Odd problem on IR Test.

[Raptor0014]

Doing some testing today on a Wet Room installation.

Fed from its own small CU. And consists of

2 x seperate 50A (for 10.5kw shower) circuits
1 x 20a radial for underfloor heating. This runs direct from CU to a fused spur and then from the fused spur to the UFH controller.
1 x 20a radial for 2 x sockets in a storage cupboard and a fused spur from this for the heated towel rail.
1 x 6a light circuit.

Everything fine until I got to the L-N IR test. It kept giving me a reading fluctuating between 0.08 & 0.37 MOhms. L-E and N-E just fine.

Nothing is going bang as it would with a L-N dead short. So it’s not that.

The 2 fused spurs have neons as do the 2 pull cords for the showers All of these I ensured were switched off as I know neons can cause an issue.

However this didn’t solve the issue on the global IR and I’m scratching my head.

The board is a Fusebox RCBO board. I tried doing the test on the supply side of each RCBO to see if that made a difference. But it didn’t.

I thought it may be the UFH controller so I disconnected that completely yet still got the same problem.

I have traced every single cable as it is still at the first fix stage. Sockets etc have just been added as the plasterer and tilers etc aren’t in for another month. Nothing is damaged. All the connections are sound and terminated correctly in the sockets and spurs, the shower cables have a wago on each conductor and are taped up. Towel rail isn’t yet connected but the supply cable is terminated in the fused spur.

What I can’t work out is why every single circuit is being affected by whatever is causing this fault.

Anyone ever experienced this? I could understand one circuit having the issue, but not every circuit.

 

[James]

I would disconnect all cables from the rcbo and test again.

 

[Raptor0014]

That’s the only thing I can think to do when I go back tomorrow. I’ve never had an issue before with Fusebox RCBOs and can’t see why it would cause an issue now.

 

[Ursanelec]

Might be the SPD giving you a false reading, if there is an SPD fitted

 

[Raptor0014]

Might be the SPD giving you a false reading, if there is an SPD fitted

There is an SPD but I had the SPD MCB switched off.

 

[GBDamo]

Are you doing a global IR? Or have the board fully terminated and relying on turning off each circuit to test?

For IV I'd always be individually IR testing circuits, before I teminate them into a board.

I too can't work out how this is happening unless you are.

And if you are think things like lighting drivers, fans etc.

Might be a faulty RCBO.

 

[Raptor0014]

Are you doing a global IR? Or have the board fully terminated and relying on turning off each circuit to test?

For IV I'd always be individually IR testing circuits, before I teminate them into a board.

I too can't work out how this is happening unless you are.

And if you are think things like lighting drivers, fans etc.

Might be a faulty RCBO.

Yes I’m doing a global IR.

Main switch off, SPD MCB off, all 5 RCBO on and all accessories on apart from the 4 switches with neons. There are no lights or fans in the installation. There are 2 standard MK sockets that are terminated correctly. The fused spurs are terminated correctly and the shower switches are terminated correctly. Only other thing is the UFH controller and I disconnected it.

Tested L-E, N-E & L-N from the isolated side of main switch.

I’ve never had any issue doing it this way before. Just seems odd with this board.

I then tried each individual RCBO from the load side and got the same result.

I’m back there today so will take out each circuit and test at the conductor. I had to leave yesterday to get the kids from school otherwise I would have carried on.

 

[GBDamo]

Yes I’m doing a global IR.

Main switch off, SPD MCB off, all 5 RCBO on and all accessories on apart from the 4 switches with neons. There are no lights or fans in the installation. There are 2 standard MK sockets that are terminated correctly. The fused spurs are terminated correctly and the shower switches are terminated correctly. Only other thing is the UFH controller and I disconnected it.

Tested L-E, N-E & L-N from the isolated side of main switch.

I’ve never had any issue doing it this way before. Just seems odd with this board.

I then tried each individual RCBO from the load side and got the same result.

I’m back there today so will take out each circuit and test at the conductor. I had to leave yesterday to get the kids from school otherwise I would have carried on.

Just a quick check with the board isolated is to look at the RCBOs.

With them off check incoming Live terminal to outgoing Live terminals for continuity and outgoing L-N. Shouldn't be any.

I'm thinking there may be a rouge RCBO that's causing all your issues.

Either that or there's something you've missed and disconnecting each circuit from its RCBO and IR testing will narrow it down.

 

[R-fur]

Are the neons on the load side of the switch? I have seen spurs wired with the in and out reversed.

 

[loz2754]

SPD neutral still connected?

 

[buzzlightyear]

Test the Tails only feeding out. On each circuit. Not test from the rcbos, if all clear then you know if rouge rcbo.

 

[Raptor0014]

SPD neutral still connected?

Yes it’s still connected. Don’t normally need to disconnect it, just turn off the MCB.

 

[loz2754]

Yes it’s still connected. Don’t normally need to disconnect it, just turn off the MCB.

Yes but if it is a faulty SPD, disconnecting line and neutral will eliminate the SPD as the source of the problem.

 

[timhoward]

Out of interest is this an EICR or IV? I'm guessing EICR as you have loads connected.

Tested L-E, N-E & L-N from the isolated side of main switch.

The board is a Fusebox RCBO board. I tried doing the test on the supply side of each RCBO to see if that made a difference. But it didn’t.

Were they the compact double pole RBCO's or the taller single pole RCBOs? In other words were you repeatedly sending 500v between every circuit's N and the CPC bar for each circuit you tested?

I'm also considering that the N-E test would have had been sending 500v to the N and PE terminals of the SPD I think, as N wasn't isolated? I currently can't see what harm that would do though.

My working theory is that they are single pole RCBOs, and the N-E test has damaged something connected in an unusual way.

My general fault finding rule is that if all else fails, start looking at what does work. It can help focus the mind! So in this case everything out of RCBO's, leave the CPC's in for now, and see what happens. If still no logic disconnect CPC's. If still no logic disconnect bonding.

For some reason I'm half expecting everything to test clear until you put it back in...!
All the best with it, I'm sure you'll crack it tomorrow.

 

[Raptor0014]

Been back today. Turns out to be 2 of the RCBOs.

With all circuits disconnected from RCBO they all get the expected >999 Mohm across L-N

The 2 x 20A RCBO however give a reading of 0.08Mohm, even with the conductors removed. The 2x 50A & 1x 6A don’t.

So I’m going to speak to Fusebox on Monday and ask why. I could see 1 being a faulty one. But 2 and from the same supplier so likely the same batch. It’s odd that they would give that reading.