More Strange RCD Faults

[securespark]

Hi Guys

Can I pick your collective brains once more, please?

I attended an RCD fault today. Has me a tad stumped.

Supply, TN-S with Ze of 0.16.

16th Ed. split-load board.

RCD tripping, but never, according to the customer, at night. Trips sometimes during the day without any obvious action preceding the trip, but trips every time when upstairs lights (usually several together) are switched on.

So it seems as if whatever is tripping the RCD it is related to a certain load on the upstairs lighting circuit.

However, the upstairs lighting is on the incomer side, not the RCD.

I have IR'd the board using method 2 & got 40 Meg. I realise this figure is not top-end, but neither does it seem to indicate any faults capable of tripping a healthy RCD.

I have tested the RCD and got 34/12ms & a ramp of 23.5mA.

The only obvious fault I've found at the board was that the main earthing conductor had dropped out of the 'bar. A Zs taken from a RF circuit was 0.5 prior to the conductor being replaced & 0.22 afterwards.

Help!!

 

[oharaf]

I know this is an obvious one, but are all the neutrals in the correct bars?

 

[RF Lighting]

Have you checked the tightness of all the neutral conductors on the main switch / meter / cutout?

Is there any flickering on the lights during normal use?

Do the lights trip the RCD instantaneously?

Did you ramp the RCD?

Have you done any IR testing on the RCD side?

I take it you checked all the obvious stuff like polarity at the incommer

 

[securespark]

Difficult to be 100% certain as it is what I call a "bird's nest" installation, but certainly the upstairs lighting neutral is on the right side.

 

[333rocky333]

In the pubs, we used to find circuits with low N to E readings that failed to trip the Rcd, however when other healthy circuits were operated, the Rcd would trip.

We assumed some of the neutral current from the good circuits was passing back through the rcd via the neutral to earth due to the low N to E resistance.
Causing an imbalance and tripping.

 

[ericmark]

I'll guess on borrowed Line. Really borrowed neutral but when two way switching is used one would normally use triple and earth. But years ago it was common only to use a pair and borrow a live from next door switch.
This causes two problems.
1) RF interference with Hum on radio's and TV's etc
2) Unless the borrowed line comes from same MCB as main line an imbalance.
See http://www.ericpalmer.fsnet.co.uk/Images5/Two-way-UnBalanced.JPG the dashed red line shows the normal connection not very good. But I hope you can see what I am talking about.
It would explain tripping with lights.
Eric

 

[securespark]

Have you checked the tightness of all the neutral conductors on the main switch / meter / cutout?

Thought about a loose neutral. Ran out of time, but will check all neutral connections on upstairs lighting circuit. The main switch is OK. Meter & Cutout inaccessible but will ask UU to check their kit. [/quote]

Is there any flickering on the lights during normal use?

Not that I have noticed.

Do the lights trip the RCD instantaneously?

If several upstairs lights are on, AFAICT, yes.

Did you ramp the RCD?

a ramp of 23.5mA.

Have you done any IR testing on the RCD side?

Yup. The whole board belled out at 40 Meg to earth method 2.

I take it you checked all the obvious stuff like polarity at the incommer

Polarity both at the meter/incomer & in the field is good. Managed to get Ze & Zs readings with an automatic polarity test on both: well you know how the Alphatek works!!

 

[Jaymack]

Borrowed Neutral?

 

[Spark123]

Neutral interconnected between two circuits - one on RCD and one non RCD?
Were all switches closed when you IR tested it?
Normal lamps or CFLs?

 

[JACKC]

Had a similar problem in a scout hut I was asked to look at, it had 2 cons units. Con 1 rcd tripping. Ramp test of 22mA. Isolated all mcb's on cons 1 and ramp tested after each mcb reset giving me which circuit was leaking the most. Had small amount on each but with fluorescants operating the rcd operated. RCD tripped even when mcb's of cons 2 operated which made me look for borrowed neutral and was a waste of time as none found. Replaced the rcd and did not trip, carried out another ramp test and was at 28.6mA. Leakage on each circuit negligable and all ok. Still to this day can not explain why cons 2 operated cons1 rcd with no electrical connection but customer happy. Do you have one to swap and carry out retest of ramp and see what it operates at?

 

[securespark]

The guy who visited prior to me already swapped it out thinking the same as you and this one tests ok. So did the last one....

Personally, I've not seen a ramp of much more than 25mA. I know 23.5 is much higher than half the rating, but do you think if it were higher, that would help?

As things stand, I'm going back with a colleague next tuesday. In the meantime, I've asked the family to use plug-in table lamps (except bathroom) and not the upstairs lights just to see if the RCD trips when not using the upstairs lighting circuit.

 

[jason1958]

Clamp meter would be good.
Identify what background leakage is w.r.t. RCD ramp figure.

Identifying why it trips at night.
I assume the same combination of lights @ day means no trip?
In which case, what is coming on at night? E7? Boiler? Immersion? Thermostat? PIR light? Dryer? Washing machine? Dishwasher? PC?

IR means little without visual inspection of every ceiling rose & light-sw

If non-RCD side trips RCD-side it is 1) leakage or 2) borrowed neutral/live. You need a clamp meter to investigate 1 really.

Stick each non-RCD side circuit on an RCBO in turn.
If the RCBO trips then 1) leakage or 2) borrowed neutral/live.
May help pin it down a bit better.

 

[securespark]

jason:you have misunderstood...there is no tripping at night when the upstairs lighting circuit is not used.

 

[jason1958]

Ah, never at night, sorry.

That could indicate it is appliances used during the day generating sufficient background leakage to push the total close to the ramp figure. At night such appliances are off (washer, dryer, heating, immersion).

Clamp meter to see what leakage is?

Putting every circuit on an RCBO soon finds borrowed neutrals

 

[JACKC]

Just another thought, do they have a fish tank with a heater in it. My mates heater caused all sort of rcd tripping fault.