rcd on rcd

[consumer done over]

Hi,

If I used one of those portable RCDs and plugged it in, and it tripped would the CU RCD still trip?

I am trying to check which appliance is causing nuisance tripping without having the CU cutting out as I am told it is not too good to have the CU RCDs tripping often.

 

[Adam_151]

Discrimination cannot be garenteed between two standard 30ma RCDs (logically... how could they )

One or the other might operate, or they might both operate, it depends on the tolerances of the devices in question, you might be luckly and end up with two that'll discriminate 3 times out of 4 due to the downstream one being faster and more sensitive than the upstream one (and if you press the test button a few times, it'll make sure the mechanism is moving freely and its going as fast as it can )

[Pressing the test button will not operate any other RCD other than the one that the test button is on]

 

[experience]

i was told that by my lecturer years ago that using a plug in RCD on a RCD protected circuit could make both of them useless for RCD protection as they counter each other. I'm not sure how though

 

[Taylortwocities]

i was told that by my lecturer years ago that using a plug in RCD on a RCD protected circuit could make both of them useless for RCD protection as they counter each other. I'm not sure how though

He was talking out of his rear end.

 

[Spark123]

That is put nicely, in some instances it is unavoidable - taking into consideration caravans.

 

[333rocky333]

I have found faulty appliances in a similar way.
I used a rcd adapter that "latches on" after power failure.

Therefore hopefully the fault will trip just the adapter or both rcd's.
If the fault is elsewhere then possibly just the cu one will go off.

If the CU goes off and the rcd is still "Latched" in on position showing no sign of tripping it may possibly eliminate that appliance.

If you did use a "non latching" rcd the fault would turn off the CU trip and the power loss would turn off the other one also which may mislead you to think that appliance was faulty.

If you do get a latching one (just temporary for tracing the fault) it is not reccomended for power tools or lawnmower use, as in the event of a power cut they would re-energise without warning.
So be carefull where you use it!!

 

[plugwash]

I wouldn't worry too much about handheld power tools and lawnmowers. They almost always have trigger action controls.

It is the fixed bench tools that have latching controls that you really should have on some form of no volt trip (either as part of a RCD or sepeately)