Strange Residual Current Device tripping on lamp post

[newfutile]

During a recent EICR the RCBO keeps tripping , i measure leakage of 2.6 ma when clamped around line and neutral, when i clamp the cpc i get over 10 ma ,its a PME supply 0.09 ZE.

These RCBOs have a functional earth , the RCBO ramp tested at 21 ma .

does anyone have any idea why this is happening ? And how to solve it?

 

[i=p/u]

If IR reads all good and leakage is under the ramp test value, I'd suspect faulty equipment

 

[newfutile]

Ir reads 199 Megs minimum ,there are 4 lamp post so I have removed local fuses to try to track this down.
the cpc current is present even when the main switch is off,but its in flats communal area where all flats are supplied by a ryefield

 

[marconi]

May I have a little more detail please?

a. A sketch of the wiring of the final circuit for the street lamps and could you measure the neutral to earth voltage at the board where is the rcbo.
b. A picture and drawing of how the lamps are connected to their feed including how the lamp metal work is connected to terra firma/earth and the feed armouring and cpc terminations.
c. If the lamp posts are connected to an earth rod what are the currents in the 4 earth conductors please?

 

[pc1966]

The CPC current could be anything, basically street furniture will be an earth rod of some form so any voltage seen at the MET due to currents on the PEN will cause some current to flow.

If the usual leakage is small as it is here then probably there is an intermittent event causing it, say water getting in to a easy-drained junction box or similar. Another cause for RCD tripping can be a rapid change in voltage due to something else kicking in, but usually it needs to be a lot of current suddenly to get a high enough dV/dt to push a troublesome amount of current through the capacitance.

2.6mA works out at about 36nF at 230V, which is probably around 100m of SWA cable at a rough guess. Is that anywhere close to what you see?

 

[Biff61]

Fairly common with cheap LED lamp drivers, to make life easy, connect non LED lamp and see does it happen. then check your reading again, if they are clear and the RCBO doesn't trip then you know the LED driver is the problem.
You will only drive yourself around the bend