Caravan diversity question

[Pretty Mouth]

What allowance for diversity, if any, would you apply to caravan hookup sockets?

The circuit will have just one hookup point, with two 16A sockets. It's a fair distance from the CU, so voltage drop is going to be significant. It would be good to keep the cable CSA down if feasible.

 

[mainline]

My son, who was banished to one over the winter of 2020-21, as a Covid precaution, would probably strongly disagree.
The 2kW heater in mine is only described as an auxiliary to the gas heater, which has an output of over 3kW.

You should close the windows then, or get a van that's insulated.

 

[Pretty Mouth]

Selectivity is always going to be a problem with having two 30ma rcds one at the hook up point and one in the van, even if using 100ma S type or similar at the source.

True, but what goes on in the caravan is out of our control. Regs say we have to provide RCD protection for the socket.

 

[brianmoooore]

Selectivity between the caravan RCD and the hook up point RCD isn't important. Selectivity between the hook up point nd the source of the power is.

 

[mainline]

True, but what goes on in the caravan is out of our control. Regs say we have to provide RCD protection for the socket.

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I would Time delayed rcd at source 10mm 2 core swa and rod at the Hook up.

 

[brianmoooore]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

 

[mainline]

Selectivity between the caravan RCD and the hook up point RCD isn't important. Selectivity between the hook up point nd the source of the power is.

Its still selectivity.

 

[mainline]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

I said at the source 30ma at the hookup.

 

[brianmoooore]

As Pretty Mouth says, we have no control over the van's installation, and the van owner has no control over what he plugs into.
Another potential problem that's beginning to appear, is that Motorhomes (often DIY made) often have petrol invertor generators built in, and who knows what the changeover arrangements are for them.

 

[brianmoooore]

Selectivity is always going to be a problem with having two 30ma rcds one at the hook up point and one in the van, even if using 100ma S type or similar at the source.

I said at the source 30ma at the hookup.

What's the point if the supply cable is SWA? No harm, but little point either, since there already should be two 30mA RCDs between the source and the customer.

 

[mainline]

Can't have an S type for the 16A socket. There's no guarantee that the 'van has any kind of RCD, or it might be just a 13A extension lead to a tent plugged in.

I said at the source 30ma at the hookup.

As Pretty Mouth says, we have no control over the van's installation, and the van owner has no control over what he plugs into.
Another potential problem that's beginning to appear, is that Motorhomes (often DIY made) often have petrol invertor generators built in, and who knows what the changeover arrangements are for them.

As you have just said, “no control over the vans installation”. So not a problem.

 

[brianmoooore]

As you have just said, “no control over the vans installation”. So not a problem.

Might be, if they somehow manage to put a potential on the cpc to the adjacent van.
Never underestimate a DIYer.

 

[mainline]

What's the point if the supply cable is SWA? No harm, but little point either, since there already should be two 30mA RCDs between the source and the customer.

I assume its pme you may not get the disconnection time.

 

[mainline]

Might be, if they somehow manage to put a potential on the cpc to the adjacent van.
Never underestimate a DIYer.

They maybe a lot of things, but it's their installation.

 

[brianmoooore]

Electrocuting innocent caravanners is NOT good for business.
If the RCD at source is needed to meet disconnection times in the event of a cable fault, then fine. That's all down to design.

 

[mainline]

Electrocuting innocent caravanners is NOT good for business.
If the RCD at source is needed to meet disconnection times in the event of a cable fault, then fine. That's all down to design.

Ok so are you saying that the OP needs to install into the design a circuit that detects flow into the source ?.

Maybe have some sort of sprinkler system as well as a fire alarm and security just in case they have fire arms

 

[brianmoooore]

Ok so are you saying that the OP needs to install into the design a circuit that detects flow into the source ?.

Not as yet, since it's relatively new potential problem, that the powers that be haven't cottoned onto yet. Watch this space. Separate earth rods for each point?

Maybe have some sort of sprinkler system as well as a fire alarm and security just in case they have fire arms

If you've ever seen a caravan burn, you'd know that there's little chance for fire fighting once it gets going.
We burnt one at our barbecue a few years ago. Drove everyone10m back across the garden and onto the top of the hedge from the heat.

 

[westward10]

This is getting way out of control from what @Pretty Mouth asked can we keep this to the original query.

 

[mainline]

What allowance for diversity, if any, would you apply to caravan hookup sockets?

The circuit will have just one hookup point, with two 16A sockets. It's a fair distance from the CU, so voltage drop is going to be significant. It would be good to keep the cable CSA down if feasible.

If the specification says 16A per caravan for every caravan, then you can't apply diversity, what does the spec actually say?
it is primarily up to the designer to establish with the client the types of loads and thus supplies needed.
I would guess each would pull no more than 10Amps for any significant duration.

I would also say you could get away with 5% voltage drop as the lighting is 12v

 

[Pretty Mouth]

If the specification says 16A per caravan for every caravan, then you can't apply diversity, what does the spec actually say?
it is primarily up to the designer to establish with the client the types of loads and thus supplies needed.
I would guess each would pull no more than 10Amps for any significant duration.

I would also say you could get away with 5% voltage drop as the lighting is 12v

There's no specification. The customer has told me they want 2 caravan sockets, and told me where they want them, the rest is up to me.

 

[mainline]

There's no specification. The customer has told me they want 2 caravan sockets, and told me where they want them, the rest is up to me.

100 mtrs Buried 10mm 2 core swa should give you around 25 amps 4.9% voltage drop.

Like you said, divorce the tncs armour at the hook-up and rod it.

I would also 100ma S type rcd at source if only to protect against the hook-up being damaged and the live supply coming into contact with the TT.

 

[littlespark]

If it’s a “site” to be following the regs for caravan sites, have they also thought of the distance between units for fire regulations?

From memory, all our hookups were factory made. RCD, then a 16A type C… but that was before the regs came in that said every socket had to have its own RCD.
We had RCD, to 2x16’s and a 6 for the light on top of the hookup post.

Even the majority of statics we had didn’t trip a 16A. Except for the ones that had a 4 slice toaster, foreman grill, coffee machine, washing machine, quick boil kettle and the one woman that had her trainee plumber grandson wire in a 9kW electric shower!

 

[brianmoooore]

If it’s a “site” to be following the regs for caravan sites, have they also thought of the distance between units for fire regulations?

This is from the rules I was given for setting up my site.
"In siting hook-ups it is also important to bear in mind that the recommended minimum safety spacing between caravans is 6 metres (20 ft). Each caravan should also have at least 3 metres (10 ft) of clear space surrounding it to restrict the spread of fire."

 

[pc1966]

I'm no expert on selectivity, it's almost never an issue for the type of work I usually do, but I can see the need to get it right in this instance. The general rule of thumb I was taught was 2:1 for breakers, so I figured a B32 upstream, and B16 downstream would do it. Do you think this won't work?

For tripping on the thermal curve during overload that is reasonable enough. The problem is if/when you hit the "instant" magnetic trip point on a hard fault typically you will hit the upstream point as well and both will be past the point of no return by time the downstream MCB has opened.

That is why fuses (with some thermal inertia) or MCCB with a short-term delay (20ms is enough) are preferred as the upstream OCPD for selectivity with MCB/RCBO.

For example, if you look up the Hager tables for 16A B-curve RCBO fed from difference upstream OCPD we have:

• 32A MCB (B-curve) they are only selective to 150A faults, Zs <= 1.1 ohm
• 32A MCB (C-curve) to 280A, with Zs <= 0.55 ohm
• 32A MCB (D-curve) to 460A, with Zs <= 0.55 ohm (5s disconnection)
• 32A BSS88 fuse would 500A, with Zs <= 1.25 ohm (5s, cold table figure)
• 32A x160 series MCCB to 1.19kA, with Zs <= 0.68 ohms (5s hot, 0.54 cold)

If you are looking at a TT supply for the SWA or otherwise using a 100mA/300mA delay RCD then 2C is fine and you can also chose the OCPD on the basis of cable adiabatic, not Zs for fault disconnection (as the RCD achieves that). Then your 10mm cable could have a 50A fuse (so selective to 1.4kA) or similar.

However, fuses are a right pain for any unskilled on-site support so if selectivity is that important then coughing up for a MCCB board might well be worth it.

EDIT: Also check what your likely PSCC is at the end of cable. For example if you are looking at around 25A at 5% drop then your max fault current would be around 500A so anything selective to that or higher won't ever have issues (e.g. 32A or so fuse, RCD & 40A D-curve MCB, etc).

 

[mainline]

There's no specification. The customer has told me they want 2 caravan sockets, and told me where they want them, the rest is up to me.

Maybe worth having a read

https://www.caravanclub.co.uk/media/7704111/electric_hookup_information_jun_2011.pdf

 

[brianmoooore]

Above is over ten years old and our of date.
Makes reference to the 17th.

 

[mainline]

Above is over ten years old and our of date.
Makes reference to the 17th.

Yes i was aware of that, but thought it still worth a read.
If you have something newer or can point to anything that has changed I'm sure you will point it out.

 

[brianmoooore]

The thing that has changed is that the 16A sockets now have to be the switched and interlocked type, which will inevitably lead to damage when customers try to unplug without switching off first.

 

[mainline]

The thing that has changed is that the 16A sockets now have to be the switched and interlocked type, which will inevitably lead to damage when customers try to unplug without switching off first.

Just like the one that the OP proposes to use right at the beginning of the thread, so I'm sure he's well aware of that (which you also seem to have a problem with) any suggestions as to how you would do this installation obviously not using a RCD at the source because you have said already that it serves no purpose.


I see that this has basically been discussed before by yourself, and seem to have had a change of heart on certain things.
Don't feel the need to reply, as I think the OP has more than enough info now.

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[westward10]

If fault protection is provided by a fuse or circuit breaker then an rcd isn't required.

 

[mainline]

If fault protection is provided by a fuse or circuit breaker then an rcd isn't required.

I understand that one isn't required, but my reasoning for using one would be mainly for a damaged hook up point where the live conductor could come into contact with the TT earth.

Imo if it was a fixed CU and met disconnection times etc then I would have no problem, but these hook-ups are usually just a stake in the ground and there's always going to be the risk of driving off without unplugging.