RCD in series ( Separate from house CU)

[Traineeboy]

Hi Chaps I Just wanted to check something. If you have an outbuilding that has an RCD board ( 30mA)  installed and then it runs back to the house on a main switch RCD (also 30mA) that is connected to a Henley block would this be OK ? I know both might trip but that wouldn't be a issue for discrimination as if the main switch RCD trips it would only trip the outbuilding anyway it's all separate from the house. 

The RCD main switch at the house is only there as it's touch and go if cable to the shed will meet the 80% rule for Zs. Therefore the outbuilding supply cable being on an RCD would cover this. I know if you were coming off a house main consumer unit would would either fit an RCBO or a time delayed RCD but as discrimination isn't an issue I figure a 30mA at both the house main switch ( separate unit from house CU)  and the outbuilding would be OK. 

 

[Murdoch]

What is the cable to the outbuilding ?

 

[Traineeboy]

What is the cable to the outbuilding ?

It is a 16mm2 SWA 2 core using armour as the earth. It’s 85 meters long. Outbuilding is small so not a lot of cable in the outbuilding it’s self. The earthing is TN-S but it’s 0.72 so within limits but doesn’t leave a lot of play for R1 + R2 . It’s a 32A so only allowed 1.10 . 

 

[Murdoch]

It is a 16mm2 SWA 2 core using armour as the earth. It’s 85 meters long. Outbuilding is small so not a lot of cable in the outbuilding it’s self. The earthing is TN-S but it’s 0.72 so within limits but doesn’t leave a lot of play for R1 + R2 . It’s a 32A so only allowed 1.10 . 

you could argue that the submain doesn’t need RCD protection 

 

[Traineeboy]

you could argue that the submain doesn’t need RCD protection 

Even if I was just the RCD for Fault protection ? That’s my concern. 

 

[Fleeting]

It is what it is if you believe the rcd is required for fault protection. Ideally you would remove the rcd in the outbuilding to remove the rcd selectivity issue but it isn't the end of the world.

 

[Traineeboy]

It is what it is if you believe the rcd is required for fault protection. Ideally you would remove the rcd in the outbuilding to remove the rcd selectivity issue but it isn't the end of the world.

That was my thoughts just have a main switch CU in the outbuilding but the boss is trying to use up stock and has loads of garage unit RCD boards. But yeah I get it , it’s OK to be done as it won’t really affect discrimination as no other circuit apart from the outbuilding DB will be affected. 

 

[Murdoch]

What’s this outbuilding going to be used for ?

 

[Fleeting]

That was my thoughts just have a main switch CU in the outbuilding but the boss is trying to use up stock and has loads of garage unit RCD boards. But yeah I get it , it’s OK to be done as it won’t really affect discrimination as no other circuit apart from the outbuilding DB will be affected. 

I thought this was an existing installation. If this is a new install then it isn't an ideal situation regardless of whether your boss is trying to offload some old stock.

By the way term discrimination is now called selectivity. 

 

[Murdoch]

Why isn’t the submain not going to be connected to the Henley block and a switch fuse …. 

this sounds like a really poor design, being done on the cheap

i think the precise details of the design load, VD etc need a proper review before anything else is done

 

[Traineeboy]

I absolutely agree this is why I don’t like working with my boss as it does appear he likes short cuts. I know that’s not good but as long as I know them and keep learning the right way. 
 

Just a question on switch fuse I assume if you wanted to RCD protect the cable to the outbuilding from Henley block can you get fused switched with RCD or would that just be a mini RCD board off the Henley blocks. 
 

if it was up to me I would of ran the outbuilding cable to a spare way in the house board ( before an RCD that would affect the house) and put in on a MCB or if RCD for submain is needed I would put in an RCBO. He said that was too Fiddly getting SWA into existing board but I responded that you could run SWA gland into an adaptable box then run cables into board. Would that be a better solution? 

 

[Fleeting]

I would be looking at protecting the cable with a BS88-2 or BS88-3 (previously BS1361) fuse because if you use a circuit breaker the chances of you obtaining selectivity with the circuit breakers in the outbuilding is slim. 

 

[Traineeboy]

I would be looking at protecting the cable with a BS88-2 or BS88-3 (previously BS1361) fuse because if you use a circuit breaker the chances of you obtaining selectivity with the circuit breakers in the outbuilding is slim. 

Ok got ya. If a main switch couldn’t be put in via Henley block direct to supply then could you use a BS1361 in a fused main switch that was ran from a circuit breaker in the main board ? 

 

[Fleeting]

No you need to try and eliminate the circuit breaker. Ideal set-up; from meter to main switch, then service terminal blocks from here you split to the consumer unit and the separate switch-fuse for the outbuilding. 

 

[Traineeboy]

No you need to try and eliminate the circuit breaker. Ideal set-up; from meter to main switch, then service terminal blocks from here you split to the consumer unit and the separate switch-fuse for the outbuilding. 

when there is not a main isolation switch install how to you put Henley blocks in to split the tails without pulling the fuse ? 

 

[Andy™]

when there is not a main isolation switch install how to you put Henley blocks in to split the tails without pulling the fuse ? 

officially, get DNO / meter company to fit an isolator. unofficially... well plenty options that best not be shared on a public forum...

 

[binky]

officially, get DNO / meter company to fit an isolator. unofficially... well plenty options that best not be shared on a public forum...

or very carefully, ideally with electrical rubber gloves on.

If there's enough slack in the meter tails, it is possible to cut and fit henley blocks working live, with main switch 'OFF' to prevent any sparking, but I just wouldn't bother these days as the seal fairy has usually been before me  :innocent

 

[Andy™]

or very carefully, ideally with electrical rubber gloves on.

If there's enough slack in the meter tails, it is possible to cut and fit henley blocks working live, with main switch 'OFF' to prevent any sparking, but I just wouldn't bother these days as the seal fairy has usually been before me  :innocent

you been watching me?