Gypsy Caravan

[Martynrosscampbell]

Looking for your opinions please.

I have a gypsy caravan in a field, currently no electrics installed.
I am thinking of running a 6mm armoured from mains supply in barn. 60m cable run. protected with 100mA rcd and 25A MCB to a 32A round pin socket next to caravan location.

wiring 3 double sockets on radial circuit in 4mm flex with PVC boxes and flex glands in the caravan with a hager 25A 30mA RCD at the start. obviously a length of flex from RCD to 32A socket. I will use blue arctic grade flex.

Questions are on bonding the Steel frame of the caravan? 4mm singles back to earth terminal in RCD enclosure? Also interested to hear your thoughts?

Thanks

 

[ericmark]

Caravans are a special case and there is a section of the regulations devoted to caravans.

There are a number of differences to lets say a garden shed two major ones are no TN-C-S supply and double pole isolation. Also the requirement for 2 x 30ma RCD's one on the supply and one in caravan which seems daft.

However there are also some exceptions which may apply in your case. One is definition caravan under Scope it states:-

They do apply to those electrical circuits and equipment intended for the use of the caravan for habitation purposes.

I have often wondered what a caravan is for if one is not going to live in it? However where the caravan is not going to be a typical holiday van then the regulations may be different.

Caravan. - A trailer leisure accommodation vehicle, used for touring, designed to meet the requirements for the construction and use of road vehicles (see also definitions of Motor caravan and Leisure accommodation vehicle).

Now as to if a Gipsy caravan meets the requirements for the construction and use of road vehicles is open for debate. It would clearly have lights and brakes including suspension to comply and often they are really a museum piece not a living van any more.

32A supply seems rather high normal for caravan is 16A so this makes me wonder what the supply is for. The words Gipsy caravan make me think of a horse drawn unit but there were also some very ornate large Showman's caravans like the Hobby which looked like a large touring van but was in fact too wide for British roads.

So can you tell us a little more about it? Specially why so much power is required.

 

[flameport]

6mm cable will be far too small for a 25A load over a 60m distance.
Why do you want a 100mA RCD to 'protect' the cable?
A 25A MCB is an unusual choice - why that rating?

Bonding the caravan frame - to what and why? What earthing arrangement will be used for the cable and the caravan itself?

 

[Martynrosscampbell]

Thanks for your replys,

The caravan like you say is more of a museum piece. its not likely to be going anywhere for a long time. there are no lights, in fact you would only know its a caravan if you saw the wheels.

I was taking care of double pole isolation via the 25A RCD next to the first socket and was hoping to get away with the TNCS supply thing!

The reason for the 100mA RCD was because like you say 2 30mA's seem like bad practice to me. circuit discrimination and all that! I suppose creating a separate TT system would eliminate nuisance tripping of both RCD's?

On reflection a 25A supply does seem a bit keen. I was just allowing for things to change in the future. The cable calculation website I used claimed that 6mm would be just about right for a 25A circuit over that length hence the 25A breaker. I think ill go for a 16A circuit run in 4mm armoured then maybe a 2.5mm radial. biggest load is a toaster. wood burner for heat and gas for cooking and kettle.

If for instance, you had a TT supply at the origin would you still need a separate rod at the hook up point. doesn't effect what im doing here. just curious!

 

[Martynrosscampbell]

Also re the bonding. the caravan is timber framed but the floorboards are supported by a steel frame which is then connected to the axle's. would it not be good practice to run an earth wire to this and bolt it on?

Just so you know where im coming from. I am a qualified spark. I've just started my own business after 10 years domestic experience. Just never really come across a job like this before. certainly not where I've had to make the decisions! I've found that because I've been house bashing so long all that stuff I learnt in college is a distant memory!!

 

[ericmark]

Personally I would treat as a caravan and use two 30ma RCD's and TT. But with no lights it's not a caravan so you can treat it like any garden shed if you want. I would guess the metal is not connected in any way to inside the van and would be like having a metal cloths line next to a shed although technical could be considered part of the structure it is not really connected.

However at the end of the day only you can decide.

I think the idea of two 30ma RCD's is to allow for either odd caravan or odd site to break the rules and not have them fitted.

Again it's not a caravan site so does not come under rules for caravan site.

I lived in a caravan for 4 years and I ran of a 6A supply. Electric heating, water heating, kettle, microwave, fridge and lights. OK I had to be careful to switch off one device before switching on another but I lived like that for 4 years then another 6 months inside a barn Sizewell Power station and new Seven bridge so even a 16A is a little over kill for that van.

 

[viewer]

Eric's the one with the expertise, but, from a lay point of view, there would be questions I would ask my self:
Is the chassis completely isolated? - it must be bolted to the superstructure somehow.
Is there any possible way in which the chassis could have a different potential to the ground around the van?
Can animals walk under the van and touch the chassis?
Are there any disadvantages to bonding the chassis to the earth terminal of the RCD?
I'd suggest the last question may be the most useful.

 

[ericmark]

Eric's the one with the expertise, but, from a lay point of view, there would be questions I would ask my self:
Is the chassis completely isolated? - it must be bolted to the superstructure somehow.
Is there any possible way in which the chassis could have a different potential to the ground around the van?
Can animals walk under the van and touch the chassis?
Are there any disadvantages to bonding the chassis to the earth terminal of the RCD?
I'd suggest the last question may be the most useful.

I think the question is similar to bonding in a bathroom. We always bonded but as both plastic pipes and RCD's hit the scene we realised having things bonded is not always the best approach.

It is two fold if bonded and it becomes live it will open the protective device however should you touch a live part and a bonded part the current through the body would be far higher than with something not bonded.

So it's all down to a risk assessment as to if to bond or not. Clearly we can't see the situation and so we can't assess the risks. However unlike a modern caravan with metal cladding this old unit has very limited metal work.

The big question is if the TN-C-S supply is used or a TT supply. With a TT bonding is OK but I would not want to bond any exterior metal work with a TN-C-S supply.

If you want to follow regulations the first thing is to decide what the item is?

Section 708 Electrical installations in caravan / camping parks and similar locations
Section 711 Exhibitions, shows and stands
Section 717 Mobile or transportable units
Section 721 Electrical installations in caravans and motor caravans
Section 740 Temporary electrical installations for structures, amusement devices and booths at fairgrounds, amusement parks and circuses

Each section has different rules and until you classify it you don't know how to wire it. I would assume it's not being lived in but why is it there? The farm could be one of these places where they allow children to pet the animals and the requirements would be very different to if used as a store room with some lights in on a farm.

These is even Section 705 Agricultural and horticultural premises to consider and to be frank only some one on site can decide what is required. We can give some pointers but to me this is one that if I was doing I would be studying the BS7671 very carefully before deciding.

 

[flameport]

As this is 60m+ from any buildings in the middle of a field, exporting the TNCS earth is totally unacceptable regardless of what is connected to the end of it.

 

[Martynrosscampbell]

As this is 60m+ from any buildings in the middle of a field, exporting the TNCS earth is totally unacceptable regardless of what is connected to the end of it.

Maybe im being a bit slow. I did drink far too much last night! But why does the distance make it totally unacceptable?

It is going to be lived in for a while but then it will be more like a summer house. that's the best explanation of it I think. it will also have a deck/balcony built around it.

 

[ericmark]

PME is a system where earth rods or other earthing methods are used at intervals so that in the case of a failure of the neutral conductor the neutral - earth is held within set limits reducing the resulting damage and danger however there will be a gradient and extending the supply without earth rods could mean the gradient becomes too large.

A garden shed at the end of the garden where all the gardens are surrounded by houses would unlikely be a problem however if beyond that garden there are just fields then likely you would need a TT supply.

It has been that way as long as I can remember now 60 so since you claimed to be an electrician I had assumed you knew the problems with exporting a PME earth and a TN-C-S is really the same as PME as far as those outside the supply industry are concerned.

As to any magic distance at which we will not export a TN-C-S I don't know of any quoted distance it's more the reverse where within hands reach we would not use a different earthing system.

 

[SimonH2]

The reason for the 100mA RCD was because like you say 2 30mA's seem like bad practice to me. circuit discrimination and all that!

It doesn't work like that - in general you can't get discrimination between RCDs by different trip current levels.
So Mr Gardener is trimming the hedge or whatever, nicks the cord, and manages to touch something live. A fault current will flow - and this is NOT limited in magnitude by the RCD. This fault current is likely to be significant if there's direct contact - so both your 100mA and 30mA RCDs trip.

Where discrimination is needed between RCDs, then one will be a time delay version. So, for example, the 30mA RCD will trip "very quickly", while the upstream one (which may, for example, be a 100ms delayed version) won't trip before the faster one has tripped and removed the fault.

 

[westie101]

I really would have thought that, by now, anyone involved with the electrical industry would have been fully aware of the risks posed by the loss of a DNO neutral (on the main or service) on PME/TNCS systems.

It is only by knowing these that folk have a chance of making the correct decisions regarding this type of earthing

 

[ericmark]

The reason for the 100mA RCD was because like you say 2 30mA's seem like bad practice to me. circuit discrimination and all that!

It doesn't work like that - in general you can't get discrimination between RCDs by different trip current levels.
So Mr Gardener is trimming the hedge or whatever, nicks the cord, and manages to touch something live. A fault current will flow - and this is NOT limited in magnitude by the RCD. This fault current is likely to be significant if there's direct contact - so both your 100mA and 30mA RCDs trip.

Where discrimination is needed between RCDs, then one will be a time delay version. So, for example, the 30mA RCD will trip "very quickly", while the upstream one (which may, for example, be a 100ms delayed version) won't trip before the faster one has tripped and removed the fault.

That's theory. In practice when a worker put a nail in the wall of a porta cabin to hang his coat on and it hit the cable
the 30ma trip went out in cabin.
the 100ma type S went out is distribution board.
the settable RCD set at 500ma and 30 seconds delay went out in the companies main distribution board.
the settable RCD set at 1A and 60 seconds delay went out in the supplies distribution board.

Where they is a leakage rather than full short circuit then yes there will be discrimination but full blown short circuit no in practice they all go out.

 

[ericmark]

I really would have thought that, by now, anyone involved with the electrical industry would have been fully aware of the risks posed by the loss of a DNO neutral (on the main or service) on PME/TNCS systems.

It is only by knowing these that folk have a chance of making the correct decisions regarding this type of earthing

Unfortunately we are now taught to read a rule book not how electrical systems work and I have seen where a caravan supply built into the house wall was set up as TT even though there was earthed metal work bonded to house within arms length of where the caravan was parked. It said TT in the book so that's what was installed.

The why it seems is no longer taught only a set of rules to obey.