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Topic Title: Generator with floating earth supplying mobile unit?
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Created On: 26 April 2014 08:29 PM
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 26 April 2014 08:29 PM
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SCB197

Posts: 2
Joined: 26 April 2014

Hi there,

A review of section 717 & 551 has left me a little confused with the requirements of earthing arrangements for a mobile unit supplied a generator with a floating earth?

Generator:
Single phase, 9KVA, with distribution board containing fault protection, I'm unsure if additional protection/RCD has been provided. I have confirmed with the manufacturer that the unit "as standard" is supplied with a floating earth.

Mobile unit:
Horse box/catering van type vehicle, with single phase installation, containing the following circuits, lighting, ring socket outlet & external distribution socket outlet for tools/lighting. The installation is provided with fault protection/MCB, additional protection/RCD & an earth spike. The earthing spike is not commonly used as the generator & mobile unit are parked on hard standing ground, (car park or similar).

My question is, given the earthing arrangement at the generator:

I would presume that the generator earth should be connected to neutral, to provide an earth fault current path, in order that disconnection times are met?

Secondly, should the generator have an RCD, i would presume it should be provided with a functional earth? If this cannot be provided by an earth spike, then perhaps a handrail or fence in a car park? In addition, i would presume the same would apply to the RCD in the mobile units consumer unit?

Any help with this would be greatly appreciated.
 27 April 2014 12:02 AM
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Fm

Posts: 668
Joined: 24 August 2011

A large copper plate with a trailing conductor
Park over the plate with the tyre on top
 27 April 2014 09:23 AM
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John Peckham

Posts: 7459
Joined: 23 April 2005

FM

What would be the function of this large copper plate?

SCB197

Welcome to the forum.

The generator probably will not be cable of producing enough earth fault current to operate the circuit breakers for fault protection. In addition with a separated winding there is no earth fault path. Hopefully your RCD is rated at 30mA? A 30mA RCD is needed for additional protection and can also give you fault protection. I see a lot of these arrangements on catering vehicles and trailers. Despite previous fabricated test certificates providing satisfactory test results for Zs and RCD times the first time I see them I have to put in a neutral earth link. If the installation is going to be used on a generator only I put a link in between the earth bar and the neutral on the incoming side of the front end RCD. If the trailer or vehicle is used also on a mains supply I modify a lead for use with the generator by putting a link inside the supply plug and then a separate lead for use when plugging in to the mains.

BS7430 says to use a separate earth electrode with generators over 3kA but I have no worked out why this is suggested as the electrode is not in the earth fault path and there is no need for synchronisng purposes.

An inspection and test is required before putting the installation in to service as often the installation has been carried out by non-competent persons.


Hope this helps?

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

http://www.astutetechnicalservices.co.uk/
 27 April 2014 10:49 AM
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GeoffBlackwell

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Take a look at this from the HSE.

The copper plate in not relevant as when you return to the vehicle it will be on bricks (if your lucky), the tyre will be slashed and the copper plate will be at melting temperature in some furnace far far away .

Regards

Geoff Blackwell
 27 April 2014 12:36 PM
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ebee

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Well that copper plate is useful then!

It`s a peace offering to your local "second-hand metal trading executives who travel around a lot".



-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 27 April 2014 02:56 PM
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Fm

Posts: 668
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Rcd reference point
 27 April 2014 03:06 PM
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Fm

Posts: 668
Joined: 24 August 2011

If its also feeding equipment outside the mobile unit, I would say an earth connection is required
 27 April 2014 04:08 PM
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John Peckham

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FM

How does the earth electrode provide a reference point for an RCD? Also how will an earth electrode make any useful difference to equipment outside the mobile unit?

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

http://www.astutetechnicalservices.co.uk/
 27 April 2014 07:06 PM
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Fm

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 27 April 2014 07:11 PM
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Fm

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 27 April 2014 08:04 PM
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davezawadi

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It appears that the ECA are showing the usual level of competence.
A floating system cannot operate an RCD in case of an earth fault, unless there is already one fault present!
So the advice offered is not satisfactory.
RCDs will only operate to disconnect supplies if they are earth referenced (eg there is a neutral - earth link in place).
It is true that the first fault on a floating system may not be dangerous, but this depends on too many other factors to be considered safe.
Imagine you have two appliances plugged in to the generator, each class two but each with an earth fault one to "live" and one to "neutral". Between the casing of each will be the full supply voltage, and no RCD will trip if you touch both, but the shock will be severe! If the supply is earth referenced, touching the live one and pretty much anything else will trip the RCD, and the neutral one is not very dangerous and if you get 30mA or so will also trip the RCD. Isolated generator supplies should only feed one appliance from a safety point of view.
You may think that class one appliances will be safe because of the earth connection, and they will as long as there is only a single fault present, and there is negligible leakage current to real earth. But f it rains there may be as problem!

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


Edited: 27 April 2014 at 08:16 PM by davezawadi
 27 April 2014 10:13 PM
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SCB197

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Joined: 26 April 2014

Thank you for the comments, appreciated.

Having inspected & competed a couple of tests on the generator in question this afternoon, (Pramac), I've gleaned the following additional information:

John was correct, the main switch on the generators distribution board was a 30mA RCD, providing fault protection. Each socket outlet was provided with B type breakers, again most likely for overload protection.

1. resistance between conductors, (measured at socket outlet):
a. PE - N = 0.6 ohms.
b. PE - L = 0.6 ohms.
c. L - N = 0.7 ohms.

Sot the supply is center tapped.

2. P.D. between conductors, (measured at socket outlet with Fluke multimeter and Fluke T110 AVI).
a. PE - N = 115 vac.
b. PE - L = 115 vac.
c. L - N = 230 vac.

Firstly, it would appear the information provided wasn't accurate as the unit doesn't provide a floating earth.

Secondly, with the generator in this configuration, it would appear that it is producing a two phase supply, (115-0-115 or L - N)?

In order, that the correct of fault protection can be achieved, can anyone offer any additional advice or perhaps point me int he direction of the most suitable guidance note, (GN 5, 8 or both)?

Thanks.
 30 April 2014 12:55 AM
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mapj1

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Well, if really then you have a 120 -0-120 centre ground system, note that of course, any 13A fusing will not help with any fault not in the live side, so check RCD is double pole, and either DP fusing or a double pole breaking MCB or two is needed - ouch! At least the RCD does something for you on first fault.
Given the lower touch voltage, increased disconnection time would be acceptable from a safety of life perspective, but probably not desirable or necessary.
Some first principles sums and further head scratching will be required as its not the normal case and the standard tables values won't apply.
really its the RCD that provides most protection, single pole breakers or fuses may provide overload protection if too many otherwise satisfactory kettles are plugged in, but not any kind of reliable fault disconnection.
Had it been a floating windings genset then I would have recommended fitting an NE bond behind the output socket or as second choice in the first plug and labelling it as such, however don't do that here!
Double check the voltages with it on rather than just winding resistances before getting in too deep.
Let us know how you get on.
regards M.
edited for clarity in the monring.

-------------------------
regards Mike

Edited: 30 April 2014 at 11:20 AM by mapj1
 30 April 2014 09:21 AM
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AJJewsbury

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a. PE - N = 0.6 ohms.
b. PE - L = 0.6 ohms.
c. L - N = 0.7 ohms.

If this is just a simple generator with direct connections to copper windings (i.e. no electronics/inverter providing regulation), I'm a bit curious as to why the PE-L/N values are so far adrift from half of the L-N one. If the whole winding (including connections) was 0.7 Ohms, then if it were centre tapped I'd expect PE-L (or N) to be around 0.35 Ohms - even allowing couple of 0.05 Ohms for joints 0.6 feels a bit high. What sort of meter was the check done with?
- Andy.
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