RCD tripping

[electrified]

I agree with Bernard.

I would be very surprised from your description if it wasn't a neutral to earth fault after the RCD. Sure, it could be a bad RCD but you said it tested fine so I find that unlikely.
One thing not mentioned yet is that a neutral to earth fault usually means a low resistance or short between CPC and neutral wires, but it could also just be a neutral wire touching anything "grounded".
I would disconnect the lights and extractor wiring from the RCD and do a normal insulation resistance test between neutral and CPC etc. on all the cables downstream of the RCD and take it from there.

disconnected cable going out from rcd, IR tested at 500V with everything disconnected

N-E= >999Mohm
L-N= >999Mohm
L-E= >999Mohm

so every reading is good but it’s still confusing as to why it’s tripping.

 

[mikeyd]

Those readings look ideal to me for that cable.
But I dunno? I'm not a sparky. A bit more explanation might be good, and we do love pictures.
Was it just one cable you tested?

As I understand it, you have a fused connection, with built in RCD. That is supplying some lights and an extractor.
It's difficult to follow which cable you tested.

 

[oldbutnotdead]

Couple of thoughts;
Is the RCD FCU one of those magnetic ones that drops out if supply is interrupted?
When you say 'everything disconnected' was that the loads as well- if yes you need to test the loads as well. LED gear and extract fan might not take kindly to 500v testing, can your gear test at 240 as well (or have you got access to a portable appliance tester?)

 

[electrified]

Those readings look ideal to me for that cable.
But I dunno? I'm not a sparky. A bit more explanation might be good, and we do love pictures.
Was it just one cable you tested?

As I understand it, you have a fused connection, with built in RCD. That is supplying some lights and an extractor.
It's difficult to follow which cable you tested.

The 1.5mm twin and earth comes straight from the RCD fcu down to the 2 gang switch, one switch feeds an extractor fan and the other switch feeds 3 downlights. This makes it 3 cables at the switch, one that feeds it and the other 2 are loads going off.

That’s all that is on the circuit it’s a short run.

So what i did was i took the 1.5mm twin and earth out of the rcd fcu that is going to the switch and did an IR test on all conductors in which all came back as >999Mohm.

 

[oldbutnotdead]

The 1.5mm twin and earth comes straight from the RCD..........

So what i did was i took the 1.5mm twin and earth out of the rcd fcu that is going to the switch and did an IR test on all conductors in which all came back as >999Mohm.

Well that answers one question. Were all the faceplates etc on the switches, fan etc (if the screws are a bit long it's dead easy to nip a core with them). Any sign of that sort of damage on the 1.5 on the load side of the RCD? No stray copper poking out of the terminals when it was connected?
EDIT Not trying to teach granny to suck eggs but you have verified the meter?

 

[electrified]

Couple of thoughts;
Is the RCD FCU one of those magnetic ones that drops out if supply is interrupted?
When you say 'everything disconnected' was that the loads as well- if yes you need to test the loads as well. LED gear and extract fan might not take kindly to 500v testing, can your gear test at 240 as well (or have you got access to a portable appliance tester?)

When i said everything disconnected that is the 3 down-lighters taken off the circuit yes. the extractor fan is not in yet, the 1.5mm 3 core and earth sticks out of the wall with all conductors separated in 4 wago connectors. my meter can read at 250V. The rcd fcu is a knightsbridge and has ‘30mA RCD Type-A Latching’ written on the packaging.

 

[oldbutnotdead]

When i said everything disconnected that is the 3 down-lighters taken off the circuit yes. the extractor fan is not in yet, the 1.5mm 3 core and earth sticks out of the wall with all conductors separated in 4 wago connectors. my meter can read at 250V. The rcd fcu is a knightsbridge and has ‘30mA RCD Type-A Latching’ written on the packaging.

Ah. Hook the downlights back up, see what IR readings you get at 250v
EDIT And yes that device does drop out on loss of supply so that's a possible (if other work is going on in the house or there are occasional power cuts or severe dips)

 

[electrified]

Well that answers one question. Were all the faceplates etc on the switches, fan etc (if the screws are a bit long it's dead easy to nip a core with them). Any sign of that sort of damage on the 1.5 on the load side of the RCD? No stray copper poking out of the terminals when it was connected?
EDIT Not trying to teach granny to suck eggs but you have verified the meter?

Well that answers one question. Were all the faceplates etc on the switches, fan etc (if the screws are a bit long it's dead easy to nip a core with them). Any sign of that sort of damage on the 1.5 on the load side of the RCD? No stray copper poking out of the terminals when it was connected?
EDIT Not trying to teach granny to suck eggs but you have verified the meter?

The switch face

Ah. Hook the downlights back up, see what IR readings you get at 250v

Will this damage them? They are intergrated luceco leds. I will only be able to test between L-N as they are double insulated. If possible can i just test these individually with my meter?

 

[electrified]

Well that answers one question. Were all the faceplates etc on the switches, fan etc (if the screws are a bit long it's dead easy to nip a core with them). Any sign of that sort of damage on the 1.5 on the load side of the RCD? No stray copper poking out of the terminals when it was connected?
EDIT Not trying to teach granny to suck eggs but you have verified the meter?

The 2 gang switch was secured to the backbox during this test and readings still came back good.

 

[oldbutnotdead]

The switch face

Will this damage them? They are intergrated luceco leds. I will only be able to test between L-N as they are double insulated. If possible can i just test these individually with my meter?

An L-N fault wouldn't cause the RCD issue. You SHOULD be OK if you reconnect them to the circuit then connect L and N together at the RCD end then test 240v L + N to E.
Is anyone doing anything else in the house that would be dropping supply to that FCU? Any power cuts, dips etc?

 

[electrified]

An L-N fault wouldn't cause the RCD issue. You SHOULD be OK if you reconnect them to the circuit then connect L and N together at the RCD end then test 240v L + N to E.
Is anyone doing anything else in the house that would be dropping supply to that FCU? Any power cuts, dips etc?

Ok. No nothing that would cause dips or anything. By the way the rcd trips at 24ma on a ramp test. As you mentioned earlier about the magnetic rcd types that trip when power dips, how do i know if i have this type of rcd?

 

[oldbutnotdead]

Ok. No nothing that would cause dips or anything. By the way the rcd trips at 24ma on a ramp test. As you mentioned earlier about the magnetic rcd types that trip when power dips, how do i know if i have this type of rcd?

That 'latching' word in the description- I just stuck the whole description you gave into Google, got a match on TLC site and in the description it says 'Latching (resets after power loss)'.

 

[electrified]

That 'latching' word in the description- I just stuck the whole description you gave into Google, got a match on TLC site and in the description it says 'Latching (resets after power loss)'.

yes it is where i got it from and it’s that one. so this ‘latching’ rcd will trip sometimes if the supply is disturbed voltage drop?

 

[oldbutnotdead]

yes it is where i got it from and it’s that one. so this ‘latching’ rcd will trip sometimes if the supply is disturbed voltage drop?

Yep.

 

[electrified]

Yep.

this could explain why it’s trips if there is no use for maybe 3-4 days, so voltage drop with naturally cause the rcd to trip from say large loads on at the same time?

And i’m guessing rcds and rcbos at the fuse board don’t trip when there is voltage drop as it isn’t ‘latching’