Why a separate earth for caravans

[Calebp43]

I am an apprentice and was just wondering the reason why you have to put a separate earth into a caravan? My friend has asked me to do a hobble and bring a supply to his caravan from his board. I do need to earth the caravan separately right?

 

[timhoward]

Many people plug in their caravans without thought for this, but if you are providing a supply specifically for a caravan then yes, it shouldn't be connected to a PME earthing setup.

Regarding why:
On a TNCS earthed supply there can be a difference in potential between the combined installation Neutral and Earth and real earth due the return neutral current.
Inside a house this is relatively safe.
Outside there are two particular scenarios to consider:
1 - standing on real earth and touching anything earthed can give a tingle. Not dangerous, but un-nerving!
2 - very rare scenario, but if the supply Neutral breaks, then worst case the turn Neutral current goes to the cutout, is linked to earth, and is then looking for a return path through everything that is earthed. The degree to which it will find one depends on each unique situation, e.g. whether there's a metal bonded water pipe. But standing outside on real earth and touching anything earthed is worst-case putting 230v between your hand and your feet with you in the middle as you could be the return path.

So on caravan sites, and supplies to caravans, the installation earth is not connected to the caravan earth, to avoid these scenario's happening.

 

[Calebp43]

Many people plug in their caravans without thought for this, but if you are providing a supply specifically for a caravan then yes, it shouldn't be connected to a PME earthing setup.

Regarding why:
On a TNCS earthed supply there can be a difference in potential between the combined installation Neutral and Earth and real earth due the return neutral current.
Inside a house this is relatively safe.
Outside there are two particular scenarios to consider:
1 - standing on real earth and touching anything earthed can give a tingle. Not dangerous, but un-nerving!
2 - very rare scenario, but if the supply Neutral breaks, then worst case the turn Neutral current goes to the cutout, is linked to earth, and is then looking for a return path through everything that is earthed. The degree to which it will find one depends on each unique situation, e.g. whether there's a metal bonded water pipe. But standing outside on real earth and touching anything earthed is worst-case putting 230v between your hand and your feet with you in the middle as you could be the return path.

So on caravan sites, and supplies to caravans, the installation earth is not connected to the caravan earth, to avoid these scenario's happening.

So if they have TNS or TT then I can use the installation earth?
Also for the cable I'm probably gonna use 3 core 10mm SWA. But what if I used 2 core and used the armouring as the main earth? Crazy talk?

 

[littlespark]

How’s the wording of the regs on this?

I’m sure it says a rod for a caravan park, not for just a caravan plugged into a house…

 

[loz2754]

It would be helpful to establish whether we're talking about a caravan or static mobile home, as the requirements are different for each.
If it's a caravan it's section 721.
In section 721 there's no specific reference to type of supply or earthing arrangements.
There are however, 5 pages detailing all of the requirements for caravans. For example the requirements for main bonding, RCDs, and each final circuit needing double pole overcurrent protective devices.

To the OP - are you certain that you have the requisite skills and knowledge to undertake this task in a special location?
If you're in any doubt, please ask someone to mentor you personally on this project.

 

[timhoward]

I don’t have regs here but I thought section 708 scope was worded vaguely and one interpretation was it could apply to any situation where supply has provided for a caravan even if it wasn’t on a caravan park. Very happy to be proved wrong though!

 

[loz2754]

I don’t have regs here but I thought section 708 scope was worded vaguely and one interpretation was it could apply to any situation where supply has provided for a caravan even if it wasn’t on a caravan park. Very happy to be proved wrong though!

I'm not sure I would agree with this interpretation, at it says "caravan parks, camping parks, and similar locations", all of which are open to the public.
However there would be no harm in applying the requirements of this section to a caravan supply on private ground.

The main part that would apply is the prohibition on the use of a PME supply.

 

[timhoward]

I'm not sure I would agree with this interpretation, at it says "caravan parks, camping parks, and similar locations", all of which are open to the public.
However there would be no harm in applying the requirements of this section to a caravan supply on private ground.

The main part that would apply is the prohibition on the use of a PME supply.

I (obviously) defer to anyone that has the regs open especially as I’m in a pub!
Maybe the OP can clarify the exact circumstances.

 

[loz2754]

I (obviously) defer to anyone that has the regs open especially as I’m in a pub!
Maybe the OP can clarify the exact circumstances.

What, you don't take your regs into the pub! Shame on you.

 

[littlespark]

Still like to know if this is for a tourer or a static…. But the 10.0mm mentioned on #3 suggests something big.

 

[timhoward]

What, you don't take your regs into the pub! Shame on you.

Not tonight! I’m in proud dad mode as my teenage daughter has a gig and is doing great!
Anyway, back to the topic in hand….!

 

[pc1966]

So if they have TNS or TT then I can use the installation earth?

Technically yes, but many "TN-S" supplies are actually off segments with a PME conductor, just they look separate at the DNO cut-out.

Personally I would TT it unless the main supply was already TT.

Also for the cable I'm probably gonna use 3 core 10mm SWA. But what if I used 2 core and used the armouring as the main earth? Crazy talk?

The two issues with using the SWA armour are:

• If using a 16A or 32A "commando" socket you will struggle to have any reliable connection to the armour. For hard-wiring to a fixed unit with glands no problem
• The end of cable Zs for disconnection has to be met. For TT supply (RCD before the SWA run) then no problem as RCD will disconnect on any sort of armour impedance, but if TN to the caravan point you ought to check it is met.

See also this article:

Electrical installations in caravan/camping parks, caravans and motor caravans

As the warmer weather approaches and with summer around the corner, thoughts turn to holidays. Chief Engineer Geoff Cronshaw reminds us of the requirements of BS 7671:2008+A3:2015.

bit.ly

 

[Calebp43]

So if they have TNS or TT then I can use the installation earth?
Also for the cable I'm probably gonna use 3 core 10mm SWA. But what if I used 2 core and used the armouring as the main earth? Crazy talk?

Does anyone have a reply to this?
I'm just curious.
Also yes I am confident enough to carry out the work. I just ask a lot of silly questions on this forum that play in my mind because I don't like asking my colleagues silly questions lol

 

[Calebp43]

Technically yes, but many "TN-S" supplies are actually off segments with a PME conductor, just they look separate at the DNO cut-out.

Personally I would TT it unless the main supply was already TT.


The two issues with using the SWA armour are:

• If using a 16A or 32A "commando" socket you will struggle to have any reliable connection to the armour. For hard-wiring to a fixed unit with glands no problem
• The end of cable Zs for disconnection has to be met. For TT supply (RCD before the SWA run) then no problem as RCD will disconnect on any sort of armour impedance, but if TN to the caravan point you ought to check it is met.

See also this article:

Electrical installations in caravan/camping parks, caravans and motor caravans

As the warmer weather approaches and with summer around the corner, thoughts turn to holidays. Chief Engineer Geoff Cronshaw reminds us of the requirements of BS 7671:2008+A3:2015.

bit.ly

Yet again you have came to answer stuff. Thanks pc
(And everyone else. Pc just seems to be a regular question answerer xD)

 

[Calebp43]

Still like to know if this is for a tourer or a static…. But the 10.0mm mentioned on #3 suggests something big.

Sorry it's a static. And about 30m run I'm cable

 

[davesparks]

How’s the wording of the regs on this?

I’m sure it says a rod for a caravan park, not for just a caravan plugged into a house…

ESQCR needs to be considered here which prohibits the TNCS earth being connected to tbe metalwork of any caravan or boat.

 

[davesparks]

So if they have TNS or TT then I can use the installation earth?
Also for the cable I'm probably gonna use 3 core 10mm SWA. But what if I used 2 core and used the armouring as the main earth? Crazy talk?

Without actually being on site an seeing the installation we can only guess at what you can or can't do.

I assume by 'main earth' you mean use it as the CPC? Yes generally it is acceptable to use the armour as the CPC, that is after all how the cable is designed to work. But you need to do the calculation to confirm its suitability, if you are unsure/want us to check your calculation then post the relevant information along with what your results were and we can have a look and help you.

If you do need to set up a seperate TT earthing system then you will need to consider how you are going to isolate the CPC of the SWA from the new TT earth you create.

 

[Calebp43]

One of the lads I work with is saying that the caravan will need a pitch. You know like a box with a caravan hookup point like a commando socket type thing.
I have looked in regs book and page 316 (721.510.3) says "where there is more than one electrically independent installation, each independent installation shall be supplied by a separate connecting device and shall be segregated in accordance with the relevant requirements of BS 7671"
Seeing as this person only has 1 caravan that leads me to believe that they won't need a pitch? I can run the armoured straight into the consumer unit in the caravan.
Have I misenterperated anything there?