Advice Sought On Caravan Supply Wiring

[wildbill92]

Hi there, after some advice, or rather reassurance on an issue a friend is having and planned upgrades.

Scenario:

Farmer-they do their own electrics, if bulb lights-jobs a good'n. So before saying get a professional spark, which I try to say, it's not gonna happen, I'm an electronics technician myself, so lack of 17th edition quals aside, it'll be better than if he did it.

Farm, supply comes via overhead lines to green supply box on his land, underground cable to 'SHED 1' which before the CU Tees off at the main switch to CU's located in 'SHED 2' and 'SHED 3'. - I would guess its a TN-S earthing system as no visible additional earths.

His static caravan is fed via a buried cable inside plastic pipe that is wired from an MCB on SHED 1's CU to the caravans CU.

The lights have always dimmed for the duration the kettle is on- i told him voltage drop because I believe the cable to the caravan is probably thin T+E over a distance of maybe <40 feet. (I'll check the MCB rating and cable next time i'm there)

He wants to fit an electric shower, and provide power to his shed next to the caravan which will house a tumble dryer and washing machine and light.

I proposed 40m of 10mm cable to a new caravan CU, giving 53.56A, based on in a conduit and temperature factor. Voltage drop 1.1%@50amps all approx.

From the caravan CU, a cable of at least 4mm should allow 36A if clipped, then in a large pipe to a garage type CU in the shed. This is just in case the tumble dryer and washing machine are both on and drawing large KW, probably power tools might be used here too.

The BG garage unit comes with a 40A 30mA RCD , then 32A and 6A MCB. The 40A is higher than the cable rating, so I'll need to change this for one below 36A. Then more 4mm from the 32A to the double sockets.

Back in the caravan, a shower drawing 8500W/230 = 37A could use 6mm cable, although probably will be using 10mm in conduit over <4m length. Hopefully protected by appropriate RCBO.

Anything glaringly wrong? All cables will be adequately protected by MCB's of a correct rating as I'm aware it'll be possible to overload the 10mm supply cable to the caravan with everything on. But either way its better than the current (I think) 4mm supply cable, and ideas to just spur the shed supply off the caravan ring main(facepalm) running the tumble dryer and washing machine!

Thanks in advance for any advice, corrections.

Forgetting to add lots as I'm tired, Insulation Resistance test and earthing tests etc will be performed.

 

[apprentice87]

I would guess the earthing would be TT, so, if you cannot find any rods, there is none!!

john...

 

[wildbill92]

I originally thought being in its rural site and overhead supply it would be TT, but then thought surely it can't be as I cant recall any local earthing rods on each of the seperate buildings. Will have to check that one. A nice earth loop test might have shocking results.

 

[Andy™]

overhead will not be TNS. more likely TT, but possibly TNCS

 

[NozSpark]

My opinion is....

If the farmer doesn't want it done correctly (including rectifying any faults) then don't even think about doing anything,,,, leave well alone

The risk to your liability insurance just isn't worth it,,, and if you don't carry the correct insurance and something goes wrong then I'm sure that their solicitors will have all your worldly possessions as compensation/fines

 

[ProDave]

Apart from anything else, regardless of what earth is supplied or not supplied by the DNO, you are supposed to feed a caravan with it's own TT earth.

So get banging a rod or 2 in next to the 'van.

 

[steptoe]

There might not be any rods,

As the sheds steel frame? Could be using that as an earth electrode.

 

[Andy™]

or it may be TT and not have any earth electrode at all...

 

[wildbill92]

I'm not doing it on a professional capacity, electronics not electrics is my forte. But when you hear of a friend wanting to run an electric shower off a 4mm cable that also will feed a caravan and outbuilding alarm bells ring.

I was under the impression the caravan chassis should not be earthed where it's sited provided there is earth at the supply provided. Correct me if you know otherwise. At caravan parks where a temp hookup cable is used they are often earther where the hookup point is mounted in the ground if not the supply.

I'll check the earthing arrangement the next time i'm there, aside from that, anything wrong with the proposed other than the obvious?

I

By caravan its permanant, I believe this negates the usual need for the TT system as with a movable caravan?

 

[Andy™]

a little knowledge is dangerous.

 

[steptoe]

mmmmm

have a look at earthing requirements for farms.

 

[NozSpark]

It doesn't matter if you're doing it in a professional capacity or not,,,,, it'll be you who's liable should anything hit the fan

 

[wildbill92]

This forum is ******* useless. ill leave him to it. A little knowlege? anything is an improvement believe me

 

[Andy™]

whilst you may be an 'electronics technician', you are not an electrician, and you do not know enough to do the job safely.

but yes, best leave him to it. at least when

its his fault and no comeback on you

 

[Sharpend]

Sorry to say you getting involved is exactly the reasons these people don't call upon a professional.

 

[wildbill92]

How do you think the current mess of undersized cables and inappropriate breakers got there in the first place?

Did you read my original post? Farmers! . They don't care what insulation resistance or any correction factor for fixing/temperature of cabling etc.

It will go ahead anyway, and in light of this being the DIY secton(granted this job should not be) any advice based upon the original post instead of the standard response of 'get a spark in' would be appreciated.

While I do know a million and one things about components and electronic systems and spend my days testing for earth leakage and for fault conditions, I admit, I'm clearly not familiar with all ins and outs of what goes 'your side of the plug' to BS7671 etc

 

[steptoe]

exactly why I said there didnt HAVE to be earth rods for it to be TT

but, you need to be a bit more cautious as it is a farm a lot more other things get involved than simply adhering to the normal for a domestic,

 

[SPECIAL LOCATION]

The BG garage unit comes with a 40A 30mA RCD , then 32A and 6A MCB. The 40A is higher than the cable rating, so I'll need to change this for one below 36A. Then more 4mm from the 32A to the double sockets.

Isn't the 40A you mention the max breaking load of the 30ma RCD....

Nothing to do with cable ratings?????

Back in the caravan, a shower drawing 8500W/230 = 37A could use 6mm cable, although probably will be using 10mm in conduit over <4m length. Hopefully protected by appropriate RCBO.

Thanks in advance for any advice, corrections

.

8500w @ 230v Shower...?

Most manufactures quote their power rating @ 240v not 230v..

so at 230v power is less than 8500w

:C

 

[wildbill92]

My bad, they may well quote the power at 240v.

Any helpful advice?

 

[apprentice87]

Hi wildbill,

I have not got a copy of the regs in front of me, but from memory, there is nothing that says a caravan HAS to be TT'ed. What the regs say, [like i said, as far as i can remember] is that it MUST NOT be connected to a TNCS supply. They also refer [again, so far as i can remember] to "caravan like structures" So unless you HAVE got a TNS supply, which i doubt, you will have to install rods.

Please do be careful though.. I myself am not an electrician, but i have a fair idea..

What i can tell you though, is that Mr Farmer will not want to pay for the cables and accessories you need, so you will end up with a "half baked" installation, but, if something goes wrong, the farmer will be the first to say that "He did it, him there!! it is his fault"

Now then, cable sizes, you are going about this the wrong way. You do not decide what cable size you need based on the current... It is based on the OCPD employed..

First off, you figure out what you think maximum load on the circuit will be, THEN you select your OCPD. Next you decide what correction factors apply to your cable and multiply these together. You then divide the rating of the OCPD by the figure you came up with, and this tells you what current carrying capacity cable you need.

It can be more complicated than this though, as more than one correction factor might apply to the same section of cable which complicates matters.

Now, you have to check the cable complies with the requirements for volt drop, but it might be easier to do this first, as it is often the limiting factor, instead of actual current carrying capacity.

There is still more to worry about, you still have to make sure that the R1 + R2 of the resultant circuit will be low enough, that in conjunction with Ze [or Ra] you end up with a figure for Zs at the extremities of the circuit low enough to operate the OCPD fast enough to meet required disconnection times for the type of circuit.

You also need to check the cable CPC size, from the point of view of thermal effects. What i mean is, you have to check that in the event of a fault, that the OCPD will operate before the cable overheats and is damaged. Google "adiabatic equation"

You need to get hold of a copy of the wiring regs, these explain all the requirements in great detail and you will find them a lot of help..

It is all complicated stuff, and i have SOME grasp of it, but I would not be wiring up someone else's property, i am VERY much a beginner....

Try if you can to design the whole installation from the cutout onwards, and try to show your calculations and what accessories you plan to use and why, and i am sure that members on here will come up with some constructive criticism for you...

Do remember what Steptoe said though.. There are special requirements for farms and there are also special requirements for TT installations in general too..

john...