Overhead supplies??

[La Poste]

Greetings.
I am helping to re-wire a bungalow that has an earth rod.
The supply cable is overhead, it's the same for the whole street.
I took a walk up the road and noticed all the overhead cables converged on one single pole.
On the pole was written PME and three cables ran down the pole into the ground, I am assuming these cables are supplier earth rods as they didn't look fat enough to be power cables.
I spoke to a friend and he said it looks like the supplier is going to upgrade the whole area to PME in time.
However I was told at college a while back in 16th edition that overhead cables should be earthed via TT system because overhead cables are more likely to suffer a neutral fault and hence you would lose your earth aswell. I was told this a while ago and I could have got this fact wrong.
So my question is if the supply is overhead is it better to use a TT system or to use a PME system?
I just wonder what would be the best earthing arrangements for this scenario.
I understand that any TT system must be RCD protected to max.
Thanks.

 

[cagedanimal2010]

ask the suppliers what the earthing arrangement is, they will give you a straight answer either way.

 

[Spudnik]

Yup, the DNO is your first call, and ask for a PME enquiry.

Its possible, that PME is already available in the house.

NORMALLY, if the overhead cable coming into the house in singular, then its ready, but if there are two coming in, then it isnt. Yet.

The above, is in no way certain, and a call to the DNO is a must.

 

[La Poste]

Thanks mate.
I was chatting to an old boss today and he said TT was the worst of the lot.
He said RCD's are just a backup measure and can fail so the best earthing is the one with the least resistance and hence will trip your fuses in the least possible of time.
With a resistance of an earth electrode of 200 Ohms an earth fault short circuit current will flow of just over 1 Amp hence not tripping any of your MCB's, with RCD failure this would leave a potentially fatal voltage hanging around on any faulty metal service waiting for someone to touch it, with a PME if the RCD's fail and there is an earth fault the earth fault current will be so big it would trip the fuses in no time.
What he said really made sense to me and I just wondered what other peoples thoughts are on this.

 

[rocker]

I think this is why we have double - RCDs on TT supplies.... what I want to know is why is the incomer supposed to be 100mA, why not just have two 30mA?

 

[La Poste]

I think this is why we have double - RCDs on TT supplies.... what I want to know is why is the incomer supposed to be 100mA, why not just have two 30mA?

Maybe it's got something to do with whether you have a metal CU or maybe I'm barking up the wrong tree.

 

[cagedanimal2010]

i thinks its to do with discrimination?!?

 

[La Poste]

Well I was told today that if you have a fully insulated CU you can use a two way split RCD board both at 30 ma. No need for a 100ma RCD anywhere.

 

[Spudnik]

Well I was told today that if you have a fully insulated CU you can use a two way split RCD board both at 30 ma. No need for a 100ma RCD anywhere.

Correctamundo

 

[cagedanimal2010]

cant comment really, i was under the impression if you have a tt supply you have to have a 100mA rcd. that might be for non fully insulated boards tho.

 

[La Poste]

Well 100ma does not give you life protection so what's the point?
100ma can kill you, 30ma is a safe level.

 

[vambo]

You don't need double rcd's on a tt if every circuit is protected by rcbo's/split load, well this was what i thought. Can understand your point though if the consumer unit is metal clad this an exposed conductive part of the installation not protected by rcd.
The reason it is 100ma is because of discrimination the 30ma will trip first not disconnecting the whole supply leaving other circuit's energized for safety.

 

[vambo]

btw 30ma can kill you any ammount of current can kill you depends on so many factors, voltage, body resistance or you could have a dicky heart for example.

 

[rocker]

Well I was told today that if you have a fully insulated CU you can use a two way split RCD board both at 30 ma. No need for a 100ma RCD anywhere.

You miss my point - with a TT installation, there should be a 30mA RCD (or two on a split board) in the board, and also a 100mA stand-alone RCD covering everything i.e. on the meter tails.

 

[cagedanimal2010]

i agree with rocker!