Garage CU problem

[Tinkler]

I have a garage CU (mains incomer) supplied from the the house CU through a 32A MCB on a non-RCD protected circuit. I noticed that the garage unit RCD was not tripping during a fault state, be that with a short form L to N or Earth...what happens is that the garage unit and the house unit MCBs trip together...the "test" button on the RCD trips it as expected. The unit is a GET GIC240R 40A...2 circuit (16A and 6A) with a 30mA sensitive DP RCD.

Many thanks.

 

[Steve]

The MCBs may be clearing the fault quicker because you're creating dead shorts - is this what you're doing? If so, stop. Its dangerous. And you run the risk of blowing the supplier's fuse. The 2 MCBs trip together because they cant discriminate in such a high fault current.

If you want to test the 30mA RCD without creating dead shorts, get a 60w lightbulb and connect it between live and earth. The RCD will trip, but the MCBs wont.

 

[Tinkler]

The MCBs may be clearing the fault quicker because you're creating dead shorts - is this what you're doing? If so, stop. Its dangerous. And you run the risk of blowing the supplier's fuse. The 2 MCBs trip together because they cant discriminate in such a high fault current.

If you want to test the 30mA RCD without creating dead shorts, get a 60w lightbulb and connect it between live and earth. The RCD will trip, but the MCBs wont.

Have done as you said and it trips fine...not silly enough to repeatedly create shorts; fully expected something to trip somewhere between fault point and supply to house...had to know if the RCD was working as supposed to. Couldn't understand why it (RCD) didn't trip when I accidently cut into the hedge-trimmer flex; an instance when a dead short was created. So in which "real" situations would the RCD alone trip?

Many thanks for your help.

 

[Steve]

Your hedge trimmer is double insulated. There is no earth in its wire. So there was no earth leak for the RCD to trip on. However, it did create a dead short so tripped your MCB / blew the plug fuse. If you mowed your mower's cable, it might leak to earth through the wet grass, tripping the RCD.

RCDs detect earth leaks. They monitor imbalance between live and neutral. So if any current "goes missing" along the way, it will trip if the missing current rises above 30mA. This gives good protection against electric shocks. It only takes 50mA to kill someone.

 

[Tinkler]

Your hedge trimmer is double insulated. There is no earth in its wire. So there was no earth leak for the RCD to trip on. However, it did create a dead short so tripped your MCB / blew the plug fuse. If you mowed your mower's cable, it might leak to earth through the wet grass, tripping the RCD.

RCDs detect earth leaks. They monitor imbalance between live and neutral. So if any current "goes missing" along the way, it will trip if the missing current rises above 30mA. This gives good protection against electric shocks. It only takes 50mA to kill someone.

Thanks for the explanation.

 

[Paul Barker]

What current will cause it to trip? Anything up to 30mA and beyond. It may trip for instance at 22mA it may not trip until 29mA but it will certainly trip at 30mA. It won't trip at 15mA if it is working correctly.