Rcd discrimination on circuit

[Bradley6969]

Hi, I’m installing some caravan boxes with 32A socket which is protected by 32A 30ma RCBO inside the box and then plug and play 32A socket inside the caravan to consumer unit which has main switch and RCD circuits can some on explain the discrimination please?

 

[Spoon]

Why do you need a RCD in the CU if there is one in the caravan box?
Can you not change the RCD in the CU for an isolator?

 

[Risteard]

There isn't a way of assuring Selectivity (discrimination) in this case. One or other or both RCDs could operate if there is a fault. However it will provide redundancy.

 

[westward10]

This is where Sections 708 and 721 clash with regards to additional rcd protection as both require it.

 

[Octopus]

If you have 2 x 30mA RCD's in series, you have no discrimination!

 

[Risteard]

If you have 2 x 30mA RCD's in series, you have no discrimination!

Indeed, but with caravan parks you also don't have any choice.

 

[Bradley6969]

Thanks guys

 

[Pete999]

If you have 2 x 30mA RCD's in series, you have no discrimination!

Unless of course the one in the take offpoint is 100ma and a type S

 

[westward10]

You have to consider them as separate systems and try to maintain discrimination within each system. As both require 30ma rcd protection you are going to compromise discrimination when they come together, not much you can do about it.

 

[Octopus]

Indeed, but with caravan parks you also don't have any choice.

And some sparks do this on purpose saying its safer!

 

[westward10]

You could look at it the other way and say discrimination between the two systems is not relevant as they are electrically separate until they come together.

 

[Risteard]

And some sparks do this on purpose saying its safer!

As I say, it does provide redundancy so you could argue that technically that is safer. Ask yourself if there is any danger with the lack of selectivity here - does it actually matter which device operates (or whether both do), and realistically it doesn't.

 

[Octopus]

On a caravan site, having a RCD on the supply means then its not assuming a RCD in a caravan - so the best of both worlds.