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Topic Title: PME
Topic Summary: What Happens ?
Created On: 03 May 2010 06:38 PM
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 03 May 2010 06:38 PM
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Ampman

Posts: 1041
Joined: 06 February 2006

Evening ,

Can somebody please explain what actually happens when a PME supply loses its neutral ?

I know the earth potential rises to a high voltage .

But i ask this question as every seems scared to export the PME but even if it isnt exported

What happens to cpcs , earth pipe work etc....

In a house that has a pme supply .& loses its neutral ?

& if it is so dangerous to have a PME supply just in case the neutral gets lost ,

Why is it still common .& is meant to be the best ?

Many Thanks
 03 May 2010 06:45 PM
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alancapon

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Joined: 27 December 2005

The way it is supposed to work, is that because all the bonding is in place, all the metalwork within the property will rise to the same voltage, possibly as much as 230v. With everything being live, it should be difficult to find something at a lower voltage to get a shock from. As nothing will work with the phase and neutral at the same voltage, the local DNO are called, as there appears to be no power. They will turn up and fix the fault.


Regards,

Alan.
 03 May 2010 07:03 PM
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Pactrol

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Hi alancapon

I don,t think it quite works like that as depending how far the nearest earth
is thats still connected to the transformer & of course the ground conductivity you could find things still work. with a reduced voltage. return path via the bonding. but isn,t that the idea.
 03 May 2010 07:17 PM
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alancapon

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My description is what is supposed to happen. Obviously with a metal gas or water service pipe, then the neutral current will be diverted down that instead and things may still work. It also depends on where the neutral is lost - the service to a single property is usually regarded as the worst case. The important thing, is that everything that should be earthed within the property will be at the same voltage, minimising the chances of a shock. However, the class 1 appliance outside, or my favourite, the outside tap connected with a copper pipe, well that is a different story.

Regards,

Alan.
 03 May 2010 07:33 PM
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Pactrol

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Hi alancapon

Im not saying your not right but. I think the latter more likely & would help exsplain why we have to install such heavy pme bonding cables
though I will say I don,t consider pme as safe as TNS. just the rec trying to save a few bob on their end by passing tha cost to the customer having to install bonding everywhere.

 03 May 2010 07:43 PM
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adder6

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Originally posted by: alancapon

My description is what is supposed to happen. Obviously with a metal gas or water service pipe, then the neutral current will be diverted down that instead and things may still work. It also depends on where the neutral is lost - the service to a single property is usually regarded as the worst case. The important thing, is that everything that should be earthed within the property will be at the same voltage, minimising the chances of a shock. However, the class 1 appliance outside, or my favourite, the outside tap connected with a copper pipe, well that is a different story.



Regards,



Alan.


Hi that's a great post thanks. So please for my benefit alone, can you tell me if I have understodd this properly?

1)If all the bonding has been properly carried out, and all the cpc's are of good quality proven with good Zs readings and the like - does that mean that even if everything is all non-conducting at 230Volts because it's all bonded togther there will never be a potential difference between them, even if they each have a potential difference to ground?

2)The outside tap thing - if the ground's wet or someone say leant on a washing line post which was wet in the ground could this possibly mean they get up to 230V through them between the post and the tap even though the premises itself doesn't have a good earth now the neutral has gone? Could a soggy ground and say metal buried object act as a decent enough earth to get a fatal current through someone?

3)If you lose the neutral but you have well sunken gas pipes and they start to conduct would this cause a brown out in the house? And could gas pipes carrying current become an explosion hazard?

Help! Thanks
 03 May 2010 07:45 PM
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alancapon

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The bonding cables need to be a reasonable size, because they may need to be able to carry diverted neutral current for a while, and also need to be able to carry fault current from the network. Even without a broken neutral, some current will flow in the bonding conductors if one or more of the service pipes are metal.

Regards,

Alan.
 03 May 2010 07:59 PM
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perspicacious

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Has anyone having carried out a Ze test, done the same test but on the MET before reconnecting the earthing conductor?

Using a mA clamp meter on the various conductors connected to the MET (including the earthing conductor) with the main switch on and off, will reveal quite a few answers that this topic has generated....

Regards

BOD
 03 May 2010 08:02 PM
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Pactrol

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Hi

to elaborate a bit on alancapons exspanation the idea is that any thing in cantact with the ground will have a potential this can be anything from zero to full voltage in the event of a lost neutral so by bonding them all together in theory they should all be the same so you shouldn,t get a shock by touching say the tap or whatever though this is not guarenteed & there are concerns where livestock are due to having four points of contact with the ground where theres a fault this has been known to kill cattle in some places such as dairys pme is not permitted.
 03 May 2010 08:04 PM
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adder6

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Hi is this what you'd call a parallel path? Could it be possible that the presence of the main earthing conductor is lowering the Ze due to test current running down the bonding to ground independently?

Would this have consequences for acceptable Zs readings at appliance outlets if they are in part reliant on bonding to gas and water work for a decent reading? (I'm out on limb with this last question something just 'occured' to me but you'll have to fill in my blanks).
 03 May 2010 08:15 PM
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adder6

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Originally posted by: Pactrolthe idea is that any thing in contact with the ground will have a potential this can be anything from zero to full voltage


Hi I think I understand this. I've made a picture - it's bad!I get the impression this is a bit like a potential divider, with me being the upper resistor of fixed value, and the varying quality of contact with the ground acting as a variable resistor. I've drawn the pic so the diamond thing at the top is the tap at 230V to ground.

So going with this potential divider idea, does this mean that the better the contact with the ground due to moisture and whatever else, the lower the value of the variable resistor, and thus the more and more voltage is across me rather than it?

Is this why cattle have a hard time due to having four contact points so their variable resistor is of a lower value due to their being four paths (four parallel resistors if you like) rather than two?

Thanks.

Link.
 03 May 2010 08:19 PM
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Pactrol

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Hi

Im not quite sure I understand your question but if your looptesting with the bonding connected then yes your zs will be lower but if that bonding is a permnent fixture the its not a problem. unless your relying on structual steel work that may be removed. as for Ze if possible isolate the supply & disconnect the bonding thought I appreciate thats not always possible
 03 May 2010 08:21 PM
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adder6

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I've got that in my head now, thanks again for replying
 03 May 2010 08:26 PM
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Pactrol

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hi
yep thats about the size of it
 03 May 2010 08:28 PM
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adder6

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Great thanks, I'm actually beginning to learn properly now cheers
 09 May 2010 06:11 PM
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simongallagher

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To answer part of the original question, PME is considered safer because it is self monitoring. If the Earth/Neutral breaks, the consumer should notice and call the DNO, or an electrician who will advise to call the DNO to fix it.

However, in a SNE system, if the earth connection is lost, you would never know as you still may have your neutral connection. The casing of your toaster becomes live, but there is no earth so it stays live.

PME cables are earthed with rods at both ends (at the sub and Pot end/other sub) to try to reduce the effect of a break in the N/E.
IET » Wiring and the regulations » PME

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