Cable theft delays to reduce?

[Annetts key]

Again, thanks for the interesting and detailed reply. I don't think the thing I saw a circuit diagram of was quite the same, but it was similar. However, the precise details don't matter - the important part is that these devices are designed to detect a filament lamp, which either takes a given current when it's working, or takes no current when it fails.

The type of relay used to detect the lamp current has hysteresis. I don't have figures to hand, but the current for the armature to move to close the normally open / front contacts is higher than the holding current.

And the minimum holding current is part of the specification. Below this holding current value, it's a requirement for the normally open / front contacts to open. That is, if the signal is consuming less current than the specified minimum holding current, the GECR contacts will be open and hence the interlocking considers the signal to be unlit (or "black" to use the more common railway term).

Yes, LEDs have a multitude of failure modes including those you describe and a reduction of light output over time. I can't tell you exactly how the proving circuitry in LED signals works because that's not briefed out.

I can tell you that one failure mode of one make of LED signal head results in the signal being lit as brightly as normal, but consuming insufficient current to satisfy the minimum holding current for the GECR.

As part of routine maintenance, the S&T technician is supposed to check that the signal light output is sufficient. And this type of signal is only supposed to have a defined lifespan before the head or the LED module(s) in the head are renewed. The lifespan for the type I worked on was ten years.

 

[MarkyT]

Yes, LEDs have a multitude of failure modes including those you describe and a reduction of light output over time. I can't tell you exactly how the proving circuitry in LED signals works because that's not briefed out.

Different manufacturers may have different concepts and circuits internally.

I can tell you that one failure mode of one make of LED signal head results in the signal being lit as brightly as normal, but consuming insufficient current to satisfy the minimum holding current for the GECR.

That's a right-side failure at least.

As part of routine maintenance, the S&T technician is supposed to check that the signal light output is sufficient. And this type of signal is only supposed to have a defined lifespan before the head or the LED module(s) in the head are renewed. The lifespan for the type I worked on was ten years.

There are special light meters for testing the output of their products in the Unipart Dorman LED Signal Handbook for example.

https://www.unipartdorman.co.uk/wp-content/uploads/2022/09/led_signalling_handbook.pdf

Dorman made the first UK LED signals before they became part of Unipart Rail. Their new head went through a painstaking acceptance process involving a long-term test site at Reading (old platform 5, London end). One of the boffins involved told me they had problems meeting colour stability requirements, particularly for the yellow aspect at varying temperatures. This was solved by directing warm air rising from the heat sinks over the circuit board to maintain a more constant temperature.

In heritage, the South Devon Railway is now 100% LED lit, with a custom-designed unit replacing a traditional semaphore oil lamp. The lamps operate at 12V, and an optional detector module can be included in the circuit, which uses a light-dependent resistor to monitor light output rather than current. It has a voltage-free contact that breaks to indicate a fault condition.