RCD test button not working

[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!"

 

[brianmoooore]

If a RCD isn't tested regularly, the hair trip mechanism can become stuck in place and the contacts of the test button can tarnish. If you can get it to trip, and then 'exercise' the mechanism, by tripping and resetting it several times, they can usually be brought back to life.
Note that anon working test button is a fail when an electrical condition report (EICR) is undertaken, and will result in the installation being declared 'unsatisfactory'.

 

[Negrita]

Thanks for all your advice I really appreciate it - so trying to trip the MCB via a socket tester would show if the problem was with the test button and if the RCD was working or not?
Now I will explain the three years - The Consumer unit is in Thailand! and was installed by a "professional" electrician (at least as professional as you will get in Thailand when we built the house - EICR in Thailand -non existent) 8 years ago.
I tested the RCD at least twice a year until - Covid struck and I was in the UK and my Thai partner was in Thailand we were apart for just over 2 years and asking her to check it would have been a useless exercise and I must admit when I got back in January I forgot to check it until now!

 

[brianmoooore]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

 

[Negrita]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

I understand what you are telling me and of course if I find I need to replace it I will get a "professional" to do that.
This is the tester I am buying - I'll let you know and thanks again

 

[Negrita]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

 

[Negrita]

Sorry to bother you again but I have just noticed that the RCD in my consumer unit has a variable trip can be set to 5, 10, 20, 30 (currently 20) and the socket tester is available in 30 Ma or 5 Ma RCD tester should I buy the 5 Ma socket tester not the 30 Ma tester as being more sensitive?

 

[westward10]

Can you show a picture of the RCD.

 

[Negrita]

Can you show a picture of the RCD.

Sorry for the delay - it's dinner time over here

Can you show a picture of the RCD.

 

[westward10]

The 30ma tester should trip it. Being set at 20ma anything above this which 30ma is should cause it to trip.

 

[Negrita]

The 30ma tester should trip it. Being set at 20ma anything above this which 30ma is should cause it to trip.

Thank you