Advice to Electricians - Cable Capacitance issues with LED Lamps

 Ever installed an LED lamp that stays lit? We have the answer...


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Have you ever installed some LED lamps which have stayed lit even after being switched off?

In short the final circuit cable capacitance causes this phenomenon. It is more common in conventionally wired intermediate and two way switching circuits due to the wide spread use of 3+E cable and the extra parallel capacitance.

A mains powered LED lamp, such as GU10, B22 or Edison fitting, is essentially several LEDs driven by a constant current driver powered by a mains rectifier and smoothing capacitor.

If the charge/discharge cycle (repetition rate) is not long, the LEDs appear to be 'ON' continuously, nevertheless the energy is limited so the LEDs are only just lit. Once more lamps are added the loading on the cable capacitance is such that the individual smoothing capacitors don't reach "firing" possibility.

A tungsten halogen filament lamp doesn't have this difficulty because the filament is eternally loading the cable capacitance and any acquired charge is unable to conquer the thermal inertia.


How to rectify the problem


160K ohm  1/4w resistorA straightforward alternative is to eternally discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across L and N on the final circuit, it is good practice to do this as close to the effected lamp as possible, if this is done across the changed L and N at the lamp holder terminals it enables simple changing out of circuit (or disconnection) for testing.

Do not attempt to use a single 330K ohm resistor because this may not read due to voltage failure.

If the final circuit has a large number of changed lamps it might be pragmatic to connect the chain resistors between long-lasting L and N at a convenient point e.g. at the DB to decrease the number of resistors required, yet the number of points that will need treatment will depend on how the cable capacitance is dispersed on the closing circuit.

Added points especially where 3+E is used for intermediate changing and 2 way may well necessitate additional pairs of resistors.

A notice should be put at the DB stating which final circuits are effected and which lamp holders have been changed, why and how this solution is implemented.

A standard GLS filament lamp doesn't suffer this problem because the filament is forever loading the cable capacitance and any charge that is acquired is not able to overcome the thermal inertia.

The tolerance of an individual LED lamp may be higher than another due to slight manufacturing imperfections which are to be expected and normal in mass production. It is therefore possible that some lamps from the same batch may suffer the phenomenon and others will not. This is not a fault with the lamp.

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