Advice appreciated

[Swanniejack]

Hi all, I am new to this forum so firstly HI ??

I am looking for some advice after buying a property

I have a TT system feeding a split load RCD protected DB in main house, I then have 10mm SWA sub main coming off that board protected by a 50A MCB

This then supply's an extension with another split load RCD DB containing a ring main, Cooker, Hob, Oven, Lights and 2 static caravans which aren't heavily loaded with appliances etc

My question is that I have a 30ma RCD at source, a 100Ma RCD at extension consumer unit supplying Caravans which in turn have their own enclosures with RCDs.

Obviously Discrimination is all to pot, any advice on how to resolve this? TIA

 

[Swanniejack]

Hi all, I am new to this forum so firstly HI ??

I am looking for some advice after buying a property

I have a TT system feeding a split load RCD protected DB in main house, I then have 10mm SWA sub main coming off that board protected by a 50A MCB

This then supply's an extension with another split load RCD DB containing a ring main, Cooker, Hob, Oven, Lights and 2 static caravans which aren't heavily loaded with appliances etc

My question is that I have a 30ma RCD at source, a 100Ma RCD at extension consumer unit supplying Caravans which in turn have their own enclosures with RCDs.

Obviously Discrimination is all to pot, any advice on how to resolve this? TIA

 

[littlespark]

If theres a 30mA RCD at source, i see no reason to have a 100mA downstream.

The caravans will have 30mA inside as well?

 

[pc1966]

A diagram would help.

Basically on a TT system you pretty much need to have an up-front RCD for the sub-main. However, that should not be one of the 30mA RCD in you first DB, but it should be a 100mA or 300mA delay ("type S" selective sort).

Depending on your make/age of first DB, it it were me and I wanted the cheapest reconfiguration I would put in a 100mA Delay RCD as the main incoming switch, then take the 50A MCB off that (i.e. on the feeds to the pair of 30mA split load RCDs) so you 2nd DB has selectivity with its own 30mA RCDs.

If the caravan sockets have their own 30mA RCDs, and if it is SWA from the 2nd DB to them (so the cable itself is not needing additional protection), I would take them off the supply-side of it, say via another MCB so a caravan trip/test will not take out other circuits off the 2nd DB (or indeed 1st DB).

That is not how is would design it though! Still up-front delay RCD, but switch-fuse for sub-main and all RCBO boards in both cases.

From the opening post you seem knowledgeable but, just in case anyone is wondering, in England and Wales new circuits or CU modifications are notifiable work under Part P of building regulations so really needs a competent-scheme registered spark (or paying for certification separately).

 

[Swanniejack]

A diagram would help.

Basically on a TT system you pretty much need to have an up-front RCD for the sub-main. However, that should not be one of the 30mA RCD in you first DB, but it should be a 100mA or 300mA delay ("type S" selective sort).

Depending on your make/age of first DB, it it were me and I wanted the cheapest reconfiguration I would put in a 100mA Delay RCD as the main incoming switch, then take the 50A MCB off that (i.e. on the feeds to the pair of 30mA split load RCDs) so you 2nd DB has selectivity with its own 30mA RCDs.

If the caravan sockets have their own 30mA RCDs, and if it is SWA from the 2nd DB to them (so the cable itself is not needing additional protection), I would take them off the supply-side of it, say via another MCB so a caravan trip/test will not take out other circuits off the 2nd DB (or indeed 1st DB).

That is not how is would design it though! Still up-front delay RCD, but switch-fuse for sub-main and all RCBO boards in both cases.

From the opening post you seem knowledgeable but, just in case anyone is wondering, in England and Wales new circuits or CU modifications are notifiable work under Part P of building regulations so really needs a competent-scheme registered spark (or paying for certification separately).

A diagram would help.

Basically on a TT system you pretty much need to have an up-front RCD for the sub-main. However, that should not be one of the 30mA RCD in you first DB, but it should be a 100mA or 300mA delay ("type S" selective sort).

Depending on your make/age of first DB, it it were me and I wanted the cheapest reconfiguration I would put in a 100mA Delay RCD as the main incoming switch, then take the 50A MCB off that (i.e. on the feeds to the pair of 30mA split load RCDs) so you 2nd DB has selectivity with its own 30mA RCDs.

If the caravan sockets have their own 30mA RCDs, and if it is SWA from the 2nd DB to them (so the cable itself is not needing additional protection), I would take them off the supply-side of it, say via another MCB so a caravan trip/test will not take out other circuits off the 2nd DB (or indeed 1st DB).

That is not how is would design it though! Still up-front delay RCD, but switch-fuse for sub-main and all RCBO boards in both cases.

From the opening post you seem knowledgeable but, just in case anyone is wondering, in England and Wales new circuits or CU modifications are notifiable work under Part P of building regulations so really needs a competent-scheme registered spark (or paying for certification separately).

Thanks for the sound advice.

I agree it's not the best setup I'm just thinking of the cheapest way of doing it without jeopardising on safety.

Also the inconvenience of having to reset RCD in house (Incoming supply end)

Question: I know it's not ideal but would a 100ma RCD in main incoming board and second submain board ensure that if there was excessive earth leakage it would activate locally in either caravans as apposed to house or submain board, they are both buried and wired in SWA so 100ma is adequate protection ?

Cheers ??

 

[Mikegh]

So do you get discrimination with a standard 100ma in series with a 30ma or does it have to be TD upfront ?

We don't series rcds here so I don't know much ,.I thought it was something like 2×30ma upfront gave discrimination ?


I suppose you can't use non-TD because the standing leakage is a variable on a main rcd so has to be TD anyway upfront

 

[loz2754]

Yes that's right. 100mA S type (time delayed) upfront. The ratio is usually considered to be 3 to 1, 3 X 30=90, so closest equivalent is 100mA.