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.

 

[Spudnik]

i.e. on the meter tails.

No need if an insulated CU is used.

 

[cagedanimal2010]

No need if an insulated CU is used.

just to clarify
if a metal clad cu is used 100mA is needed.
if double insulated cu is used 100mA is not needed.

 

[Spudnik]

100mA on the tails and 30 on the circuits.

 

[cagedanimal2010]

yup cool, cheers jason! up-until tonight i had put in 30mA split loads(double insulated), with 100mA over tails(on a tt system), should save me some dogh!
cheers again!

 

[rocker]

No need if an insulated CU is used.

Ooh nice.

Mind you, I think I will still use one for safety, TT earth fault loops can be high and as discussed RCDs can fail, I prefer to have that back up and I always price them into TT jobs anyway. Come to think of it, I may start putting them on all installations. It can't hurt, right?b

 

[Des 56]

Rocker

If you had a metal clad consumer unit,as long as the tails are bushed at entry theres no requirement
Only just reminding you that if you fit an 100m/amp Rcd it would have to be time delayed and they are around £ 80 a pop

 

[vambo]

can only hurt the customer's pocket! i certainly wouldnt bother if the tail's are in good condition and it's not in a area likely to get fiddled with by kids etc. RCD's can fail hence the quaterly test's that no one ever does!

 

[cagedanimal2010]

Has any one got the reg to link to the "double insulated cu not needing 100mA protection"? Cheers!

 

[La Poste]

Has any one got the reg to link to the "double insulated cu not needing 100mA protection"? Cheers!

Could someone show me the reg that says it is needed?
Cheers.

 

[vambo]

not got the redbook with me at the mo sorry. anyone else know?

 

[pushrod]

Think you have a choice of mA value depending on Zs (table 41.5), although if you have 2 rcds you have to ensure discrimination (411.5.3).
I think 100mA for the upstream RCD is sensible to minimise nuisance tripping. Not sure about circuits over 32A