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Discuss When PME is NOT recommended.....why?? in the Electrical Forum area at ElectriciansForums.net

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Rockingit

Forum Mentor
Something that's always bugged me, and I've never fully understood WHY (just know that's it what the regs say) is why certain installations are prohibited from using TN earthing, only TT (caravan sites etc etc).

Anyone care to enlighten me?
 
W

WPD Lad

I can post something from work tomorrow on earthing arrangements for different developments, steel framed divided buildings, flats caravans etc all have regs. ESQC regs etc

If there is external metal work on the caravan then no PME can be given, if it's going into a brick enclosure then we can provide it
 

rich.250

Electrician's Arms
I think caravan hook ups are classed
As temporary supplies hence tt, I was told tt is used for
Temp supplies, agriculture. Due to the increased risk and to limit the pefc.
 

GLENNSPARK

Electrician's Arms
well surely if theres exposed metalwork in contact with the ground and this is essentially acting like an extrainious conductive part and the install was PME..in an area that could be subject to disturbance...like a building site...then the risk of dangerous potentials would exist....so by TTing it all.....would keep potentials equal wouldn`t it??.....
 

Rockingit

Forum Mentor
This is where it gets to the bit I've never figured out why it's safer: in a tn system (assuming that everything is bonded properly) you've effectively got everything that can conduct under a fault all tied together back to one place using a system of actual copper cable, the resistances are near irrelevant mathematically so you get a nice fast disconnection time under ADS.

But in a TT, it's all a bit left to chance with much bigger resistances and slower disconnection times (hence why needing RCD backup).

Maybe our expert friend from the Orient can assist?
 

telectrix

Scouser and Proud of It
Respected Member
it's only TNC-S that is banned from caravan sites and the like. think it's more to do with what would happen if the neutral were to be lost. as the earthing is connected directly to the neutral at the cut-out, you have lost the earth as well so .............
 
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GLENNSPARK

Electrician's Arms
Hmmm....so basically it falls to us to safeguard against the DNO messing up. That's a first.
well...its only like the scenario my boss n I were faced with the other week....that job with the shared supply and i couldn`t get a Ze on it......bloody DNO not ensuring there was not adequate earthing for THEIR APPARATUS.....and a cutout with both the knockouts missing.....of course client was charged for a PME connection to be supplied and we were slated as "know nowts" by this obnoxious little bint on the end of the phone for pointing out these failings to em...
 
W

WPD Lad

well...its only like the scenario my boss n I were faced with the other week....that job with the shared supply and i couldn`t get a Ze on it......bloody DNO not ensuring there was not adequate earthing for THEIR APPARATUS.....and a cutout with both the knockouts missing.....of course client was charged for a PME connection to be supplied and we were slated as "know nowts" by this obnoxious little bint on the end of the phone for pointing out these failings to em...
Has their ever been a DNO earth at the property? If not, they're not obliged to give you one, if there was one (legit one) then they're obligated to maintain it!
 

Rockingit

Forum Mentor
OK then, here's a different angle to it: If for arguments sake we're talking about a caravan park, TNC-S provided to the supply, if we ignore the supplied earth but bang down our own rod and use that, does that a) still satisfy regs/EQSR and b) achieve the same end result? When is a TT not a TT? Given that it won't be that far (in theory) to the DNO's nearest PME point in the ground anyway.
 

SPARTYKUS

Electrician's Arms
Answer: anywhere where the IET don't manage to convince you that the laws of physics changed in the 70's & its ok to suddenly use your return path as your earth too.
 
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