A Point for Discussion

[westie101]

I answered a question about Caravan site Electric Hook Ups (EHU) on another forum, which got me thinki8ng about the permissible voltdrop on a site

Now BS7671 has two different permissible figures, 3% for lighting and 5% for power I think.

Given that the designer of the site installation has no means of knowing the exact installation design of the caravans being used how can a site system be designed
and
given that caravans do have 230V lighting

What figure should be used as it can only initially be checked at the EHU point?
Should a designer just ignore the individual caravan installations as they are outside the control of the site?

Though as some of these double as site lighting are they power or lighting point?

The same would apply to loop impedance as well I guess

 

[JohnW2]

I answered a question about Caravan site Electric Hook Ups (EHU) on another forum, which got me thinki8ng about the permissible voltdrop on a site ... Now BS7671 has two different permissible figures, 3% for lighting and 5% for power I think. ... Given that the designer of the site installation has no means of knowing the exact installation design of the caravans being used how can a site system be designed and given that caravans do have 230V lighting ... What figure should be used as it can only initially be checked at the EHU point? Should a designer just ignore the individual caravan installations as they are outside the control of the site? ... The same would apply to loop impedance as well I guess

Other than the application of 'wild guesses', I don't think the designer has any other options, has (s)he?

In a sense, it's no different from a standard sockets circuit in a house. The designer (and installer/tester) can ensure that Zs, and VD under maximum design load, are acceptable at the socket, but that doesn't stop a user plugging in some equipment via a very long, maybe low CSA, flex - so the designer can never say anything (other than minimum bounds) about what the Zs and VD will be at the equipment.

In the situation you're discussing, there might perhaps be some scope for debating where is the 'origin of the installation' (from which VD has to be measured), but even that would not help with Zs. Certainly within premises, VD has to include that in any 'distribution circuits' as well as the final circuits.

Kind Regards, John

 

[westie101]

It's one of those anomalies of the process. That could result is some really strange low volts at the end user (the caravan).

As you say a guess, though a calculated one, would probably be the only way of providing an effective design. If the reasoning was recorded in an appropriate way I doubt there would be any comeback

 

[JohnW2]

It's one of those anomalies of the process. That could result is some really strange low volts at the end user (the caravan).

True, but I think the real anomaly is that the guidance is all in terms of the amount of VD (relative to the origin of the installation), without reference to the fact that the voltage at the origin could be anything between 216.2V and 253V (or even outside of those limits for brief periods). Admittedly, there's nothing a designer can do about this, since (s)he cannot be sure what the supply voltage is always going to be.

As you say a guess, though a calculated one, would probably be the only way of providing an effective design. If the reasoning was recorded in an appropriate way I doubt there would be any comeback

I suppose that's about the best anyone can do. Maybe it could be coupled with some notices which somehow indicated that the EHU supply was not suitable for supplying 'high impedance connections'?!

As I implied, it's really just an example of the fact that 'proper design' simply is not possible when plugs/sockets are involved, since the designer has no control over what gets 'plugged in'.

Kind Regards, John

 

[Robin0577]

I think I would take the approach of designing for caravans which themselves comply with the regs, and any that don't can take pot luck...

So that gives us;

Maximum load per point - 16A

Cable from hook up to caravan, 2.5mm2 3 core flex, maximum 25m in length.

Internal cabling to meet the requirements of the internal protective devices, usually B type breakers no greater than 16A

I would think it should then be possible to certify that "the installation complies with the regs, provided it is used with compliant caravans"

Any thoughts on the legal acceptability of taking that approach?

 

[JohnW2]

I would think it should then be possible to certify that "the installation complies with the regs, provided it is used with compliant caravans"

Yes, I suppose that's conceptually similar to my comment/question:

Maybe it could be coupled with some notices which somehow indicated that the EHU supply was not suitable for supplying 'high impedance connections'?!

Any thoughts on the legal acceptability of taking that approach?

As I've been discussing with westie. I think that something like that is probably the best a designer can do, or be expected to do, given that they have absolutely no control over what may/could get plugged in to the circuit.

Kind Regards, John

 

[Robin0577]

I would think it should then be possible to certify that "the installation complies with the regs, provided it is used with compliant caravans"

Yes, I suppose that's conceptually similar to my comment/question:

Maybe it could be coupled with some notices which somehow indicated that the EHU supply was not suitable for supplying 'high impedance connections'?!

I'd say it's conceptually almost identical to your comment, the only difference being that "caravans complying with BS7671" is a more rigid/quantifiable definition of what may or may not be plugged in.