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#142533 - 01/29/05 05:10 PM PME Good or Bad?.
Trumpy Online   content


Member

Registered: 07/05/02
Posts: 8558
Loc: SI,New Zealand
OK Pauluk,
You've asked for it now!.
The thread that PaulCornwall posted, has raised some questions about the actual integrity of the system.
Now that we have a few newer members here in the Area, anyone like to throw thier 2p/2c, or even 2 Euro's in, on what they think is good or bad about this system?.
Pauluk, now I'm making a silly and wild assumption, that the PME system, is virtually the same as our MEN system here in NZ, apart from the Earth stake at every house.
If they are, I would also invite new member Kiwi into the fray!.
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#142534 - 01/31/05 06:04 AM Re: PME Good or Bad?.
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yes, as I see it it is essentially the same as the Aussie/Kiwi MEN system, except as you say for the absence of a local earth rod here.

The only other main difference as I understand it is that you make the bond in the main panel whereas we keep N and E separate in the panel and bond to the neutral at the cutout on the supply side of the meter.

The main advantage oF PME is the much lower loop impedance for an earth fault and the fact that the earth is not the sole path for fault current.

The disadvantage is the potential (no pun intended ) for a bad neutral to result in all the bonded metalwork rising to 240V and thus the importance of minimizing the chances for a p.d. to exist between adjacent items.

PME was originally used in Britain only in certain rural areas, but has become much more widespread in the last 20 years or so. As I think I've said before the entire network in my local area has been adapted (extra earth rods etc.) to allow a PME connection to anyone who requests it, although the majority of systems are still TT.

I would certainly like to see earth rods installed at every house supplied under PME, as you have and as is done in the U.S. (where all services are the equivalent of PME, of course).

Actually the comparison brings up the other point of the differing approaches to distribution and how this can have an effect on PME arrangements. In my neighborhood we have the usual huge 3-phase xfmr sub-station with 4-wire 240/415 distribution which feeds dozens of homes. In the States, we'd have a much larger number of smaller transformers with each feeding just a few houses, and of course with each xfmr having its own independent neutral/grounding system.

Any thoughts on how those different approaches affect the PME system?

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#142535 - 01/31/05 04:00 PM Re: PME Good or Bad?.
gideonr Offline
Member

Registered: 12/18/04
Posts: 152
Loc: Edinburgh, Scotland, UK
At work a few years ago they built a new building, where outside the laboratory doors there were simple switches controlling contactors to turn off all the lab power at night. Then someone in a digger put the bucket through the neutral, cue lots of fried contactors, costing tens of thousands.

PME also causes difficulties when a radio amateur wants to connect their transmitter inside to an earthed antenna outside.

I agree with the local earthing idea, only that it needs to be mandatory as a voluntary lone earthing rod would be subjected to a lot of current in a phase imbalance situation.

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#142536 - 02/01/05 12:16 AM Re: PME Good or Bad?.
kiwi Offline
Member

Registered: 12/04/04
Posts: 347
Loc: christchurch new zealand
OK Trumpy, I'm "in the fray". I believe the "Bathee" fault described by PaulCornwall would have been much less likely to happen in the MEN system. The extra earth reference in the MEN system would make this fault highly improbable. Differing earthing and plumbing systems make these type of faults hard to diagnose. Add to that mix the actions of cowboys ( both electrical and plumbing ) and it becomes even more complex. I'm guessing the best thing to come out of this discussion is the lowered likelihood of it happening again.

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#142537 - 02/01/05 05:57 AM Re: PME Good or Bad?.
Trumpy Online   content


Member

Registered: 07/05/02
Posts: 8558
Loc: SI,New Zealand
There's some good comment's here.
Strange thing about having an electrode though, is that once it's installed, it's expected to carry the same current 30-40 years down the track, as it was the day it was installed.
I've pulled the odd 1/2 (or worse 1/4) of an electrode straight out of the ground in some places around here after 30 odd years, due to corrosion.
Now you can't tell me that that electrode will give a 10 ohm maximum connection with respect to the Neutral.
BTW Paul,
Didn't you post some diagrams of the UK system a couple of years back?.
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#142538 - 02/06/05 01:31 PM Re: PME Good or Bad?.
djk Offline
Member

Registered: 12/26/02
Posts: 1269
Loc: Ireland
Paul/Trumpy:

Sounds like our system in Ireland more like the Aussi or Kiwi system

The normal system here has a local earth rod connected to the earthing system in the house which would be bonded to the PoCo neutral at a point just before the meter. The PoCo neutral's then earthed at the each distribution pillar or regularly at every odd pole if it's overhead.

(That being said, many over head supplies here would be TT, it depends on the local cable set up)

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#142539 - 02/06/05 04:35 PM Re: PME Good or Bad?.
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Quote:
BTW Paul,
Didn't you post some diagrams of the UK system a couple of years back?.


UK PME / TN-C-S System

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#142540 - 02/07/05 02:14 PM Re: PME Good or Bad?.
aland Offline
Member

Registered: 05/20/04
Posts: 187
Loc: United Kingdom
djk, Believe we refer to this one as TN-C-S (PNB). PNB stands for Protective Neutral Bonding. Its only used here where a single consumer is supplied from a distribution TX, usually used when the consumer owns the TX and metering is carried out at high voltage. It becomes the owners responsibility to earth the CNE Conductor. (Sorry missrd your reference to before the meter) Your still refering to PME. Just as an asside any one use PNB then.

[This message has been edited by aland (edited 02-07-2005).]

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