RCD test - Tripping 3rd RCD downline at half current

[highjon]

Any ideas on this guys?
I was at a job today where the customers garage intermittantly trips out at the house DB.
Tested the garage ring, end to end, linked out then insulation test - all ok
When I done the 1st RCD test on half current, it tripped out the main RCD in the house - on half!
Supply is from the house DB which has a main 63a 30ma rcd - this feeds to an adjacent rcd in its own enclosure(40a 30ma), which connects to the SWA to the garage. The garage has its own consumer unit with a 25a 30ma RCD and various mcbs.
Why would the main house RCD trip at half current? When I am testing from the garage another 2 RCDs downline.
This is the problem the guy has been having, the main RCD trips out randomly and he has to turn off the garage circuit to get it to reset.
ps - when I say garage, it's more of a gamesroom with arcade style pinball machines(3a each) and a large video game with twin screens and motorcycles.
I dont think the load is anything to do with it though
cheers for any ideas
John

 

[tony mc]

Why have more than one RCD on the same cct as you will not get any dicrimanation.

Tripping on half could be that you have loads connected to the cct which are drawing mA and putting it over 15 mA .

It could be the main RCD is faulty aswell take off all loads and test it on its own.

 

[telectrix]

beat me to it. tony. was going to suggest taking garage off house RCD. all depends if he has a non-RCD way in the board.

 

[davelerave]

how many rcd's have they got in series there

oh ya -3
crazy setup anyhow

 

[Chr!s]

Any ideas on this guys?
I was at a job today where the customers garage intermittantly trips out at the house DB.
Tested the garage ring, end to end, linked out then insulation test - all ok
When I done the 1st RCD test on half current, it tripped out the main RCD in the house - on half!
Supply is from the house DB which has a main 63a 30ma rcd - this feeds to an adjacent rcd in its own enclosure(40a 30ma), which connects to the SWA to the garage. The garage has its own consumer unit with a 25a 30ma RCD and various mcbs.
Why would the main house RCD trip at half current? When I am testing from the garage another 2 RCDs downline.
This is the problem the guy has been having, the main RCD trips out randomly and he has to turn off the garage circuit to get it to reset.
ps - when I say garage, it's more of a gamesroom with arcade style pinball machines(3a each) and a large video game with twin screens and motorcycles.
I dont think the load is anything to do with it though
cheers for any ideas
John

The first rcd is seeing the whole istallation(regarding imbalance) the next a little less and so on.

You probably have a conductor current between 5-10ma through capacitance alone, regarding capacitance the first rcd will see the lot, then you have equipment that may of been connected while testing, again this could be in the region of 5-10ma and the first rcd will see the lot. And then theres leakage due to parallel resistances so when you come to test at 1/2 it trip the first rcd.

 

[davelerave]

ya-if they're series

the standing leakage will accumulate 'upstream'

more appliances/lights andf lower overall IR

 

[sparkiegaz]

i thought as u go down stream u reduce the milliamp of the rcd,,,,,,

 

[Sintra]

But you can't go lower than 30mA for additional protection. Ideally the op needs to use a non RCD circuit to feed the garage in SWA then protect the garage with an RCD.


Sent from my iPhone using Tapatalk.

 

[Bradzz]

sounds like you need to get shot of the first board and replace it with a dual split rcd board, as you pointed out the board has a main switch that is 63A RCD, so if a circuit trips out the lot goes out, also if you was to change board then the garage would be protected from the main board.
if im reading what you have put up correctly.

 

[highjon]

Thanks for all the replies, make sense
I did inform him it was a crazy setup, he said the builders done it all while building the garage.
I did test the main RCD which passed fine, until I introduced the garage circuit - it then tripped at 40ms
The cable run to the garage is about 25m underground so capacitance does come into the equation.
I told him I would need to remove the RCD and put on a main switch, which worried him as he has a fear of power surges from bad experiences.
Will return tomorrow and reconfigure the supply circuit protection
cheers
John