Installing a Garage CU question..

[rcampbell]

Hi all, if all the circuits of the main CU are RCD protected can I connect up an RCD protected garage CU to a spare way on the main CU?

Thanks!

 

[TonyCE]

Ideally not, to allow discrimination of the protective devices.

 

[m4tty]

As above there will be no discrimination. In a fault one may trip or both.

 

[sparkytim]

Yes you can if discrimination is not a big concern, but No point in having RCD in Garage unit IF RCD protected at main CU.

Benefit can be IF freezer in Garage then you may not be aware RCD has tripped if its in the garage and not checked every day. With RCD at House end shared with other circuits then you will know.

 

[Megaohm7]

This must be happening on many occasions where a garage cu is already in place and the main cu is updated to 17th ed. Discrimination can be an issue there are ways to deal with this of course.

 

[ian932]

Yes you can if discrimination is not a big concern, but No point in having RCD in Garage unit IF RCD protected at main CU. Benefit can be IF freezer in Garage then you may not be aware RCD has tripped if its in the garage and not checked every day. With RCD at House end shared with other circuits then you will know.

+1

If you have one in the house and one outside then the chances are they are both going to be 30mA so there is no discrimination here as either one could trip depending on the manufactures tripping point set in the device.

I've seen it before when testing a 2way garage cu for 1x 5x the house one goes 1st making it impossible to test without bypassing the house one temporally to get a set of readings.

 

[ianmacd]

+1If you have one in the house and one outside then the chances are they are both going to be 30mA so there is no discrimination here as either one could trip depending on the manufactures tripping point set in the device.

I've seen it before when testing a 2way garage cu for 1x 5x the house one goes 1st making it impossible to test without bypassing the house one temporally to get a set of readings.

And what difference would that make?

 

[steptoe]

chances are a 500mA RCD is NOT going to trip on a 30mA test.

 

[ian932]

Dont follow your question Ian please expand

---------- Post Auto-Merged at 21:19 ---------- Previous post was made at 21:17 ----------

Steps where is 500mA coming into the OP's scenario?

 

[ianmacd]

chances are a 500mA RCD is NOT going to trip on a 30mA test.

Hmm. Thats a worrying statement.

Time delayed is the only recognised way of achieving discrimination with RCD's

 

[platty668]

If the garage is protected by the RCD in the house why not replace the Garage RCD with a double pole isolator as used in older CU's as a main switch

You then have a spare RCD and discrimination is not an issue.

 

[steptoe]

Hmm. Thats a worrying statement.Time delayed is the only recognised way of achieving discrimination with RCD's

that is why I said 'chances' ,

you asked ian932 what difference it would make them both being 30mA,

so I simply said that usually a 30mA will fire off long before a 500mA, normally about 470mA before.

 

[sparkytim]

+1If you have one in the house and one outside then the chances are they are both going to be 30mA so there is no discrimination here as either one could trip depending on the manufactures tripping point set in the device.

I've seen it before when testing a 2way garage cu for 1x 5x the house one goes 1st making it impossible to test without bypassing the house one temporally to get a set of readings.

Trick to prevent this is to test the Garage RCD between Load L and supply N at the Garage RCD. It sees an inbalance but the Supply end RCD does not.

But you only need one set of tests so if Supply end RCD tests ok then no requirement for Garage RCD

 

[Adrian]

by fitting a rcd in the house and garage you still have protection in the garage if one of the rcd goes faulty and we know how often that happens

 

[Sidewinder]

Adrian,

Backup or redundant additiona protection.

I regularly install this for medical jobs.