Crazy one from the weekend

[phil d]

Went to give my mate a lift with a job over the weekend, doing some much needed repairs on a rather scruffy commercial job. Part of it was replacing some light fittings, the whole place was lit with pendants containing CFL's, now he reckoned he got a belt after he touched 2 earths on a fitting, the circuit was live, he split the earths and got a belt. Ok so here we go, we disconnect the earth at the CU and measuring between this cable and either the earth bar or the neutral bar we were getting around 140 volts on a Fluke multimeter, the more lights you turned on the higher the voltage, peaking at around 170 volts, measure the current and it was around 30 milliamps, old 3036 board no RCD's, we plan to change this board shortly. we IR the circuit and are getting around 35 meg at 500v, start breaking it down and testing the cable between fittings and it's megging clear. we find a couple of rough joints and replace them, also we remove a couple of redundant fittings, power it back up and we have the same readings as before. Anyway to prove a point I end up getting an RCBO and wiring it in using a couple of bits of cable, ramp testing it with nothing on the outgoing terminals and it trips at 22 milliamps, I connect this into the circuit and go to the furthest fitting where I conduct a ramp test and get 22 milliamps tripping current, clearly despite the meter readings I am not actually getting what I appear to be seeing. I have had induced voltages on cables before but never anything like this, if I had the perceived 30 milliamp leakage down the earth then it would/should have taken out the RCBO, but it didn't, wonder if the Fluke is faulty? 

 

[user 28595]

The Fluke probably has very high input impedance so any induced voltages are read. With a high value shunt across the cct, say 100k ohms, you may see expected readings.

cheers, Paul

 

[Phoenix]

Seems quite normal to me, a disconnected earth on a lighting circuit that has any flouro battens or HF fittings, or basically anything slightly 'leaky' will go to somewhere around half supply volatage when measured with a high impedance instrument. Yes 30mA would trip the RCD, but I'd doubt you'd have that much.... how were you measuring it, and if auto ranging instrument could have been 30 micro-amps?

Just this afternoon I located an open circuit cpc on a row of flourescent battens down a corridor with the use of a volt stick...

Why the courgette was your mate opening the cpc to a cirucit that was energised?

 

[phil d]

Seems quite normal to me, a disconnected earth on a lighting circuit that has any flouro battens or HF fittings, or basically anything slightly 'leaky' will go to somewhere around half supply volatage when measured with a high impedance instrument. Yes 30mA would trip the RCD, but I'd doubt you'd have that much.... how were you measuring it, and if auto ranging instrument could have been 30 micro-amps?

Just this afternoon I located an open circuit cpc on a row of flourescent battens down a corridor with the use of a volt stick...

Why the courgette was your mate opening the cpc to a cirucit that was energised?

God only knows why he did what he did, but the strange thing was there were no florrys or anything on this circuit, only those horrid PL lamps in the lampholders. As I said I've seen strange things but never had this before, still every day's a school day.

 

[Rob_the_rich]

The cpc for the lighting circuit might be carrying 8mA leakage current from another energized circuit, by being connected to shared metalwork. Your RCBO would not have "seen" the 8mA because it didn't come from it's live conductors. 

 

[Lurch]

only those horrid PL lamps

So, fluorescents then.

 

[phil d]

So, fluorescents then.

What I meant was there were non in the traditional sense, i.e metal box, they're all in pendants so no earth. Anyway there are a couple of other issues that have been mentioned and I'm wondering whether it's coming in from outside on the service cable, so I'm going back at weekend to do some more tests. I'm suspecting there may be a fault in the supply somewhere.

 

[Tony S]

Phil, can you let me know the results please?