Conductor insulation breakdown

[burnzybubbles]

I’m currently problem solving a lighting circuit issue on a retail park.

newish install (12months old) IR of 12M Ohms across all three conductors, getting some very odd IR readings overall, I got a low reading between two separate disconnected cables, sent my meter off to be calibrated and came back fine, was due the 20th anyway.

I’m struggling to isolate the whole of a circuit because it means the block paving needs to come up and the cables to be dug up.

Split one circuit in half and lights removed, removed from the DB and got my 12M Ohm reading.

no lights on and the circuit holds fine, start plugging lights in and the earth leakage increases, running a TT supply so RCBO only, getting 47mA on start up with all lights dropping to 35mA constant (used MCB for testing purposes) remove lights and test separately on a test rig, and the earth leakage is 0.1mA.

I haven’t dug up every JB on this circuit but every one I’ve checked has been fine on all the other circuits I’ve been investigating, so leads me to assume it’s not a termination issue, plus these light have previously been working without any disturbance of the circuit in between.

my question is, has anyone else seen earth leakage under load through the cable? That’s what my tests point too but I’m going to rock the boat with my findings so I just want to get other people’s opinion before I let my findings known.

thanks

 

[DT1991]

Is all this controlled by one protective device?

 

[Pete999]

Is this a three phase installation? 12 Meg aint a bad reading is it, not perfect I agree? was the test done between phases or to N or to E ? last question so sorry, why is isolating the circuit proving to be so difficult?

 

[telectrix]

damp maybethe issue here, due to all the wet weather recently/

 

[burnzybubbles]

Thanks so far guys, the supply is 3ph but the circuit is 1ph.

it’s a Schneider acti9 DB so individual 30mA 10A type C RCBO’s for each lighting circuit, which has been ramp tested to 28mA

lighting circuit of which there is 4 problematic ones are all oblong wall wash up-lighters sunk into the paving, current circuit is 9 lights in total iirc 25w each and a run of about 35-40 meters.

the reading of ~12M Ohms was taken between L-N, N-E, E-L

Reason for the reluctance to dig them out for isolation is the ground workers mixed sand and cement together for the sleeves that they are wired into making it tedious getting everything free.

We first assumed damp but we are currently working on a circuit under a canopy and relatively dry, all the JB’s have been dry so far, only thing we have found has been a couple of failed plugs on the light fittings but the current circuit has had all the fittings removed.

Now the only thing left to mention is it’s wired in YY type cable that doesn’t have a BS number, it was discussed during instillation and apparently agreed to use by others, it’s not damaged in any way mechanically that I’ve seen so far and it’s fairly well protected.

current thought is to dig up the first light properly, IR the disconnected cable, then add a light and note the increase in earth leakage?

 

[DT1991]

I agree with Tel.

When a heavy down pour occurs these readings will rapidly decrease.

Edit I'm trying to picture this installation you say you have access to the JBs?

 

[burnzybubbles]

They are IP rated JB’s I have access if I dig them out, I’ve currently got the middle light dug out with JB accessible.

would it account for the rise in earth leakage as fittings are applied though, I was seeing and increase of ~4mA per fitting when fitted to the circuit, circuit will hold with 3-4 fittings on circuit.

They are wired through a 100mm duct into 150mm sleeves for each light JB terminated and fly lead with plug for light coming out, Then buried and paving put on top with the light fitting.

 

[DT1991]

Earth leakage and poor IR to earth are two different things.

YY is not suitable to be buried under the ground. This may be your problem.

 

[burnzybubbles]

So back to the question, are you in agreeing that the issue is the cable is broken down and leaking current to earth?

I agree it shouldn’t be buried.

 

[Pete999]

I’m currently problem solving a lighting circuit issue on a retail park.

newish install (12months old) IR of 12M Ohms across all three conductors, getting some very odd IR readings overall, I got a low reading between two separate disconnected cables, sent my meter off to be calibrated and came back fine, was due the 20th anyway.

I’m struggling to isolate the whole of a circuit because it means the block paving needs to come up and the cables to be dug up.

Split one circuit in half and lights removed, removed from the DB and got my 12M Ohm reading.

no lights on and the circuit holds fine, start plugging lights in and the earth leakage increases, running a TT supply so RCBO only, getting 47mA on start up with all lights dropping to 35mA constant (used MCB for testing purposes) remove lights and test separately on a test rig, and the earth leakage is 0.1mA.

I haven’t dug up every JB on this circuit but every one I’ve checked has been fine on all the other circuits I’ve been investigating, so leads me to assume it’s not a termination issue, plus these light have previously been working without any disturbance of the circuit in between.

my question is, has anyone else seen earth leakage under load through the cable? That’s what my tests point too but I’m going to rock the boat with my findings so I just want to get other people’s opinion before I let my findings known.

thanks

It really does point to the YY cable being used "not a suitable choice" however what's done is history.
Rather than start digging you need to Electrically Isolate the cable and test each length of cable used in this case, I can't in all honestly see why you can't isolate the cable, this doesn't mean digging it up.
Test and eliminate cables as you go, a tedious job I know but it will make you look more professional in your Customer's eye's if you apply logic to your fault finding, rather than digging up in places, where the fault MAY NOT be, good luck with it anyway. could come and give you a hand if you pay my fuel and food bill.

 

[DT1991]

I dont know if this is the cause from just what we have been discussing just throwing a few ideas out

 

[Strima]

YY is not suitable for burial even in ducting. It is permeable to water and over time will absorb quite a lot causing insulation breakdown under test.

Who agreed to use YY? Who was responsible for the circuit design and subsequent construction and inspection & testing? i.e. who's paying to replace it all with SWA? Has there been a departure on the EIC?

 

[burnzybubbles]

Rather than start digging you need to Electrically Isolate the cable and test each length of cable used in this case, I can't in all honestly see why you can't isolate the cable

just to clear it up, to get to the ends of each cable I have to dig them out the sleeve that’s filled with sand and cement, but as my reply earlier I think I will do the first one, to be honest, if it is the cable I’ll have to dig them all out anyway to get to the cable duct at the bottom.
[automerge]1574105315[/automerge]

YY is not suitable for burial even in ducting. It is permeable to water and over time will absorb quite a lot causing insulation breakdown under test.

Who agreed to use YY? Who was responsible for the circuit design and subsequent construction and inspection & testing? i.e. who's paying to replace it all with SWA? Has there been a departure on the EIC?

this is all before my arrival, so without digging I don’t know the answers, it’s a big M&E company that are responsible and that will also be replacing the cable.

I don’t know the reasoning for the use of YY it does require a 5 core due to the fittings using Dali, but you can buy 5 core SWA so it’s no excuse.

 

[Pete999]

Yes but you should be able to locate the Faulty cable without exposing the duct, unless of course you intend to replace the YY cable all around.

 

[DT1991]

Does the circuit only contain floor luminaries?