UK Lighting

Confused.com · Dec 12, 2019

Head scratcher

wired external up and down wall lights with 3c &E via a switch for both sensor and switchable lights joining the loop feed through where it's not required. I've an intermittent fault that I'm struggling to identify. I'm not having any issues with the switched lights as when the loop permanent feed is disconnected from source, the switched fittings work as they should. Done the usual checks for loose connections, water ingress, damaged cables etc but cannot see anything untoward. Thought maybe one of the sensor lights could be faulty so changed both (only 2 wait sensors out of 8 fittings), but still having the issue. 6amp MCB supplying the circuit with all fittings being 10w LED downlights or 10w GU10 LED lamp. Can't be a load issue as the fault still occurs with only the minimum amount of lights switched on.
No timing as to when the circuit will trip as had them all on for around 6 hours before I left last night with nothing tripping to receive a message from the customer complaining of the lights being out when she arrived home from work!!!!

marconi · Dec 14, 2019

Because you don't say it affects both sensor and non-sensor lamps, and they have both been left powered on for 6 hours without tripping, I assume you mean that the mcb trip occurs when the non-sensor lamps are already on and then sometime later the sensor lamps turn on. I think the problem is caused by inrush current when the sensor LED lamps turn on; the 6A mcb sees the high inrush current as a short circuit and trips. Quite a common problem with LED lamps. Made worse when the non-sensor lamps are on already and then the sensor lamps turn on because of the 'peaky' current waveform of the LED drivers all adding up.

What we need to do is limit the inrush current which means introducing either a power resistor or Negative Thermal Coefficient (NTC) Thermistor in series with the sensor lamps. The question is where to put them? The NTC Thermistor would get hot (say 150C) and the power resistor warm say 50C. Could a small box be placed near each sensor light to hold the surge suppressor? Or is there any space inside the sensor lamps?

James · Dec 14, 2019

marconi said:
Because you don't say it affects both sensor and non-sensor lamps, and they have both been left powered on for 6 hours without tripping, I assume you mean that the mcb trip occurs when the non-sensor lamps are already on and then sometime later the sensor lamps turn on. I think the problem is caused by inrush current when the sensor LED lamps turn on; the 6A mcb sees the high inrush current as a short circuit and trips. Quite a common problem with LED lamps. Made worse when the non-sensor lamps are on already and then the sensor lamps turn on because of the 'peaky' current waveform of the LED drivers all adding up.

What we need to do is limit the inrush current which means introducing either a power resistor or Negative Thermal Coefficient (NTC) Thermistor in series with the sensor lamps. The question is where to put them? The NTC Thermistor would get hot (say 150C) and the power resistor warm say 50C. Could a small box be placed near each sensor light to hold the surge suppressor? Or is there any space inside the sensor lamps?

If this is indeed the problem, what about changing to a c type Mcb or even a 10A b or c type?
Obviously the circuit will have to be suitable to make that change but let’s be honest in the uk it’s rare to find a lighting circuit that can’t take a 10A C type and still comply.

marconi · Dec 14, 2019

If allowed James's idea is a better first step towards a fix - wish I had thought of it first

telectrix · Dec 14, 2019

~
#2 for the C type.

Confused.com · Dec 14, 2019

That's the next step tbh. Just wanted to be 100% certain there wasn't anything I may be overlooking before increasing the MCB, although I'm pretty sure there's a damaged cable between 2 of the fitting be it a screw or something but the fact it's intermittent is making it difficult to trace.

DPG · Dec 14, 2019

Confused.com said:
That's the next step tbh. Just wanted to be 100% certain there wasn't anything I may be overlooking before increasing the MCB, although I'm pretty sure there's a damaged cable between 2 of the fitting be it a screw or something but the fact it's intermittent is making it difficult to trace.

Don't change the protective device to mask a damaged cable.