Garage Consumer Units

[Jonny72]

Ok, most new Consumer unit seem to come with a 63A RCCB and two MCBs. Most house consumer units now will also have RCD of some type be it a dual board or RCBOs. I know there can be issues with Selectivity having an upstream and downstream RCD. One Should really be S type (more expense). Should one 30mA RCD be removed? I am really not bothered which RCD activates as long as one does in event of providing additional protection. Just seems to add to cost replacing one with a simple on/off isolator. I am just concerned neither work.

 

[Jonny72]

You will need RCD protection somewhere is virtually every case. If it is fed via SWA cable so no need for protection on the distribution cable then it makes sense to have just a MCB at the house CU and then RCD and any other MCBs at the garage.

However, if you can't be sure of the sub-main installation then having RCD/RCBO at house and then just MCB at the garage is a better choice.

Two 30mA RCD in series without selectivity is poor design, not enough to worry about on an EICR beyond C3, but not something you should aim to do unless you have a very good reason.

Having said that, having MCB in series or RCBO+MCB will have negligible fault selectivity, even if they are OK on overload due to a 1.6:1 or greater difference in rated current. Again, not good design, but usually an acceptable trade-off if the final circuits in the garage are not critical as done in practically every case.

Thanks great advice and that's my plan. So MCBs in parallel not a good idea either?? That warrant a C3? Don't want to design code breaking into the circuit. Cheers Jonny

 

[westward10]

You have an HNC in electrical engineering so the design of this should be straight forward.

 

[Jonny72]

You have an HNC in electrical engineering so the design of this should be straight forward.

I was used wiring and designing high end avionic equipment and power supply in trainee fighter aircraft a past life so slightly different area of expertise. I do have an NVQ3. I come on here to get advice as I know I don't know everything. Before anyone says leave it to professionals - And I am updating my skill base having completed C&G 18th Edition and CG Initial Verification and Periodic Testing. I do concede I get stuck and use this forum to bounce ideas with the helpful people on here of which there are plenty. Thanks Jonny.

 

[Jonny72]

Indeed, making the typical TN assumption here that end of distribution Zs is low enough for MCB disconnection in under 5s.

No doubt will be, TNCS and last Ze was 0.22 ohms.

 

[Jonny72]

You will need RCD protection somewhere is virtually every case. If it is fed via SWA cable so no need for protection on the distribution cable then it makes sense to have just a MCB at the house CU and then RCD and any other MCBs at the garage.

However, if you can't be sure of the sub-main installation then having RCD/RCBO at house and then just MCB at the garage is a better choice.

Two 30mA RCD in series without selectivity is poor design, not enough to worry about on an EICR beyond C3, but not something you should aim to do unless you have a very good reason.

Having said that, having MCB in series or RCBO+MCB will have negligible fault selectivity, even if they are OK on overload due to a 1.6:1 or greater difference in rated current. Again, not good design, but usually an acceptable trade-off if the final circuits in the garage are not critical as done in practically every case.

And thankyou that was great advice. J

 

[Jonny72]

You will need RCD protection somewhere is virtually every case. If it is fed via SWA cable so no need for protection on the distribution cable then it makes sense to have just a MCB at the house CU and then RCD and any other MCBs at the garage.

However, if you can't be sure of the sub-main installation then having RCD/RCBO at house and then just MCB at the garage is a better choice.

Two 30mA RCD in series without selectivity is poor design, not enough to worry about on an EICR beyond C3, but not something you should aim to do unless you have a very good reason.

Having said that, having MCB in series or RCBO+MCB will have negligible fault selectivity, even if they are OK on overload due to a 1.6:1 or greater difference in rated current. Again, not good design, but usually an acceptable trade-off if the final circuits in the garage are not critical as done in practically every case.

I knew there was an issue with RCDs in same circuit. I did not know MCBs would be an issue and quite surprised. I agree no other way of doing it. I am sourcing a garage unit with 2 MCBs and a 2 pole isolator. This will be run of the main CU which is a dual board with 32A MCB and RCCB type A, thankfully. Thanks for your help. You are very knowledgeable. I see alot of ppl using those plug RCDs when using power tools on circuits with RCBOs or RCCBs. Wonder what potential effect that would have?

 

[pc1966]

I see alot of ppl using those plug RCDs when using power tools on circuits with RCBOs or RCCBs. Wonder what potential effect that would have?

It is perfectly safe - at least one of the RCD will trip - it is just inconvenient to have to reset two devices, and a possible hazard if the upstream device also feeds something like stair lights that you don't want failing.

 

[Mikegh]

Ya that's the main disadvantage of the upstream RCD

Loss of lighting on a socket fault in shed

 

[pc1966]

Ya that's the main disadvantage of the upstream RCD

Loss of lighting on a socket fault in shed

If its a workshop shed then probably best option is emergency lights, that way any cause of power failure results in some lighting, not darkness and power tools still running!

 

[leesrubberroom]

just buy the cheapest garage board , buy a seperate 80a switch and swap it out. in the grand scheme of a job it adds about a tenner to the material cost

Yep, that’s what I do. That spare RCD has always come in handy down the line.