Annex electricity on a submain

[Jupiter01]

I have a submain in the annex which runs off an appropriately sized MCB on a RCB board in the house.
The annex has it's own 3 way RCD board which again has been sized appropriately: 32amp MCB for a small ring circuit constructed from 2.5mm T&E cable and a 6amp MCB for a lighting circuit run off 1.5mm T&E cable. Everything is working fine.

Q. I can understand that if there was a fault/overload on any circuits within the Annex, the associated MCB/RCD will trip. Are there any situations (other than a fault on the cable between the house and annex) where the MCB/RCD in the hosue would trip?

Thanks in advance.

 

[DetlefSchmitz]

Yes, just about anytime there's a fault or overload in the annex. Slightly less likely on the 6A MCB, but you carefully didn't say what main CU MCB value you used.

 

[ericmark]

This is the situation with most caravans, they have a 16 amp MCB on their supply, if you use a 10 amp socket radial it is hardly large enough, with 6 amp for lighting and with 16 amp in caravan the supply one goes first.

It is a balance, with a 50 amp MCB feeding the annex, a fault could rupture the DNO fuse first, and as you reduce is supply MCB size it gets more and more likely the house MCB will trip first.

With a 40 amp in the house it is unlikely the house MCB will trip first, 32 + 6 = 38 amp so theroy annex will trip first, but if the house consumer unit is warmer than the annex then still the 40 amp could trip, but unlikely.

The 16 RCBO's in my CU add up to well over the DNO fuse of 60 amp, but in real terms it is unlikely to rupture, one as the RCBO's are unlikely to get fully loaded, and two the solar panels and battery are also feeding in, however I could not rent out my annex as the main CU in my case is in the annex so access 24/7 is required. Unlikely I will need assess, but I have to plan that access may be required.

 

[Jupiter01]

you carefully didn't say what main CU MCB value you used.

The house has a 50amp MCB

 

[Jupiter01]

This is the situation with most caravans, they have a 16 amp MCB on their supply, if you use a 10 amp socket radial it is hardly large enough, with 6 amp for lighting and with 16 amp in caravan the supply one goes first.

It is a balance, with a 50 amp MCB feeding the annex, a fault could rupture the DNO fuse first, and as you reduce is supply MCB size it gets more and more likely the house MCB will trip first.

With a 40 amp in the house it is unlikely the house MCB will trip first, 32 + 6 = 38 amp so theroy annex will trip first, but if the house consumer unit is warmer than the annex then still the 40 amp could trip, but unlikely.

The 16 RCBO's in my CU add up to well over the DNO fuse of 60 amp, but in real terms it is unlikely to rupture, one as the RCBO's are unlikely to get fully loaded, and two the solar panels and battery are also feeding in, however I could not rent out my annex as the main CU in my case is in the annex so access 24/7 is required. Unlikely I will need assess, but I have to plan that access may be required.

Trying to get my head around the above @ericmark.
Am I right in thinking that in my situation (since I only have 32+6 in the annex) it will never exceed 50amp and therefore, the annex end will only trip in the event of overload? I have noted the exception of the house end warming up. Is this broadly true?

 

[Murdochcat]

What is configuration of the CU for the house?

Single up front RCD, dual RCD, RCBO board, MCB board ?

 

[Jupiter01]

What is configuration of the CU for the house?

Single up front RCD, dual RCD, RCBO board, MCB board ?

Dual RCD Board

 

[Murdochcat]

So a fault in the annex would almost certainly trip the main house RCD

 

[Murdochcat]

Dual RCD Board

The time is well overdue that the regs to ban use of dual or single RCD boards

 

[EFLImpudence]

Yes, we need to replace the CUs we replaced a few years ago before something else new is invented causing the CUs to need to be replaced again.

Ah - perhaps this is why they used the word 'replace' in the notification regulations.

 

[Jupiter01]

So a fault in the annex would almost certainly trip the main house RCD

regardless of the MCB ratings?

 

[Jupiter01]

On a related note, when does a MCB trip without the corresponding RCD knocking everything else out? I am asking this question in relation to a standalone CU as opposed to the added complexity of a sub-main.

I appreciate that an RCBO board takes this problem away...

 

[Murdochcat]

regardless of the MCB ratings?

RCD's aren't tripped by overload only .......... neutral to earth faults too.

 

[EFLImpudence]

On a related note, when does a MCB trip without the corresponding RCD knocking everything else out?

When an overload occurs.

I appreciate that an RCBO board takes this problem away...

Not really a problem if the cause is an overload.

 

[Jupiter01]

RCD's aren't tripped by overload only .......... neutral to earth faults too.

Confused as @ericmark response suggests that an overload only trips the MCB? Not the RCD too? Is that the case?