RCD test button not working

[dazza1981]

Hey guys i've got an update for you. There was a ring main with a N-E fault on the same RCD. had to put in a new leg, guessing the door had a fixing through the cable, couldnt pull it at all, site agent wasn't too happy, flats were basically being commissioned. When i removed the ring from the neutral and earth bars the rcd would trip on test. The lighting circuit was functioning properly now aswell.
so in conclusion the N-E fault on the ring main was stopping the rcd from functioning properly.

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

 

[appspark]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

 

[dazza1981]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

I know what you mean i had N/E fault the other week in a big cottage and it took me best part of a day to find it and rectify it...... i swear i must have done 1000 laps of that house lol

 

[jaypp]

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

Indeed.

 

[Midwest]

posssible N-E fault. remove all load from RCD the try the test button again. then get mft on it and do a proper test.if it then works, you have a fault on a circuit.

I've had same situation, found neutral earth fault on radial on a rewire doing IR test. Put that circuit to one side to get the rest of the house powered up, except I mixed it up with another radial and actually connected it up! Went on to do remaining tests, but Tester would not do ZS nor RCD tests on that side of the CU. Test button also wouldn't work. Went back to the beginning and found my mistake.

 

[Negrita]

Hi guys I'm an amateur but I have the same problem. Can you explain what you mean by an N-E fault (an earth cable on the neutral and vice versa?) Also if I swich of each MCB one by one and try the test button would that identify which circuit the fault was on? I won't fix it myself but I like to understand what's happening and where.

 

[brianmoooore]

First of all, there is a very low resistance between the earth and neutral wires where they enter your property. If you have a TNC-S (PME) earthing system, they are actually connected directly together at the service head.
If there is a short circuit or bad leakage between the earth and neutral wires anywhere in your property, some of the current flowing normally in the neutral wire on an unrelated circuit anywhere else in the property will find its way through this N-E fault, back through the earth wire to the service head, and then flow into the incoming neutral, so bypassing the N side of the RCD, while the full current is passing through the L side.
Very difficult to diagnose without the correct equipment, but hopefully should become a problem of the past, as RCBOs which switch both N and L (far from all do at the moment) become more common.

 

[Negrita]

Thank you.
So can this be caused by an appliance that is plugged in and has a fault?

 

[Lucien Nunes]

Yes, appliances can be equally responsible.

 

[Negrita]

So unplugging all appliances and retrying the test button might be a good first step - or the house has 11 mcbs might turning them off one and trying the test button by one identify a duff (technical word) circuit?

 

[Lucien Nunes]

Turning off normal single-pole (SP) MCBs won't narrow it down, as they don't switch the neutrals. Hence Brian's comment that over time, with the increasing use of RCBOs with double-pole switching that do isolate the neutrals, troubleshooting will get easier.

 

[brianmoooore]

Thank you.
So can this be caused by an appliance that is plugged in and has a fault?

And turned off at the socket, in most cases. Some sockets have double pole switching, but they are rare.
This is something I've spent hours on the 'phone, trying to explain customers, while trying to avoid being dragged out on 'emergency' calls.

 

[Negrita]

So how common is it for the actual RCB0 to fail?

 

[brianmoooore]

Not unknown, but rare, as long as the test button is pressed a couple of times a year.

 

[Negrita]

Thank you - but what if it hasn't been pressed for say 3 years - I am just buying a socket tester with a "RCD test button" I will try this on a circuit to see if that trips the MCB (I assume that "safely" shorts the live and earth) I will explain the "3 Years later!"