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Topic Title: Supplies for Mobile Applications
Topic Summary: Regulation concerning external supply for mobile application
Created On: 03 October 2012 11:01 AM
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 03 October 2012 11:01 AM
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paulrhwilliams

Posts: 8
Joined: 21 September 2008

I have a mobile laboratory, based on a ISO 20ft steel container, which has been kitted out with the following circuits:

A132A inlet has been specified for the electrical inlet to allow the following configurations to be operated;
Two 32A ring circuits each supplying x6 13A socket outlets.
One 32A isolator provision for warming oven.
Two 6A lighting circuit.
One 6A fire alarm circuit.

Obviously, there is some diversity to be applied, depending on the appliances used on the sockets. These are planned to be mostly computer equipment or chargers. The oven will not be used at high power either.

I would like some guidance as to the Regulation for configuring the hook-up, especially whether an existing 32A board with an external connector (like on a campsite) can be used.

I am being told that a separate Earth spike is required, but this doesn't seem right.

Thanks
 03 October 2012 12:06 PM
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AJJewsbury

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Sounds like you're treating it rather like a touring caravan - which sounds about right to me. If you're main supply is PME, then it's legally prohibited to use that for a caravan earth, so a spike does sound reasonable. Section 708 of the regulations is probably worth a read (although one or two bits might not apply as you're not really on a campsite).
- Andy.
 03 October 2012 12:54 PM
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paulrhwilliams

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Andy,

Thanks for this.

If we incorporate a supplementary Earth spike (I assume it is supplementary, as the supply will probably still contain an Earth), can it be connected to a standard PME supply?

I don't have my 7671 to hand at the moment, so I am a bit blind.

Thanks again
 03 October 2012 02:03 PM
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spinlondon

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No, you would have to arrange it so that the unit is TT, and divorced from any PME earth.
It would not be a supplementary earth, it would be the main and only earth.
Best bet would be to disconnect at the unit end, the earth in the lead you use to connect the unit to a mains supply.
 03 October 2012 03:03 PM
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paulrhwilliams

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OK, thanks for this.
 03 October 2012 03:12 PM
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AJJewsbury

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And don't forget 30mA RCD protection for the 32A socket. (I presume it's 32A not 132A).
- Andy.
 03 October 2012 03:32 PM
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paulrhwilliams

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Do these restrictions only apply if its out in the open? If its housed in a shed or similar, do different Regs apply?
 03 October 2012 04:09 PM
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AJJewsbury

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Do these restrictions only apply if its out in the open? If its housed in a shed or similar, do different Regs apply?

The socket or the "caravan"?

- Andy.
 03 October 2012 04:30 PM
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paulrhwilliams

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the container
 03 October 2012 04:39 PM
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AJJewsbury

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As far as I know the regs don't explicity address having 'mobile units' indoors - so I presume it'd be down to engineering judgement. If it was within an area where bonding could reasonably establish an equipotential zone (e.g. floor has a dpm below it), I don't imagine that the restriction on PME would need to apply.

Any circuit <= 32A that could be expected to supply mobile equipment outdoors would still need 30mA RCD protection though.

- Andy.
 03 October 2012 05:19 PM
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spinlondon

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Tsk tsk Andy.
 03 October 2012 05:32 PM
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AJJewsbury

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Tsk tsk Andy.

32A cct rather than 20A skt? I did try to choose my words carefully..

- Andy.
 03 October 2012 05:38 PM
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spinlondon

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No requirement for the circuit to be protected, and no mention of the word 'expected', which was removed for the 17th.
Then of course, is a transportable unit considered as mobile equipment (moveable while in operation, or easily moveable between places whilst still connected to the supply)?

Edited: 03 October 2012 at 05:48 PM by spinlondon
 03 October 2012 06:04 PM
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AJJewsbury

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No requirement for the circuit to be protected, and no mention of the word 'expected', which was removed for the 17th.
Then of course, is a transportable unit considered as mobile equipment (moveable while in operation, or easily moveable between places whilst still connected to the supply)?

It was intended as a more general comment - a place where they're in the habit of moving electrical installations in 20' containers around and have 32A sockets could well be the sort of place where it might be anticipated that someone might take it upon themselves to run a lead outside - not necessarily to a container, but to anything. You never know what these boffins might get up to .

I'd suggest that if the designer considers that a possibility, they'd need to make provision under 411.3.3 (ii). Yes, 'expected' was just my short way of saying that.

Point of the point was to say don't ignore RCD protection just because the socket is rated >20A.

- Andy.
 03 October 2012 06:05 PM
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OMS

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There is no reason why you can't connect it straight to any system without an electrode (for a mobile lab an electrode will be a bloody nuisance).

If you do happen to have a PME supply point then a competent person needs to have supervision of it and a suitably qualified person needs to have deemed it suitable and effective as a means of earthing.

Personally speaking, I would have addressed all of the potential supply configurations for your lab by means of a simple isolation transfomer at the cabin intake along with a few RCD's including a 30mA unit for the sockets just in case someone has a desire to take a wandering lead outside.

Essentially figure 717.6, page 251 of the big green bumper fun book - wave it under the noses of the ISO pod designers and tell them to get a grip

regards

OMS

-------------------------
Failure is always an option
 03 October 2012 06:53 PM
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paulrhwilliams

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OMS, this is interesting. I'll certainly take a look in the latest (2010?) issue.

Can you give me a bit more detail on your proposal?

Thanks
 03 October 2012 07:56 PM
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OMS

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Originally posted by: paulrhwilliams

OMS, this is interesting. I'll certainly take a look in the latest (2010?) issue.

Can you give me a bit more detail on your proposal?

Thanks


my pleasure - it'll be 2011, or the original 17th of 2008 - Section 717. I think it's also covered in Guidance Note 7 - might be an easier read

Get a suitably rated isolation transformer fitted in the ISO 230V:230V - from there the DB in the iso should have one or more RCD's (ideally 30mA for the socket outlets). You won't need an electrode

At the supply end you can just get your clients to provide a 32A or 63A socket and you provide the lead to your ISO inlet coupler. The clients supply ideally should also have an RCD to protect it (usually an RCD socket as you'll have access to that for plugging in and you won't need access to thier building for resets etc.) external cabinets, lockable with flex channels to allow you to lock the door when the flex is plugged in are the norm

i don't know what your set up is, but presumably you don't just rock up in the lab unannounced as see if you can cadge a power supply.

I did a number like this in a previous life for mobile dental surgeries as one project, mobile air quality moinitoring units as another and , whilst slightly different in scope - mobile diagnostic imaging suites (MRI and CT) as well as eye surgical units - the secret is to make your kit compatible with the variable nature of the client supplies - a simple one to one isolation TX does that - if what you do is more critical then gensets and full IT earthing systems with insulation monitoring for first fault is the next level up

regards

OMS

-------------------------
Failure is always an option
 04 October 2012 11:04 AM
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paulrhwilliams

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This sounds a much better plan than the additional Earth spike. Any idea where I can get these transformers from?

Thing is that the lab supplier was supposed to make the unit compatible with a mobile environment, but have clearly missed the point a little when considering remote, global locations. I would like to advise them on the work required to meet the spec.
 04 October 2012 12:42 PM
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OMS

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OK - there are two general options

1 - you make the power supplis at each know location compatible with the unit (ie usualy TT supplies with a time delayed RCD and appliance coupler or socket outlet)

2 - you make the ISO unit compatible with any supply arrangement (including a generator) by also including a simple isolation transformer (or a more complex arrangement with insulation monitoring)

Simple seperation isolation transformers should meet BS EN 61558 - i think the last ones i had were via Claude Lyons - but we also had voltage stabilisation units as well so it was easier to deal with one supplier

I guess your lab supplier need to do a bit more reasearch to meet your brief

Regards

OMS

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Failure is always an option
 04 October 2012 12:53 PM
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paulrhwilliams

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This is really helpful ... many thanks
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