Residual Current

[ElectricMick]

Hi all, working on a 500W halogen today (just went to change bulb for inept owner) and found that there was 3V on the terminals when disconnected via the MCB. Its a TNC-S with a ZE of 0.15 and a ZS of 0.96 on that circuit. Its controlled by an IP66 switch with no earth facility. Any idea's why that 3V is still sat their with it all turned off?

 

[simonatlondon]

Inductance from a live cable, running close and parallel for considerable length buried in wall somewhere?

Similar thing with me today, TT system, 65V on cpc. Ze 6.4 ohms. But zero volts on met inside consumer unit. Cpc's were not connected at every terminal.

 

[ElectricMick]

Its all surface mounted and the cables that run alongside (2) are on the same circuit, which was off at the MCB. I must admit its had me baffled. I won't lose any sleep over it, 3v isn't much, I would just be facinated to know where it comes from as the only cables in the location are on the same circuit, which is switched off!!! Any more thoughts?

 

[ian.settle1]

Its all surface mounted and the cables that run alongside (2) are on the same circuit, which was off at the MCB. I must admit its had me baffled. I won't lose any sleep over it, 3v isn't much, I would just be facinated to know where it comes from as the only cables in the location are on the same circuit, which is switched off!!! Any more thoughts?

As said previousl you can get an induced voltage in the cable if it run next to another cable, even if it is connected to another MCB

 

[ElectricMick]

I see where your coming from now - it was probably off the other lighting circuit. They all run through some trunking before branching off in their various directions. Now it makes sense. Cheers.

 

[4x4 mark]

dragging some dim and distant memory up, would it not be eddy currants from other circuits causing the problem.

 

[ElectricMick]

Could be any or all of above I suppose - but with 3V I'm not going to dwell on it too much. Re tested today with CU all off and no current was present at the halogen. Its sneaking through from somewhere when the whole installation is energised but its not doing any harm so will leave it. I could get the customer to pay me an arm and a leg to chase and rectify but I know they won't pay for it. Thanks all for your thoughts.