark

A break in the neutral of a TN-C-S supply (external to the installation) results in load currents being carried by 'earth' cables and 'earthed' metallic paths of the installation. Any fault on the neutral can effect adjacent properties. PME (protective mutiple eathing) is a method of limiting the extent of the area effected by the fault.

TT supplies (as distinct from installations using a TT supplies) can be upgraded by the DNO's to PME (TN-C-S) but this is a network upgrade rather than an individual dwelling upgrade.

Certain installations such as caravan parks, petrol stations, marinas and even metal lamp posts in domestic gardens can not be connected to a PME supply because of the dangers that could arise in the event of a neutral break on the supply cable.
 
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S

Spudnik

but this is a network upgrade rather than an individual dwelling upgrade.
Agreed, which is why the test is performed at the CU to check and then the DNO is contacted to pay a site visit to confirm and do the necessary.
 
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so lets get this straight, u can do the test inorder to see whether it meets the regs at the cu, but u still have to contact the DNO to make sure its alright (and they'll pay a visit) in most cases wat is his decision going to be ?
 
B

Bright-spark

Only the DNO can advise whether or not pme exists for this installation.

If pme does exist then the DNO can provide an earth terminal but the responsibility for making sure that adequate earthing for the installation is in place rests with the contractor.

If the DNO provides an earth terminal then they are duty bound to maintain that earth.
 
J

jaldred

Just call the DNO - give the postcode of the property in question - they will tell you, in principle, whether a PME supply is available in that area or not. If it is, the DNO will charge a nominal fee (£35 rings a bell), appoint and attend the property (usually within a few weeks), confirm PME suitability and provide a PME terminal (link the neutral to an earth block).
You can then connect your 16mm from the PME terminal to your CU, upgrade your service bonding to 10mm etc....
Not sure why you would want to convert from a TT to PME in the first place though especially with new Regs and RCDs being required throughout the majority of all installations now, regardless of supply characteristics.
 
S

Spudnik

so lets get this straight, u can do the test inorder to see whether it meets the regs at the cu, but u still have to contact the DNO to make sure its alright (and they'll pay a visit) in most cases wat is his decision going to be ?
His decision will be to confirm that its do-able and provide the earth point.


My local DNO dont even charge for this service.:cool:
 
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