Capacitive Inductance

[Arewehere]

Hi Everyone,
I'm just hoping someone has had this problem and found a way to get over it.
I do work for water feature companies, yesterday I came across an awkward problem, they'd installed maybe 70m of 2.5 3 core SWA, down to a pond to run a pump and UV light, load total 7A.
The idea was to spur off a DSSO in the main house via an RCD spur and provide two outlets by the pond for the pump and UV.
Pretty straightforward !!
All connected, power up and RCD will not stay in, sometimes tripping the main house RCD, I know two 30mA trips in line is not ideal but it was specified. So OK I think I've made a mistake, take it apart piece by piece and check, using the insulation tester as I go.
I finally am back where I started with every end open and everything showing as close to infinity as makes no difference. I spent ten years of my career on T&R contracts with a meter in my hand all day so I'm reasonably confident with it, but this had me checking and rechecking, but the figures remained the same for cable and appliances.
All put back together the pump and light will run all the time the earth of the SWA and Sheath are not connected, as soon as they are it will trip the RCD spur as if the is a fault, but the meter will show none at 500v.
It can only be capacitive inductance, the system needs an earth as it's a buried SWA and the pump and UV need to be protected for users safety obviously.
This leaves us in an awkward situation as the cable has been buried and landscaped over some time ago, but the feature has to work.
Has anyone come across the problem of capacitive inductance and found a safe and effective way round it.

 

[mhar]

Well, if it isn''t the cable it will be whatever is on the end of it such as a pump with high earth leakage. Can you PA test it with a decent tester to determine the amount of leakage?

 

[Arewehere]

No, as I said everything tests clear, this is why I can only think it is capacitive inductance, so I was wondering if anyone had any ideas of something that can be added to diffuse the inducted voltage.
It is to do with the SWA as we've had the whole thing running on a couple of 50m extensions plugged into an RCD.
It's just a strange set of circumstances.
Thanks for getting back so quick though

 

[Wilko]

Hi - apologies, I know you’ve said you’ve tested the cable and it passes, but it sounds like it’s N-E damaged. Any joints in the run? Can you power everything up with no loads connected at the far end?

 

[Arewehere]

Hi,
No joints, they've assured me the cable was laid by hand not machine so hasn't been damaged in that way.
I'm not being arrogant but as I said I spent years fault finding etc and this has completely thrown me.
The cable with the two D.P isolators switched off when livened up will knock out the RCD in about two seconds.
Several times I've disconnected the end and tested the cable at 500v and it'll show 200+ core to core brown to black and grey and with say grey to black then brown and black, the sheath is clear to all cores.
Honestly, I didn't give up easily as I only priced for a day and it's an hour away, but I can only put it down to some kind of inductance.
By the way I'm 55 and been doing this too long and this is only the second time I've come across anything like this

 

[Wilko]

Will you be going back there?

 

[mhar]

Test the swa at 1000V?
Have you tried only using two cores and use the armour as cpc?

 

[ipf]

'Capacitive inductance' is a new one to me.
Are we talking reactance......be it inductive or capacitive?

 

[static zap]

UV light does damage stuff . (can it be left out of circuit to rule it out )..
( Is it an old fashioned fluorescent ... Inductive starting spike ? )
IR only going bad when it actually .
a) Is lit
b) IS striking to light.
c) Grumpy tube flickering its --- off/damaging balast

 

[Gromit2014]

What a fascinating (and frustrating!) problem! Is the supply TN-C-S or TN-S? 70m is a long run of cable and I wonder if, assuming at the far end there is some connection to the earth (pump casing, SWA terminated in steel box screwed to concrete etc), there is a differing earth potential between the house and the pond? This may upset the RCDs in some way?

You said that you'd powered it up with two extension leads and it went OK, but without wishing to be rude, did you ascertain that there was actually an earth present at the pond end extension lead? Extension leads take a bashing and it's not unknown for cores to become disconnected.

Another thought was have you checked end to end resistance of each core of the SWA in case it's some obscure cable manufacturing fault. Also it's not unknown for colours to change along a run of cable if they put the wrong coloured PVC in the machine at the factory. Highly unlikely but I had it when I was PAT testing a mains lead once.

Hope you get it sorted soon!

 

[Lucien Nunes]

The OP says the IR is all good and suspects that the tripping is caused by capacitive leakage. As per ipf, capacitance and inductance are different - sort of opposites - and what you are talking about is capacitive susceptance (the reciprocal of reactance, as admittance is to impedance and conductance is to resistance).

The capacitance in question is from the line core to the combined earthed parts, i.e. the earth core (as it's 3-core) and the armour. The capacitance to the neutral core doesn't contribute earth leakage as the current returns to neutral. From the link below I get a figure of 0.25μF /km for the star capacitance of 3-core 2.5mm² BS5467 SWA. Theoretically we have to exclude the neutral core, but it's the only figure I can find. You reckon it's 70m long.

C=0.25/1000*70=17.5nF
Xc= 1/2.pi.f.C =181kΩ
IΔn=230/181k=1.3mA approx.

So you are right to consider that capacitive leakage can contribute measurably to the total, but in this case it's nowhere near causing the RCD to trip on its own. Even if our figure for capacitance is out by a factor of 5, it still can't be the sole cause of a minimum 15mA leak at 230V.

 

[Faizan]

I would suggest you a following read:

Capacitive Reactance in AC Circuit - Electrical Academia - http://electricalacademia.com/basic-electrical/capacitive-reactance-ac-circuit/

 

[plugsandsparks]

I would be looking for cable damage, try another tester and suspect damp, as from my experience damp can be a real pain when it wants to and only respond to a a/c waveform, dont understand how or why but i have had happen to me.

 

[Octopus]

Op .... did you check continuity of all cores before you did the ir tests?

 

[Arewehere]

Thank you to everyone for your replies, between you all it has given me a more balanced perspective on the problem.
I'm due back there next week to hopefully complete things.
I will start with a thorough testing of everything, as we all know there shouldn't be any great mystery to 70m of 2.5 SWA running a couple of spurs to a UV and pump. I'm sure I already have but !!!
What didn't help, was the fact that the house belongs to someone famous and at lunchtime they had a number of guests for dinner, being part of the garden scenery and twice being responsible for knocking out the downstairs power and the CU being in the same room as events, may have upped the ante and made it a little more tricky to keep a logical frame of mind, for testing and fault finding.
Everyone is very nice about things, but we could all do without it !
I'll post the results, in case anyones still interested.