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Topic Title: Caravan park TT sanity check
Topic Summary: Low Zs readings
Created On: 25 July 2013 11:02 AM
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 25 July 2013 11:02 AM
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deejackson

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So I've been asked to take on the inspection of a local touring site. to get some idea of the site layout I have a look at past reports (very professionally produced by the national firm that installed it originally).... And each pitch Zs is between 0.8 and 1.2 ohms! That sounds like the TT earth has been connected to the incomer TNCS. There is a set of poorly explained figures on a separate sheet that lists TNCS 0.18 ohm, TT(disconnected) 32.9 ohm and TT(connected) 0.49 ohm - so I can probably assume that 'connected' either means 'linked to the TNCS' or it means 'with equipotential bond connected' and a link is inadvertently formed via some metalwork or service bonded to both earthing systems. Hopefully all will become clear with a site visit - but then I need to persuade the site owner that his current setup may be in error. How can a professional electrician see a value of Zs around 1 ohm on a TT system and not flag it up as at least strange and worthy of further investigation? Or am I the crazy one?

Dee
 25 July 2013 12:04 PM
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John Peckham

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Dee

The Ra/Ze may be tens of ohms on a TT installation but the Zs values may be a lot lower with bonding to services providing a parallel path to earth.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 25 July 2013 12:20 PM
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AJJewsbury

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Zs around 1 ohm on a TT system

Might not be implausible - depends how it's done. If they've TT'd the distribution system and put in a rod at every pitch you could end up with dozens (if not hundreds if it's a big site) of rods in parallel - if each one had a decent resistance the overall Ra could be quite low. Likewise the DNO's network may well have multiple connections to true earth (not just the rod at the transformer), so a low overall Zs might be possible (if not highly likely).
- Andy.
 25 July 2013 12:23 PM
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deejackson

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John...So is it acceptable that the metalwork of a caravan becomes connected to a tncs earth albeit that that connection is via bonding /services rather than a piece of wire? Electrically this doesn't seem different from making each pitch tncs, but with the addition of a rod or two? Under what circumstances should the TT earth be bonded when it is serving hookup points not within an equipotential area? I must admit that I'm more familiar with the format used in other sites I service where distribution cables are tncs and each pitch hookup has it own individual rod/TTsystem - isolated from the rest of the site.

So, yes I understand your point, but from a safety viewpoint, is it acceptable?

Dee


Andy... could be possible with many parallel rods - but seems unlikely given that this is a small site and local soil conditions don't give good earth results.

Dee
 25 July 2013 12:36 PM
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John Peckham

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Dee

The bonding to the service pipes should be to the TT earth with no connection to the supply neutral if the whole installation, internal and external, is TT.

If the caravan supply comes from a building where the installation in the building is TN-C-S it is usual to completely separate the external supply earth from the building earth. Tails in to a plastic enclosure containing overload and fault current protection in the box with a connection to an earth rod for the earth. SWAs running away connected to the TT earth. Ends of the SWA runs often have an additional belt and braces earth electrode. If there are feed pillars around the site these may have additional electrodes.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 25 July 2013 12:52 PM
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deejackson

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Originally posted by: John Peckham

Dee



The Ra/Ze may be tens of ohms on a TT installation but the Zs values may be a lot lower with bonding to services providing a parallel path to earth.


Ah, I see what you're saying. The service (eg pipe) providing the low earth impedance - from miles of buried metalwork - not by cross connection to tnc-s earth.

Dee
 25 July 2013 12:54 PM
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deejackson

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Originally posted by: John Peckham

snip
The bonding to the service pipes should be to the TT earth with no connection to the supply neutral if the whole installation, internal and external, is TT.



snip


Local practice however is buildings TNC-S, pitch hook-ups TT

Dee
 25 July 2013 01:35 PM
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John Peckham

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Yes I have I&T a site in the past where all the building on the site, offices, shop, cafe hall etc are TN-C-S but the pitch supplies are derived from the building supplies but have a separate TT earthing system. You might want to have a good look at your installation to see why the Zs values are lower than the Ra/Ze.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 29 July 2013 03:16 PM
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deejackson

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Mystery Solved. One of the static residential units on site has off peak electric heating from a TNC-S feed, but also has it's main electrical supply from the site-wide TT system. Both feeds originate from the site electrical distribution cabin - so two separate supplies into the static unit - and, of course, the earths are linked in the residential unit. Thus connecting all the TT hook-up points to the TN earth. Only been like it a couple of decades......Best answer - convert the off-peak to TT and fit an RCD consumer unit for it. Keep everything TT only.

Dee
 30 July 2013 09:21 AM
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davezawadi

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But it doesn't matter as such. Static units are not classed as caravans, so it doesn't need to be TT unless you want it to be. There are good reasons to make it TN, such as the larger loads likely to be encountered, and the convenience of having the RCD inside rather than outside for vandals to switch off! I assume that the power cables that supply the unit are buried and not a bit of blue dragging on the ground. RCD protection for all circuits is wise as BS7671 latest.

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David
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 30 July 2013 03:42 PM
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AJJewsbury

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But it doesn't matter as such. Static units are not classed as caravans, so it doesn't need to be TT unless you want it to be. There are good reasons to make it TN, such as the larger loads likely to be encountered, and the convenience of having the RCD inside rather than outside for vandals to switch off! I assume that the power cables that supply the unit are buried and not a bit of blue dragging on the ground.

It's not desirable for a PME supply to be interconnected with a TT earthing system shared with tourers though.
- Andy.
 30 July 2013 05:16 PM
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perspicacious

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"Static units are not classed as caravans"

Where is a static unit defined in BS 7671?

Regards

BOD
 30 July 2013 06:12 PM
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John Peckham

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BOD

Static caravans are not defined in BS7671 but mobile homes and residential park homes are mentioned in section are mentioned in Section 721 in a footnote saying the general requirements apply. However caravans are defined in Part 2 as "a trailer leisure accommodating vehicle, used for touring, designed to meet the requirements for the construction and use of road vehicles". So a static caravan being one not suitable for use on the road e.g not fitted with obligatory lights etc as required by the Construction and Use Regulations,would appear to be excluded from section 721.

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John Peckham

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 30 July 2013 07:12 PM
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perspicacious

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"Static caravans are not defined in BS7671 but mobile homes and residential park homes are"

Ah, so anyone using the term "static caravans" is using slang?

Regards

BOD
 30 July 2013 09:39 PM
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leckie

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 31 July 2013 06:26 AM
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mossep

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Originally posted by: davezawadi

But it doesn't matter as such. Static units are not classed as caravans, so it doesn't need to be TT unless you want it to be. There are good reasons to make it TN, such as the larger loads likely to be encountered, and the convenience of having the RCD inside rather than outside for vandals to switch off! I assume that the power cables that supply the unit are buried and not a bit of blue dragging on the ground. RCD protection for all circuits is wise as BS7671 latest.


I queried this with a guy from the ECA technical help line, and he said if it has got wheels then class it as a caravan! Every 'static' van I see has got wheels on it so I class them accordingly.

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 31 July 2013 09:21 AM
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AJJewsbury

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If a static caravan doesn't fall within sections 721 (caravans) then it presumably does fall in within 717 (mobile & transportable units, as I think we can all agree static caravans are transportable) - which also has a downer on PME.
- Andy.
 31 July 2013 10:01 AM
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weirdbeard

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Originally posted by: AJJewsbury

If a static caravan doesn't fall within sections 721 (caravans) then it presumably does fall in within 717 (mobile & transportable units, as I think we can all agree static caravans are transportable) - which also has a downer on PME.



Hi Andy, just a thought but the scope of 717 says "mobile or transportable structure......which is provided with a temporary supply"
 31 July 2013 10:02 AM
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weirdbeard

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Originally posted by: mossep


I queried this with a guy from the ECA technical help line, and he said if it has got wheels then class it as a caravan! Every 'static' van I see has got wheels on it so I class them accordingly.




 31 July 2013 10:06 AM
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AJJewsbury

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Hi Andy, just a thought but the scope of 717 says "mobile or transportable structure......which is provided with a temporary supply"

Good point !
- Andy.
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