CU change where sockets can’t go on RCD

[Chris]

Hi,

I have a job for a customer, the consumer unit is an old 3036 board and there are several holes in it giving access to live parts, I can change the consumer unit but when put on an RCD the sockets trip. IR readings on the sockets are low but the customer isn’t in a position to have them rewired at the minute (recently decorated and cost) can I replace the consumer unit, put all circuits on RCBOs but leave the sockets on an MCB?

Can this be put in the departures section?

I understand not ideal just trying to find a way to at least make this safer than it is

 

[James]

Looks like a bit of fault finding is required.

omitting an rcd on a socket circuit because of an existing fault, it is more than a departure, could be seen as criminal negligence.
in my humble opinion.

 

[pc1966]

What fault finding have you done on the socket circuit so far?

If, as is likely, it is a RFC then you can almost certainly split it in to two 20A radials to eliminate a single section of failed cable if that is the only issue. However, you absolutely must find out what the nature of the fault is in case it is something far more dangerous like old VIR cable crumbling that could well cause a fire, etc.

Edit: Beaten to it by @James

 

[James]

@pc1966 yes, but you did put a bit more technical info and effort into your post!

 

[westward10]

What are "low IR readings"?
If you are going to put departures in then the scenario leading to this must be no less safe than if the departure was not present. I am somewhat surprised you are considering this.

 

[pc1966]

As @westward10 asks, what sort of IR results are you seeing?

Folks on here are helpful so if you engage then hopefully we can help you solve anything odd. If you are lucky and it is a single fault then should not be too hard to find, but if more than one issue it can be confusing and needs more care. Your RCD/RCBO will trip due to a current imbalance, that means one or both of:

• L-E leakage
• N-E leakage

For L-E you need above 15mA so below 15.3k ohm, that is low for any insulation and deeply worrying, but could be water in somewhere, etc.

You need the same current for N-E but due to the far lower voltage that usually means a near-short, however, that could have been present for years and nobody would have noticed until an RCD was present. Often that is a trapped cable in some back-box, cut on missing grommet, etc, and might only take a hour or so to check them all for it.

IR values of 25-50k might be down to an old RCD socket outlet, so try to eliminate that if IR results look odd. Often used for outdoor stuff as that was the highest risk of a fatal encounter.

Also be aware of any hard-wired equipment on the socket circuit, like a boiler or similar, they can be trouble and at the very least should be isolated for testing (and checked separately).

 

[davesparks]

I can change the consumer unit but when put on an RCD the sockets trip. IR readings on the sockets are low but the customer isn’t in a position to have them rewired at the minute

That's a hell of a big jump to make, an RCD trips therefore a rewire is required!

If a wheel on your van gets a puncture do you go out and buy a whole new van?