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Discuss Intermittent Ring fault! Any advice please. in the Electrical Forum area at ElectriciansForums.net

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Hello all, I was hoping for some advice on this ring fault I have come across. It has been intermittent for about 2 months now.

It is a converted church to a children and family centre. Installation is within 10-15 years. TN-C-S supply.

The fault is on a 32A type C RCBO 30ma which supplys 12 DSSO in dado trunking. The run is about 30-40 metres. Wired in 2.5 t+e.

The only equipment plugged in is x4 computers and x4 monitors. X2 photocopier machines and x1 shredder. All equipment is PAT tested.

It has tripped around 6 times in the past 2 months. The tests I have carried out are end to end and the values I am getting are:

r1:0.57
rn: 0.57
r2:0.69

These are fine and with figure of 8 tests comply aswell. I have then carried out IR tests which are around 45-60 meg. I have split the ring and found the incoming legs.

RCD tests pass on x1 and x5 and also ramp test is showing it trips at 25ma.

However today I tested IR again with everything disconnected and had short circuit across all conductors. I then split the ring and tested IR again on both legs and they pass. The only suspect thing is where the cables come from above the boiler room into the kitchen (behind the units) there is mouse bait and also strawberry jam used for bait however the jam looks like it follows the cables into the trunking.

The only thing I have not tried is testing for earth leakage, although I do not have the tester for that. Could anyone help by suggesting anything else I can do? Thank you!
 
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Pete999

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Yep follow the trunking toute remove the lid and physically check the cables that are being chewed by Mickey
 
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Yep follow the trunking toute remove the lid and physically check the cables that are being chewed by Mickey
I have checked all cables in trunking and they are completely fine, will have to get these kitchen units out and then check the cables. The way they have run the cables is very frustrating as i believe they are under the floor void but no access under there due to hardwood flooring! So might have to rewire new legs surface was hoping if there are any more tests to carry out to elminate anything else out, thanks for the reply!
 

Pete999

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Isolate the cables where the exit visual range and re test, bet you find something amiss, good luck ley us know what you find please.
 
I once found a dead squirrel in a neighbours loft still attached to a lighting cable , the cable was still fully live and never tripped out. It hand chewed about 1 inch of insulation clean off before chomping down on the bare love wire.
Those old 3036 fuses took some tripping
 

littlespark

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Those old 3036 fuses took some tripping
What do you do if a fuse "trips"? Reset it?

OP; You say there was a dead short fault, but when you split the ring, the fault cleared?
Have you ruled out damaged socket? Was it disconnected from the legs when it tested fine?
 
One of the devices (x4 computers and x4 monitors. X2 photocopier machines and x1 shredder) may be intermittantly faulty and causing the RCD to trip about once a month(?). How often does the shredder get used?

As the ring measures OK then it may well be something that is plugged in that is causing the intermittant problem.
 
If you are convinced the fault is not on the part of the circuit you can physically access stick in a temporary 4mm from the board to the dado section and see if it still trips before you decide to rewire any part of it.
 
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  • #12
you're gonna find a fried mouse chewing the cable

RIP Mickey
Little -------!
What do you do if a fuse "trips"? Reset it?

OP; You say there was a dead short fault, but when you split the ring, the fault cleared?
Have you ruled out damaged socket? Was it disconnected from the legs when it tested fine?
My bad

Should have said ‘blowing’
It
One of the devices (x4 computers and x4 monitors. X2 photocopier machines and x1 shredder) may be intermittantly faulty and causing the RCD to trip about once a month(?). How often does the shredder get used?

If the ring measures OK (do check the insulation resistance of Phase & Neutral to Earth - you must be able to test that) then it may well be something that is plugged in that is causing the intermittant problem.
this was my inital thought, a faulty piece of equipment somewhere either shorting or to much earth leakage. I did IR all kettle leads individually and they all seem fine but whats not to say its a faulty piece of equipment internally. The client is fine with everything unplugged and circuit energised, so we will see if has tripped again. When IR testing i disconnected cpcs at DB so will leave them connected and IR again to see if it shorting out on a different circuit
 

Lucien Nunes

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45-60 meg
Is one leg significantly lower than another? If you isolate the cables at the ends of the hidden parts, is everything else significantly higher? Although that overall IR is not very low, some parts might be lower than others with enough of a difference to guide you to a fault.
 

James

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I wonder if you have an accumulation of leakage currents from all the IT equipment.
If you work on 2 to 3 mA per device, you have a possible leakage of 20 to 30mA from the computers, monitors and copiers before you take anything else into consideration.
 
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I wonder if you have an accumulation of leakage currents from all the IT equipment.
If you work on 2 to 3 mA per device, you have a possible leakage of 20 to 30mA from the computers, monitors and copiers before you take anything else into consideration.
That is a possibility, the ramp test showed the rcbo trips at 25ma so it could all be accumulated and therefore tripping it. I need to source a earth leakage meter to check this. I will be back there tomorrow so will follow up
 

James

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can you move the circuit off the rcd?
then split the ring and change it to 2 x 20A rcbo's
it will show if you have an intermittent cable fault by tripping the rcbo on that circuit only and leaving the other half of the old ring powered up.
if it is due to an accumulation of leakage from the IT equipment then all will be ok and nothing will trip.
 
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can you move the circuit off the rcd?
then split the ring and change it to 2 x 20A rcbo's
it will show if you have an intermittent cable fault by tripping the rcbo on that circuit only and leaving the other half of the old ring powered up.
if it is due to an accumulation of leakage from the IT equipment then all will be ok and nothing will trip.
I did this over a month ago, as one leg was showing a fault on it so i split the ring and put the good leg on a 20A rcbo radial, however it still tripped a couple weeks later. The run is 30-35Metres anyway so wouldent want to leave it on a radial permanently as would need to upgrade to 4mm. I then went back a month ago, tested both legs are they passed the IR so i put back on the 32A. And now as of yesterday thats when it tripped once again. I asked the client when it first tripped to record times and what was plugged in... did i receive any of that information... no. The frustrating thing is that the readings im getting one time are fine then the next are showing a fault...
 

James

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think about maybe using 16A rcbo's for testing the circuit.
use 2 and split the ring, this will guide you into what area to look at, you can then move the point where you have split the ring and eventually the breaker that trips will change from one to the other. this may help in finding the source of the problem.?
 

Lucien Nunes

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The frustrating thing is that the readings im getting one time are fine then the next are showing a fault...
This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?
 
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This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?
Thanks for the reply mate, i have not tried testing at 1000v yet. Will be removing the kitchen units tomorrow where the cables enter as thats where they have had rats before...
 
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UPDATE! Found the fault. Tested between neutral and earth, however left cpcs in earth bar to see if it was shorting on another circuit or pipes. Tester showed dead short. Traced cables and found they run behind the boxing in for rad pipes. Took off rad cover and there it is in front of my eyes the leg that had been squished.... therefore shorting on to the neutral.... replaced leg of ring and re tested. All good. Must of been plumber who chsnged the stats on the rad cover and squashed the cable wheb putting the cover back. The reason it was tripping every so often was because there was a play centre cabinet for the kids and everytime the kids would push the cabinet it would push the rad cover into the neutral. IR reading are 10megaohms which is not that great but the cable run they have used goes under the floor after the boxing in. All is good now, thanks for everyones help! (Also attatched some pictures)
 

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This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?
Thank you, disturbance was the kids unfortunately
 

telectrix

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looking at those pics.are you sure wet-pants didn't nail the cable after you'd questioned his parentage?
 

Risteard

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UPDATE! Found the fault. Tested between neutral and earth, however left cpcs in earth bar to see if it was shorting on another circuit or pipes.
I don't mean to be unkind, but why did you test without the cpcs connected to the means of Earthing before? It is a requirement of the Wiring Regulations that they be connected when conducting the test.
 

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