Understanding RCD trips

PaoloGotti

Hi,

I am trying to understand the general concept of an RCD and why it trips.

Today I was changing out a part in my heat pump. I turned off the isolator at the unit before starting. The isolator is two pole (L & N) with the earth non-switched and straight through.

However, when I was mucking about (water going everywhere etc) the RCD in the consumer panel tripped (herself came out complaining that the TV went off). So I reset the RCD. Went mucking about again at the heat pump and the RCD tripped again!

I am 100% sure the isolator was off as the display panel on the unit went off with me switching it off.

Can someone explain how having just an earth connected to a unit can trip an RCD? If that was the case then wouldn’t my earthed kitchen sink also cause a trip?

Cheers

Paolo.

2011

Basically if there is a path of resisatnce that is low enough between neutral & earth this can casue current to be diverted to earth rather than returning on the neutral as intended. If this current exceeds 30mA this can casue the RCD to trip. Water can often cause this resistance to be low enough, even if the unit is off. Make sense?

John.G

One would think that the 2 pole switch would break the N-E link even if shorted out at the HP.

2011

https://www.boards.ie/discussion/comment/118118667#Comment_118118667

well it depends on where the water gets to! Also sometimes when a 2 pole isolator is used it does not mean that the neutral is broken.

Tails142

Yes neutral to earth will trip an RCD when live is isolated - interesting point about the two pole switch... are you sure it's two pole? Maybe there is enough current being induced along the neutral and earth cabling back to the fuseboard to create a current to trip the RCD?

John.G

If there was water splashing around the return cable then a very large resistance L-E in the order of 7700 ohms will trip the RCD but it will take a much lower resistance between N-E to trip the RCD as there may only be a few volts at the neutral.

PaoloGotti

https://www.boards.ie/discussion/comment/118118604#Comment_118118604

Humm…no…how can there be a low resistance between N and E if N is isolated?

PaoloGotti

https://www.boards.ie/discussion/comment/118118705#Comment_118118705

I opened the isolator and it definitely switches the Neutral. It is also away from the unit and is IP rated.

Bruthal.

https://www.boards.ie/discussion/comment/118119326#Comment_118119326

Did you test the isolator to see it is in fact isolating the N?

Lenar3556

https://www.boards.ie/discussion/comment/118119326#Comment_118119326

It does sound like the neutral is not disconnected. If you have full double pole isolation there should be no potential for the RCD to trip. Check the isolator, some of those have a neutral in/out connection that isn’t actually switched.

Failing that is there by any chance other control wiring entering the unit, such as a call for heat signal?

If the above does not get to the bottom of it, I would suggest having the electrical installation and particularly the house’s earthing arrangement / neutralising link checked. There is likely a deficiency if you are managing to trip the RCD despite having double pole isolation.

PaoloGotti

https://www.boards.ie/discussion/comment/118130072#Comment_118130072

Nail, meet the hammer head!

There is a control wire calling for heat! A 230v L + N from the thermostat. At the time of works the thermostat was calling for heat.

Fair play, mystery solved!