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Caravan earth

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Old Aug 22nd, 2019, 11:27   #1
canis
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Default Caravan earth

Pure sine wave inverter producing mains voltage, mains fusebox with RCD, both earthed together. Connect to caravan chassis or not? I say not, my client doesn't agree and is insisting the chassis be earthed. He's even talking about carrying a metal stake around with him to hammer into the ground at every pitch.

Am I missing something here? Since the power supply is earthed to the mains end nodes, then connection the the physical Earth (the wet rocky orb floating in space) is not needed, right?

As for connecting the aluminium chassis to the inverter ground, won't this have some wierd effect on the vehicle -12vDC?

Can anyone with a few more brain cells than me solve this?

Thanks very much.
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Old Aug 26th, 2019, 13:11   #2
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I say not, my client doesn't agree and is insisting the chassis be earthed. He's even talking about carrying a metal stake around with him to hammer into the ground at every pitch.
Your client does not understand what he's doing... putting in a grounding spike will just make his caravan more likely to be struck by lightning....

I've just had a quick flick through the 18th edition regs and I can't find anything that will be specific to this (that's not to say there isn't something, it does cover caravans when connected to the mains) but I would say with some confidence that I expect the regs to be the same whether the power source is local to the caravan (i.e. inverter) or by hook-up (240v mains). If the latter requires the chassis to be earth bonded (which I expect it does) then you should do so in this case.

By earth bonding I mean connecting the chassis to the earth terminal of the power source (hook-up or inverter) so that any fault that causes a live cable to come into contact with the chassis will (hopefully) cause that circuit's protection to operate and isolate the supply - i.e. RCD on the hook-up or overload/RCD on the inverter.

However - the above would only apply if you are using the inverter to power the "premise" wiring in the caravan (i.e. fitted lights, appliances and sockets via the consumer unit). If you are just plugging appliances directly into the socket on the inverter itself then it's simply not relevant.

That's "as I understand it". Connecting the chassis to the inverter ground will have no effect on the vehicles electrical system unless another connection is made somewhere - but you've not going to be connecting a 240 ac supply to your brake light circuit....?
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Old Aug 26th, 2019, 14:45   #3
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Be careful with RCDs on inverters, some inverters run two phases of 110v making 230 across phases, this works most domestic electrical appliances and lights without problem.
Your RCD may require 230v on the phase to work correctly, only way to be sure is to use an RCD tester when the installation is running on invertor.
Similarly your overload device needs to disconnect at below peak current of the inverter, otherwise a fault may not pull the mcb...
Some use an earth stake with a generator but I don't think there would be any reason to in your case.
It's a can of worms using mains/inverters on mobiles!
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Old Aug 27th, 2019, 14:31   #4
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Be careful with RCDs on inverters, some inverters run two phases of 110v making 230 across phases, this works most domestic electrical appliances and lights without problem.
Your RCD may require 230v on the phase to work correctly, only way to be sure is to use an RCD tester when the installation is running on invertor.
Similarly your overload device needs to disconnect at below peak current of the inverter, otherwise a fault may not pull the mcb...
Some use an earth stake with a generator but I don't think there would be any reason to in your case.
It's a can of worms using mains/inverters on mobiles!
Would agree with most of the post but would make the following points:

- Pure Sine Wave inverters (which the OP is using) are usually of much higher quality than modified sine wave inverters and I've yet to come accross one that works in "anti-phase" as you describe, so they should be okay with RCD's, but as you say best to test.

- Ground Earthing spikes for generators are usually used when domestic wiring is connected to them (i.e. building or portacabin) and is there for equipotential bonding.
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Old Aug 27th, 2019, 17:24   #5
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Would agree with most of the post but would make the following points:

- Pure Sine Wave inverters (which the OP is using) are usually of much higher quality than modified sine wave inverters and I've yet to come accross one that works in "anti-phase" as you describe, so they should be okay with RCD's, but as you say best to test.

- Ground Earthing spikes for generators are usually used when domestic wiring is connected to them (i.e. building or portacabin) and is there for equipotential bonding.
That's interesting Tannaton, every small generator I have had is always 110-110v across phases?
I used to connect generators to installations as you describe, the 17th edition requirements to do this were plain daft requiring disconnection of suppliers earthing arrangements and reconnecting isolated TT earthing via break switching!
I don't get involved now so don't know the 18th edition on generators supplying caravans but an earth stake would not normally be used unless the generator was remote with centre tapped earthing and TT earthed, normally connecting to the generator frame suffices but many don't realise the danger of protective devices not disconnecting due to limited current, a fire risk in a caravan...
I don't touch much domestic kit now but the last two, all be it cheap, invertors I had were also 110-110v between phases, newer stuff may have caught up I suppose?
Be interesting to see if one of the new invertors would run something with a choke or transformer.
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Old Aug 27th, 2019, 18:51   #6
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That's interesting Tannaton, every small generator I have had is always 110-110v across phases?
I used to connect generators to installations as you describe, the 17th edition requirements to do this were plain daft requiring disconnection of suppliers earthing arrangements and reconnecting isolated TT earthing via break switching!
I don't get involved now so don't know the 18th edition on generators supplying caravans but an earth stake would not normally be used unless the generator was remote with centre tapped earthing and TT earthed, normally connecting to the generator frame suffices but many don't realise the danger of protective devices not disconnecting due to limited current, a fire risk in a caravan...
I don't touch much domestic kit now but the last two, all be it cheap, invertors I had were also 110-110v between phases, newer stuff may have caught up I suppose?
Be interesting to see if one of the new invertors would run something with a choke or transformer.
I don't disagree - small portable generators are all invariably two 110v generators that are switched in parallel for 110v operation and then series in anti-phase for 220v operation with neutral (and therefore earth) coming from the centre tap. This sort of power supply is also common on farms and rural buildings where a 2-phase 120v transformer will be fed from 1 phase of the grid to give 240v with neutral and earth from the centre tap (aka split phase) but in this case a TN-C or TN-C-S earth would be provided which could be supplemented with TT earthing.

I think we're saying the same thing aren't we?
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Old Aug 27th, 2019, 23:26   #7
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I don't disagree - small portable generators are all invariably two 110v generators that are switched in parallel for 110v operation and then series in anti-phase for 220v operation with neutral (and therefore earth) coming from the centre tap. This sort of power supply is also common on farms and rural buildings where a 2-phase 120v transformer will be fed from 1 phase of the grid to give 240v with neutral and earth from the centre tap (aka split phase) but in this case a TN-C or TN-C-S earth would be provided which could be supplemented with TT earthing.

I think we're saying the same thing aren't we?
Not really! Most generators I have tinkered with don't have a centre tap, that's the point.
They simply provide a floating supply with no earth reference, that's why I mentioned being careful with an installation that uses an RCD for shock protection, they don't normally work on floating supplies like small generators or invertors.
This catches people out because they test the RCD with a mains hook up and the RCD works, many don't realise it's not going to work on the invertor!
The type of sine wave on the invertor doesn't effect the floating supply output, it's still a floating (or two phases as I describe it because I think that's a clearer way to describe it) supply.
If you want an invertor with RCD protection its going to be easier to buy one with a built in RCD and an earth connection which will be already internally connected between neutral and earth like a centre tapped and earthed generator can be.
It really is a can of worms.
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Old Aug 27th, 2019, 23:58   #8
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Not really! Most generators I have tinkered with don't have a centre tap, that's the point.
They simply provide a floating supply with no earth reference, that's why I mentioned being careful with an installation that uses an RCD for shock protection, they don't normally work on floating supplies like small generators or invertors.
This catches people out because they test the RCD with a mains hook up and the RCD works, many don't realise it's not going to work on the invertor!
The type of sine wave on the invertor doesn't effect the floating supply output, it's still a floating (or two phases as I describe it because I think that's a clearer way to describe it) supply.
If you want an invertor with RCD protection its going to be easier to buy one with a built in RCD and an earth connection which will be already internally connected between neutral and earth like a centre tapped and earthed generator can be.
It really is a can of worms.
I think I get what you're saying...

However I don't see why an RCD shouldn't work on a split phase/2 phase/floating supply as opposed to conventional, fixed mains?

The RCD should trip if the leakage current exceeds 30mA (or it's rating) - i.e. if the difference in current between the live and neutral conductors is > 30mA because some of the current is taking an alternative path.

The problem with temporary installations such as a caravan is creating that alternative path... Faults within appliances with a case earth will trip the RCD in the same way but simply touching a live conductor is unlikely to create sufficient leakage to trip the RCD unless you are touching something else that will create that path. Hence it's better to earth bond the chassis, sink, cooker, water and gas pipes/systems etc.

I have found some guidance on it:

http://www.hse.gov.uk/foi/internalop...99/oc482_2.htm
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Old Aug 28th, 2019, 10:20   #9
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Think of a isolated supply like a shaver socket, that's similar to the generator/invertor output.
If you touch one phase and earth there is no fault path.....
You have to touch both lines/phases/lives at the same time to get a shock.
If you earth the supply neutral to ground, like a pme house supply and provide a earth conductor to the metalwork of the caravan then you have a return path for fault current and the RCD would work.
Look up a earth free (is it It?) environment where your protection against shock is lack of earth.

Belatedly I just skipped through the document you linked to, its excellent and describes better then I can why the RCD won't work.
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