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Topic Title: Inspection and testing a butty wagon that will run off genny
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Created On: 27 July 2013 07:57 PM
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 27 July 2013 07:57 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Evening,
I have been asked to I and T a mobile butty van. I am ok with domestic but is there anything different as it will be run off a petrol genny.
I presume most of the routine will be the same but what about the Zs as I am not familiar with this situation.
I have told the lad to ask a caravan firm if there electricians can sign it of but it has got me wondering.
Regards
Antric
 27 July 2013 08:29 PM
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aligarjon

Posts: 2802
Joined: 09 September 2005

Thats a good question. How is the RCD going to work correctly without a reliable earth.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 27 July 2013 08:38 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Originally posted by: aligarjon

Thats a good question. How is the RCD going to work correctly without a reliable earth.



Gary


Evenibg Gary,
Exactly. I believe some gennies are earthed if connected to a mains supply but petrol must need roding or do they so how specialised is the testing on these vans.
Regards
Antric
 27 July 2013 10:09 PM
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antric2

Posts: 1049
Joined: 20 October 2006

Originally posted by: jcm256

See section 7 (Read all)



Page 19





Pay attention to:

7.1.4



You have two diagrams





Link removed by moderator





Try getting free download of BS7430 frrom below if the previous will not open



Link removed by moderator

Thanks.

Edited: 29 July 2013 at 04:44 PM by IET Moderator
 28 July 2013 09:56 AM
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leckie

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Joined: 21 November 2008

I clicked on the link for the download site but although it said it was free it asks for credit card details. I didn't continue because I'm afraid I didn't fancy giving those details.
 29 July 2013 11:06 AM
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AJJewsbury

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Thats a good question. How is the RCD going to work correctly without a reliable earth.

As long as there's a N-c.p.c. link at the generator then the RCD will work as normal for faults to c.p.c. or exposed-conductive parts - it's more like a TN-S rather than TT system so there's no reliance on a connection to true earth for ADS. Think of it as TN-S on a very small planet (the van + generator).
- Andy.
 29 July 2013 03:59 PM
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jcm256

Posts: 1840
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That a good answer there, you are describing what some council's state that they realise primary RCD protection may not be possible then you should use secondary RCD protection.
If you did try to provide primary protection and the catering, van was on or moved to a tarmac cark park and nowhere to shove the earth spike I know what the van owner would say.

(This statement taken from the web site below might be out of date (or is it)



Generators
Mobile catering units frequently use single-phase generators, having an output not exceeding 10KVA, to supply power to various electrical appliances. British Standard 7430 states it is often better to use these generators in an unearthed system i.e. an earth rod does not have to be connected to the generator for it to operate safely. The installation of an RCD is recommended. Advice should be sought from a competent person regarding its installation.

http://www1.arun.gov.uk/assets...lth_Safety_Booklet.pdf
 29 July 2013 04:50 PM
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rossall

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May I remind participants in this thread of the IET's position on posting links to copyright material, which certainly includes British Standards and related material?

I have removed some links in messages in this discussion.

Regards

-------------------------
David Rossall
The Institution of Engineering and Technology
 29 July 2013 05:29 PM
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AJJewsbury

Posts: 11268
Joined: 13 August 2003

Generators
Mobile catering units frequently use single-phase generators, having an output not exceeding 10KVA, to supply power to various electrical appliances. British Standard 7430 states it is often better to use these generators in an unearthed system i.e. an earth rod does not have to be connected to the generator for it to operate safely.

I think there are two possibilities here - one with a N-PE link and one without - both could be regarded as "unearthed" in that neither are connected to true earth. One BS 7671 treats as a TN-S system and so demands ADS (section 411), the other (I think) would be regarded as a separated system supplying more then one item of current-using equipment (section 418.3). I suspect that the BS 7430 reference (especially with the comment about <=10kVA generators) is referring to the separated system (no N-PE link), as most small generators are supplied configured that way.

Does that mean that there's a conflict between BS 7671 and other standards, as BS 7671 reserves 418.3 for installations that are controlled or under the supervision of skilled or instructed persons - which I doubt a butty wagon would qualify for.

- Andy.
 29 July 2013 06:15 PM
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jcm256

Posts: 1840
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I would have thought IET Guidance note 8 page 145 would had more to say on single phase generators earthing, no they refer you to in accordance with section 551of BS7671, Figures 7717.1 and 717.2 (which are three-phase). Anyhow, thanks for the replies to anything I said on this simple but do what's right subject.
Regards
jcm
 30 July 2013 07:34 AM
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davezawadi

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I think it would be useful to review the shock paths available in this van type installation.

The primary use of the RCD protection is secondary protection for someone inside the van, touching an appliance with an earth fault, and a faulty cpc at the same time as another earthed item inside the van. This would trip an RCD in a standard TN-S connection of the generator (earth - neutral link fitted) whether there was a good connection to real earth or not, as in a car park.

Should the whole van earth system not have a good earth stake, there is a very small risk to people outside the van even if the earth bonding fails and a fault raises the external metalwork to a high potential compared with the neutral, as all of the system is essentially isolated from true earth. If a significant current can flow through an external person then the RCD will trip in the normal manner, so secondary protection is again provided.

Running as an essentially IT system (no N-E bond) means that dual faults can produce dangerous potentials between items in the van, and RCDs will not work, so the risk would be somewhat higher than TN-S connection, but still small as multiple faults are rare under a well inspected and controlled safety regimen.

Serious accidents with these setups are rare, and are probably due to poor installation, or deterioration, rather than an overall lack of protection or inadequate system design. It is very unlikely that such installations suffer risk from overload electrical fires as the generator cannot provide much more than is rated output power, unlike grid connected systems.

Regards

-------------------------
David
CEng etc, don't ask, its a result not a question!
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