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Did a kitchen refurb' a few weeks ago. One of the things the client wanted was under cabinet lighting. As the lighting circuit did not have an RCD, I powered the light from 2 x FCU's. The uc lighting was supplied by the kitchen manufacturer.

One FCU supplies 2 x led drivers (12v), with 2 x led lamps for each. This one is the one that's flashing, the other FCU set up is fine (3 lights with one driver).

Went there today for the first time to have a look, previously the client replaced the driver, for the set of lights that were flashing, which didn't rectify things. I disconnected the drivers/lights from the FCU cable, and recorded 12v across live & neutral on my tw&e from the FCU. Operated the FCU a few times and the voltage went. Thinking the switch was faulty, I replaced it. The voltage was no longer present, and no flashing. About an hour after I left the fault replicated again.

Again I found voltage across L&N, which only went after operating the FCU.

I've read here about induced voltages, snubbers etc, but could a faulty FCU switch be an issue? Can you add snubbers to 12v systems? FCU cable to lighting points run parallel with RFC, and one led cable from driver runs close to cooker supply cable. Done quite a few under cupboard LED sets, first time I've come across problem.

PS voltage only recorded with voltage tester. PPS flashing is very faint once every 5 mins, from only one of the first set of led lamps.
 
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could be insufficient load on the driver if the 3 are ok and the 2 aren't. try adding a LED.
 
I was thinking of that, but Thai instructions did suggest a minimum of one, only pictorially though. Worth a go.
 
What can you do if there is NO access to the neutral (ie you can't use a snubber)?
 
If the FCU is used as the switch, is there room to put the snubber in the back of the FCU? As the problem cleared for a while after changing the FCU is it damp building up inside the FCU from recent plastering?
 
Disconnect one leg of the elv side and put your dc amp-meter in-between and see whats being drawn and compare that with the specification on the driver.

I'll give that a try, but it's one of those plug & play's with small male-female sockets.

What can you do if there is NO access to the neutral (ie you can't use a snubber)?

Yep, I've seen from some wiring diagarms that it's a switched live and solid neutral, although I could always put the load neutral into the neutral IN at the FCU? But newbie with snubbers.

If the FCU is used as the switch, is there room to put the snubber in the back of the FCU? As the problem cleared for a while after changing the FCU is it damp building up inside the FCU from recent plastering?

Even though its a 35mm back box, think its gonna be a tight squeeze. Don't think there's any damp there, it's an old dot & dab wall.

Spoke to the manufacturers of the led lights, who were of no help. Spoke to Danglers, CAPLOAD is of no use for this problem, although they suggest their RESLOADE, but no guarantees. Also spoke to Timeguard about their ZV900, but don't think its a goer. Seems Maplins/RS online is the only place for these snubbers?

Yesterday, I connected up the lv cable's as per the set up that's working ok (only two led lights connected Tel). Got a response from the customer today, that there's no flashing since I left, but not holding my breath.

Done quite a few u/c lights like this, with my cables running parallel with other cables, in ceiling voids etc with no issues, first time I've come across this.
 
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Just an update, problem solved I hope, but with an slightly disturbing discovery.

The issue seemed resolved for a day, but then the customer told me it was happening again. So with your advice in mind, I went back and had another look. The LED were flashing at one set every 2-3mins. I disconnected the led drivers/lights and found a small voltage across live & neutral (with voltage tester only, must invest in analogue meter). I rerouted the 'supply' to the lighting point, and decided to try putting 'switched' neutral into the 'in' neutral of FCU.

Reconnected LED/drivers & re-energised circuit and turned on FCU, LED's illuminated. Switched off FCU, and......LED's remained illuminated!? Sort disbelieving what I was seeing, operated switch once again, lights still remained illuminated, until FCU made a clunking noise and LED's went off.

Suspecting faulty FCU, I replaced it with different manufactured one, and flashing.....fingers crossed, has seemed to have disappeared, I was unable to replicate the fault with the FCU switch, i.e. disconnecting neutral but not the live.

I thought I had this fault with the original FCU (same make) and replaced it, but it appears the replacement had the same fault, a rather dangerous one. Gonna return FCU to wholesalers, with a view to reporting the suspect fault to the manufacture.
 
Thanks for the update. So the current to charge up the reservoir cap in the driver was capacitive leakage from the neutral cable to earth, as the line was still energised. As you say, about the most dangerous kind of fault to have in a DP switch!
 
Maybe worth doing some IR testing of the FCU (between each combination of L, N, E and IN/LOAD side). Daz
 

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