RCD wont trip... tehn trips.....then wont reset... Whats going on please?

[Dave1982]

Hi all.

I'm getting abit confused. Help would be appreciated.

Was called to a customer who had lights dimming and kettle not working.

Arrived and found TT sytem with in line Voltage operated Earth Leakage device between cutout and fuse box.
The device was found to be faulty, large voltage drop reduced the supply voltage down to 120V so replaced this with a DB 100A isolation switch as a temporary measure, then to return with an 30mA RCD to offer earth faualt protection. Recommende a new 17th edit CU to comply but they weren't keen on the idea. So told em if RCD trips all circuits will become unoperational.

Tested RCD....just wouldnt trip. suspected a neutral to earth fault.

Removed all neutrals and found the lighting circuit to be culprit.

With a time limit of 1 hour to hunt down the cause......I figured not much hope.


Anyway Found that on removing the lighting circuit neutral the RCD tripped as the neutral was removed from the neutral bar.

On attempting to reset RCD it would not reset. But did reset once the neutral was replaced

Anyone know why please.

Thanks in anticipation

 

[truckster]

If it wouldn't trip I would think it is unliely to be an N/E fault or it wouldn't set in the first place.

If it won't set when the light neutral is out (I hate to say this because it sounds so b*****y stupid), is the light neutral acting as the supply neutral? Many rcd's wont set if they haven't got a neutral.

 

[Top Cat]

As above^^^^^^^^^^^^^^^^.

I wonder whether the 120V was you thinking you were testing between live & neutral when in fact you have no neutral and picking up live to earth through the neutral of one of the circuits. With everything disconnected check L - N and see if you have any voltage.

 

[Philpot]

I'm wid you Top Cat I also think that there is in fact no neutral on the supply. It only gets a neutral if the lighting cct neutral is connected because the lighting cct has an earth fault on it. Try dising the Lighting cct and then measure the voltage at the supply, you may find that you have zero or close to it.

 

[Philpot]

I do remember that when I started my apprenticeship there were lots of very old installations that were double pole fused, yes double pole fused. It was common for people to mix up neutrals and cross them and cause all sorts of problems not unlike the one you have.
Best of luck Philpot

 

[acvc]

It;d be good to know what you find there Philpot.

 

[Philpot]

Hi ACVC One that I remember was when the lighting CCts, Upstairs and downstairs, had the neutrals crossed/joined. This usually happened around the hall and staircase lighting. That was because the 2 way switches were in common conduit. If you pull, what you think is the L + N fuse for a cct then some lights go out but you could find a lethal voltage on the cct if certain lights were on. It was also possible to get lights in series so they were half brilliance. ....It's easier to draw than describe. The main thing was that they realised that neutral fuses were a bad idea and it was never contemplated again on any cct that has an earthed neutral.
There are however CCts that do not have an earthed neutral, like ships. This is a different story when you have to check every day or monitor for earth faults ( well that was when I was at sea). It is possible to have an earth fault on both the live and neutral at the same time.

Regards Phil