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Earthing Arrangment


shawn-b

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I've been asked to install some american swimming pool equipment which requires a transformer to convert from american to USA voltages. The question is do i get the transformer with a center earth tapping and have a complete separate earth for the pool (pool tank is metal) from the center tapping or install a TT system. 

 

interested in your views

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Yes sorry uk to usa. It's a tncs incoming but regs say that in case of a failure on the incoming supply of the earth or slight volt drop in the earth cabling could result in voltages into the system. Hence why they say a earth rod.

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The Electricity Safety, Quality and Continuity Regulations 2002 (as amended) permit the Distribution Network Operator (DNO) to combine neutral and protective functions in a single conductor provided that, in addition to the neutral to earth connection at the supply transformer, there are one or more other connections with Earth. The supply neutral may then be used to connect circuit protective conductors of the customer’s installation with earth if the customer’s installation meets the requirements of BS 7671.

Protective Multiple Earthing (PME) has been almost universally adopted by DNOs in the UK as an effective and reliable method of providing their customers with an earth connection. This supply system arrangement is described in BS 7671 as TN-C-S.

Whilst a PME terminal provides an effective and reliable facility for the majority of installations, under certain supply system fault conditions (external to the installation) a potential can develop between the conductive parts connected to the PME earth terminal and the general conductive mass of Earth. The potential difference between true Earth and the PME earth terminal is of importance when:

body contact resistance is low (little clothing, damp/wet conditions); and/or

there is relatively good contact with true Earth.

The local DNO may therefore decide not to provide a PME earthing terminal for an installation such as that of a swimming pool, etc.

As far as BS 7671 is concerned, it does not preclude the use for an installation that includes a swimming pool but recommends that an earth mat or earth electrode of suitably low resistance, for example, 20 ohms or less, be installed and connected to the equipotential bonding.

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Hello shawn-b,

So, how does this (post #5) translate to BS7671?

BTW, I know.

I am asking so that you can be helped to develop your understanding, as you are asking the question.

Do you, fully, understand the text that you have quoted in post #5?

 

What are the issues that you have, because from where I am your posts are very confusing.

 

If you don't use the exact, correct terms, then we can't decipher what you are asking, because all we have are what you have written in your posts, thus ambiguous terms, descriptions or explanations only serve to confuse things further.

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Would you connect the metal pool shell and pump equipment to the main earth. Some people are saying it needs to be earthed to the main system. Some to an earth rod on its own earth and some say an isolating transformer. I can understand that if you was to loose the main earth, any fault current could then travel to the pool. What way would people wire it to prevent this from happening.

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shawn,

Your terms are still a little confusing.

 

We need to establish some facts slowly here, because none of us have seen, nor are party to the technical details of the installation.

Is the incoming supply TN-C-S, if so, is it PME or not?

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Have you a copy of BS7671 to hand?

It has a whole section about 'special locations' in which a swimming pool falls under.

Yes we can tell you what you are asking but it would be good practice for you to tell us what you think needs to be done after looking at BS7671 section 7.

Trust your instincts. You are not a million miles off.

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I don't have a copy to hand. The incoming is a tncs. If a fault was to occur on the neutral earth connection at the cutout then the circuit would make the pool shell come live. By earthing the pool directly to an earth rod this would eliminate this problem. But could you also use the center tapping of the transformer that's needed to also create an earthing point. Or am I still missing something.

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Ok. A TN-C-S does not necessarily mean it is a PME. Only PME supplies have this restriction.

Let's guess it is a PME.

If I were you I would run a new sub-main to the pool area. Get your electricity supply sorted to the area then worry about the transformer.

You really need to have a copy of BS7671 before starting these threads because to show you is one thing. For you to understand it will make you a better sparks.

So you have a PME. You need to get a sub main outbto the rear garden. What are you gonna do?

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Is the pool even in the rear garden?

Is the pool within the main equipotential zone?

Too many unknowns here.

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The pool is to be situated in doors. The supply is a pme. The transformer will be fed via a 63amp 3 phase supply. The transformer will then drop this to 220 volt 3 phase for the American control panel. The pool construction is steel with large motors and equipment bolted directly to the steel shell

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I read it, that it says that it needs an earth rod or mat. The next question which the regs doesn't mention is because I have to use a transformer could I use use the centre tapping of the transformer for an earth instead of an earth rod or mat.

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It also says only when there is a relatively good contact with true earth? How is this possible indoors with your installation?

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Sorry I know your having to guess the installation. The pool is situated by an outside wall for future access if things need removing. The earth rod could be located just outside. What are your thoughts on using a centre tapping for earth.

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 What are your thoughts on using a centre tapping for earth.

Since we are talking about 3 phase now, I find it very hard indeed to see how you can have anything other than neutral  from your transformer secondary connected to earth

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Since we are talking about 3 phase now, I find it very hard indeed to see how you can have anything other than neutral  from your transformer secondary connected to earth

 

edit: ignore.. didnt read earlier post properly...

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see post # 14

i know... missed it before i posted

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As I see it. There is no requirement for a TT earthing arrangement as the pool is within the equipotential zone.

Why waste a perfectly good earth?

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As I see it. There is no requirement for a TT earthing arrangement as the pool is within the equipotential zone.

Why waste a perfectly good earth?

 

Not quite strictly true.

 

This is quite a complex design, and there are a LOT of FMEA's needed here that I don't think can be covered on an internet forum without a better description and understanding of the installation and it's requirements.

 

Let's take a step backwards.

You are looking to use USA sourced equipment.

Is this equipment CE marked?

If not it's illegal for sale in the EU, and YOU personally will have to certify that you GUARANTEE that it meets the requirements of the LVD, or MD as required for placing on the market in the EU.

 

This is NOT as easy as it first appears, IF you want to comply with the Statute Law of the Land, and BS7671 as a secondary compliance.

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It is all ce marked equipment

It is all ce marked equipment

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