Testing an RCD (garage)thats protected by an RCD (house)

[noidea]

Hi
I am quite new to all this testing so if the answer is obvious then I apologise.
I have installed a new CSU in my garage and rewired the lighting and sockets.
My problem is that when I test the RCD the main RCD in the house is tripping 1st before the new one in the garage. I am using the auto fuction on the tester. When I go inside and reset the RCD it trips 3 more times. However when I go back to the garage I notice that the garage RCD has also tripped.
Is there a way of testing the garage RCD without the house tripping 1st?

 

[Sparky Ninja]

Re: Testing an RCD in a garage

Normally if after a few attempts, I am unable to trip the local RCD, then I would bypass the origin RCD briefly to obtain my reading. If you do use this measure however, be sure to safely isolate the rest of the installation that the origin RCD would be covering if it is deemed necessary..

All RCD's have different tolerances and so behave independant to each other, it is likely that the RCD at the origin is more sensitive to the test. Sometimes installing a time delayed RCD can help prevent this from happening, however a TD RCD in certain situations is not acceptable.

 

[pevvers]

Discrimination is your friend...

 

[brucelee]

hi mate
what is the rated residual operating current of both rcds are they both 30 mA then as pevvers says you dont have discrimination ie upstream rcd 30 mA then downstream should be 10mA you dont need two 30mA RCDs
ie for a TT then upstream time delayed 100mA mA ie the supply downstream 30 mA RCBOs for the individual final circuits

 

[Hawk81]

if you have a 30mA rcd in the house then you have 2 options either take the rcd out of the garage board or if your suply cable to your garage is swa then you can take the garage supply from a non rcd protected way. no need for 2 rcds on the garage.