Procedure for tracking down RCD faults

[smart51]

We're coming to the end of having an extension built.  Their electrician has wired it all in but said he cannot sign it off unless we have him replace the fuse board with RCDs at an additional cost.  We agreed and he did it on Friday.  On Saturday, we found that the sockets at the front of the house have no power and the circuit he labelled as downstairs lights is tripped and won't reset.  His reply to my email asking about this was more or less '£90 per hour to fault find, min 4 hours'.

 

With all the lights off, if I reset the RCD for the faulty light circuit, the other light circuit trips as well.  For the sockets, there is only 1 circuit labelled on the box.  All the sockets in the back rooms are good.  All the ones in the front rooms are off.  This seems odd to me.  I wonder if the spare way on the board is the other sockets and he's not labelled it.  That RCD won't reset.

 

I'm an electronics engineer, though not an electrician.  I'm safe to disconnect things and reconnect them.  So what's a good procedure to track down where the fault is?

[Onoff]

The "electrician" should have carried out initial verification works and a series of tests before fitting a new board. There should then be a Schedule Of Tests recording this. 

 

Adding any RCD or RCBO to existing faulty circuits make this sort of tripping likely. An old circuit without such protection can work for years with faults that don't manifest themselves until said devices are installed. 

 

https://www.tlc-direct.co.uk/Book/8.3.2.htm

[ProDave]

You need a new electrician. Shame you did not question this here before you gave him the job.

 

He s only responsible for making the new, or altered circuits compliant with current regulations.

 

The simplest way would have been to fit rcbo's in the existing board if they can still be sourced (i.e not an obsolete make)

 

So what has he done?  Post a picture of the CU as it is now.  I am guessing a split load board with 2 rcd's?

 

What exactly is tripping?  One or both of the rcd's or one of the mcb's?

 

Once I know what you have I can advise more.

 

As for £90 to fault find his own work, the second word in my reply to that would be "off"

 

To test for earth leakage faults you need an insulation resistance tester.  As you are not doing this professionally so are not interested in a calibration certificate you can usually find one for not much money on ebay.

 

But it is also very likely your fault is not earth leakage but a borrowed neutral, very common on old lighting circuits.

[Onoff]

Guessing maybe you have a new board something  like this?

 

On the far right is the double pole isolator switch. Next is an RCD that provides earth leakage protection fed from the 3 MCBs next to it.

 

Further to the left another RCD protecting 2 MCBs / circuits. 

 

 

See the three neutral bars at the top? There should be no links between them.

 

Furthermore, the incoming circuit cabling: If you have two circuits fed from the two MCBs on the left, the circuits neutral wires should go to the far left neutral bar. Anything fed from the 3 MCBs, then those circuits neutral wires should go to the central neutral bar.

[Alan Ambrose]

>>> I'm an electronics engineer, though not an electrician.  I'm safe to disconnect things and reconnect them.  So what's a good procedure to track down where the fault is?

 

OK you'll easiy understand this then. An easy way is literally to measure the residual current 😃 in each circuit. Often things like LEDs bulbs & controllers actually have a few mA of leakage current.

 

If those, along with anything else on that circuit, add up to the trip level (or thereabouts) then ... the RCD trips. There's a bit of subtlety about type of leakage and type of RCD (e.g. DC leakage vs. AC leakage) but it's normally fairly simple.

 

So, if you have something (preferably non-contact) that will measure, say, 0-20mA then it's all quite straightforward. Actual electricians might purchase a special device for that measurement if this is something they come across often.

 

Suspect anything which is a more modern non-linear load - LED drivers, induction hobs, anything with a VFD in it, heap pumps, inverters etc.

 

bigclive on youtube, I think, has some teardowns of RCDs - they're actually quite clever bits of electromechanical engineering.

 

 

 

Edited August 12 by Alan Ambrose

[smart51]

It is the MCB that is tripping, not the RCD.  There are decorators in at the moment so don't want to knock the power off to remove the cover.  I'll do it later when they're done.  

 

This is the new board from the outside.

 

 

 

[Onoff]

24 minutes ago, smart51 said:

It is the MCB that is tripping, not the RCD.  There are decorators in at the moment so don't want to knock the power off to remove the cover.  I'll do it later when they're done.  

 

This is the new board from the outside.

 

 

They are RCBOs, a combination of an MCB and RCD.

 

24 minutes ago, smart51 said:

 

 

[Onoff]

Just now, Onoff said:

 

They are RCBOs, a combination of an MCB and RCD.

 

Just now, Onoff said:

 

As in Residual Current Breaker with Overload.

 

[ProDave]

There are several off there.  Are they all tripping?  Are they tripping individually or do 2 trip when you turn one thing on?

 

Try and get a picture square on, and one with the cover removed.

 

As it appears to all be "lights" that are tripping, I put my money on borrowed neutral (neutrals between one circuit and another connected)

[smart51]

29 minutes ago, ProDave said:

There are several off there.  Are they all tripping?  Are they tripping individually or do 2 trip when you turn one thing on?

 

Try and get a picture square on, and one with the cover removed.

 

As it appears to all be "lights" that are tripping, I put my money on borrowed neutral (neutrals between one circuit and another connected)

The decorators knocked off everything with lights in the name.  They're papering two of the ceilings and wanted to remove the light fittings.

[saveasteading]

54 minutes ago, Onoff said:

They are RCBOs, a combination of an MCB and RCD.

Perhaps , some rainy day, you electric boffins could explain why these exist as various options,  and how they are selected?

Or point to a simple explanation somewhere?

 

'Overload' I think I understand!  Residual ??? why is there current left over?

 

Ever since some of these modern boxes ( I don't want to get too technical) were installed in my old house, we have had issues. No doubt there are many old and duff and incompetently made circuits  that  the old fuse boxes coped with better than the new units. 

We have only used 'proper electricians'  so what we have rewired is probably ok, but there are still some old circuits, probably taken as spurs from the handiest/easiest point.

 

[Onoff]

So on that board you have the orange surge protector, aka SPD on the far right. Then the double pole isolator. To the left is an MCB that is there likely just for the SPD. To the left of that a line of RCBOs.

 

Inside it'll look something like this. Each RCBO should feed a separate circuit. Generally the twin and earth cable comes in at the top on each RCBO. Neutral on the left, line (live) on the right.

 

In the case of a ring main you'll have two neutrals on the left, two lives on the right (sometimes 3 in each)

 

There is just one neutral bar. Each RCBO has a blue fly lead that goes to the neutral bar. All the earths (aka CPCs) go to the earth bar.

 

Check as aforementioned the neutrals on top are in the correct RCBO. 

 

 

[Onoff]

7 minutes ago, saveasteading said:

you electric boffins

 

https://www.explainthatstuff.com/howrcdswork.html#howork

[Alan Ambrose]

>>> The decorators knocked off everything with lights in the name.  They're papering two of the ceilings and wanted to remove the light fittings.

 

It's v easy to trip RCD/RCBOs by shorting N to E when L is swtched off. I would wait until the lights are wired back in properly before spending much time on it.

 

Anyway, it's not rocket surgery - as an EE you should be able to figure it out.

 

[smart51]

The landing light is a culprit.  With the bulb removed, the lighting circuits stay on.  I've had all the downstairs lights on and off and it seems stable.

 

Anyway, I've had the cover off the board and found that the unlabelled way is used and is for the sockets in the front rooms.

 

With the bulb in but switched off, the socket circuit for the front room trips.  What's that a sign of?

 

 

[ProDave]

He has done a neat enough job of that board.

 

14 minutes ago, smart51 said:

The landing light is a culprit.  With the bulb removed, the lighting circuits stay on. ...

 

With the bulb in but switched off, the socket circuit for the front room trips.  What's that a sign of?

So you have a "borrowed neutral" on the landing light circuit.  The usual thing is the landing light get's it's L from the downstairs light circuit and it's N from the upstairs light circuit.  But a socket circuit tripping suggests the original electrician was a bit more "inventive" with where he borrowed the neutral from.

 

In any event some investigation and wiring alterations for the landing light circuit is needed.

 

This will have been a pre existing fault, the only issue here is the electrician should have found this as part of his testing before the CU change.  I think I would tell him to issue the certificate for what he has done, and find a different electrician to fix the landing light fault, one that is not going to charge £90 per hour.

[smart51]

2 minutes ago, ProDave said:

He has done a neat enough job of that board.

 

So you have a "borrowed neutral" on the landing light circuit.  The usual thing is the landing light get's it's L from the downstairs light circuit and it's N from the upstairs light circuit.  But a socket circuit tripping suggests the original electrician was a bit more "inventive" with where he borrowed the neutral from.

 

In any event some investigation and wiring alterations for the landing light circuit is needed.

 

This will have been a pre existing fault, the only issue here is the electrician should have found this as part of his testing before the CU change.  I think I would tell him to issue the certificate for what he has done, and find a different electrician to fix the landing light fault, one that is not going to charge £90 per hour.

^{The electrician is an employee of the firm of builders who are doing the rear extension.  We're stuck with him until he will sign it off, which he wouldn't do with the old board.}

[ProDave]

Do you want to tackle fixing this landing light fault yourself?

[smart51]

16 minutes ago, ProDave said:

Do you want to tackle fixing this landing light fault yourself?

Is it DIYable?  I'd guess I've have to find where the errant neutral comes from and remove it, then find where the live comes from and connect its neutral.

[ProDave]

26 minutes ago, smart51 said:

Is it DIYable?  I'd guess I've have to find where the errant neutral comes from and remove it, then find where the live comes from and connect its neutral.

That depends how competent you feel.

 

At least you can point the electrician where to look if you want to stump up his fee to do it.  It usually means rewiring the offending light, sometimes needing a new cable pulled through between the upstairs and downstairs light switches.  

 

If you want to have a go, post some pictures of upstairs and downstairs light switches with the fronts unscrewed so we can see the wiring, and the landing light fitting with it's cover unscrewed so we can see the wiring in that.

[smart51]

This is the upstairs switch.  All as expected.

 

 

This is at the bottom of the stairs.  Left switch is the landing light.  Right switch is the hall light.  Back left terminal is live and neutral from the RH twin and earth, plus live from the front right terminal.  Front left and centre are N and L to the upstairs switch.

 

 

this is the mother load of 5 twin and earth with PVC tape and everything.

 

 

The ceiling rose just has red, black and earth coming into it.

Edited August 12 by smart51

[Onoff]

I think you'll find some of what you think are neutrals are in fact black, switched lives (that should have red sleeving on them). Only old book I could find with light switches in the old colours:

 

 

 

Edited August 12 by Onoff

[Onoff]

Traditional light switch wiring in new colours:

 

 

 

 

 

Then you've intermediate switch wiring and loop at switch wiring where there's a neutral block in the back box.

 

https://flameport.com/electric/lighting_circuits/lighting_loop_at_switch.cs4

[ProDave]

1 hour ago, smart51 said:

this is the mother load of 5 twin and earth with PVC tape and everything.

 

 

Where is this third switch and which light does it control?   the two posted above that appear to be the switches for the landing light?

[smart51]

6 minutes ago, ProDave said:

Where is this third switch and which light does it control?   the two posted above that appear to be the switches for the landing light?

 

The third switch is by the front door and is for the hall light. 

 

The common on this switch is connected to both the red and black of a twin and earth.  The common on both the switches at the bottom of the stairs is connected to both the red and black of a twin and earth.  I take it this is common practice if you really just want a single wire.  So if you wanted to connect all three terminals of two switches, you'd connect red to S1 and black to S2 of both switches, then have a 2nd T&E and connect both red and black to the common of both switches.

Edited August 12 by smart51