PME Supply

[andemz]

As many will be aware, my earthing set-up is a TT earth system. This evening i opened the met box on the outside of my house soley for educational purposes. It looked very neat.

I do however have a couple of questions.

1) There is a block before the meter, which states PME on it (series 7), this block also has a part which states Henley on it. The wire from the MET comes into this block and then goes into the meter to which it connects the Live and Neutral for the consumer unit.

PME from what i understand stands for protective multiple earth and by which earth leakage current is fed back to the substation along the neutral return wire, and so to earth.

I take it the earth rod is still needed and if so will i still require my 100A Time delayed main switch. ?

2) Does PME make any difference?

 

[plugwash]

either its TT or its PME

the two are mutually exclusive

if the earth is linked to the neutral (usually done in the service cutout sometimes a seperate henly then its PME

 

[andemz]

Oh right.

How do i actually find out. The earth is coming out of the consumer unit to the rod. the big block states PME. Would some pictures help?

 

[plugwash]

yes photos would be helpfull

often with a TT that has been upgraded to PME the rod is left connected as well

 

[Lectrician]

Pictures paint a thousand words

Henly is a brand name, they make service connectors, and also very often the actuall fused service head.

If it has a green PME sticker on it, then PME is usually available from that service head, although it may not have been linked accross inside the service head as yet.

 

[andemz]

Right Sparkies have a look at these then. Give me your thoughts as discussed in previous posts above. PME or TT

That green sticker states PME system. The block above says Henley on it and states 100A TypeII B.

As you can see i've just laid some new drains etc.

 

[plugwash]

it looks like there are no earth wires connected to that block

so it would appear the supply is wired to PME standards but your system isn't actually connected as a PME

 

[andemz]

What would you suggest i do now then? Does that mean that i have a valid earthing system in place at presant, if not would it cost me to get the rec in to undertake this work?

 

[Lectrician]

The earthing system you have is satisfactory. PME is usually a more convenient system, as you do not need a main RCD (no nuisence trippping).

You would need to upgrade your main equipotential earthing to gas, water, oil etc. And you should also provide 30mA RCD protection to socket outlets if this is not already done.

The work can be carried out by any sparky, and he will leave an earth cable for the rec to connect, he will then return to site to remove the main RCD (this must remain until the new earth connection is made. Many recs do not charge to conect up a new earth or tails.

 

[andemz]

So 16mm of earth cable from the consumer unit to the outside box.

Are you saying then that a time delayed main switch in the consumer unit is not necessary and a 100A main switch would be fine.

Why do just the whole of the sockets in the house need rcd protection and not the lights. I take it that 80A 30mA is ok for such protection.

I thought that rcd protecting everything wasn't right.... I am still learning, but i'm uncovering more each day.

 

[salem2000]

Sadly if you do not have PME you will have to keep the RCD to protect the whole system. you could have your system converted to PME, but the REC's are not always helpfull, and some of them make a charge for this service.

Salem.

 

[Damocles]

The 100mA rcd is specified for TT systems because there is a chance the earth connection could be too poor to blow your fuses/mcb in the event of a short to earth. With PME there is very good connection between earth and neutral so the 100mA RCD is not needed.

A 30mA RCD is more sensitive than a 100mA one. The 100mA is considered 'good enough' for overall protection, but with less risk of nuisance tripping disconnecting all your power. However, it is still considered that 100mA could be enough to kill you under some circumstances, wheras 30mA probably would not.

It is a regulation that 30mA protection should be provided for any socket outlets which could be used to power outdoor equipment. Some bathrooom stuff also recomends this level of protection.

On the other hand, lights are generally out of reach and therefore it is considered that there is less risk of anyone getting a shock in the event of a fault. It is also considered a bad idea to have lighting power switched off in the night by a RCD, when in fact the fault is on a different circuit.