Question about level crossing lights

[py_megapixel]

Level crossings often have wig-wag lights in this pattern:

(I have labelled the lights A, B and C for simplicity of referring to them - for those unable to see the image, A is the left-hand red light, B is the right-hand red light, and C is the amber light underneath those two)

The way these would usually light up is C would light for a few seconds, then A and B would flash alternately. However on all light-controlled crossings that I've come across, for the first few seconds that A and B are supposed to be flashing, they actually start off flashing together, then go out of sync over a period of a second or two until they are flashing alternately.

See this video for a demonstration - the amber lights come on at 11 seconds, the red lights start at 13 seconds, but it takes until about 15 seconds for them to be flashing "properly"

I've wondered for a long time why these they do this - does anyone know?

 

[Ianno87]

Is it to "test" that both red lamps are functioning correctly, before continuing the rest of the lights/barrier lowering sequence continues?

 

[swt_passenger]

Is it to "test" that both red lamps are functioning correctly, before continuing the rest of the lights/barrier lowering sequence continues?

That was my immediate thought too. I just compared a few videos, cos I hadn’t noticed that sequence before either.

 

[py_megapixel]

Is it to "test" that both red lamps are functioning correctly, before continuing the rest of the lights/barrier lowering sequence continues?

That was my immediate thought too. I just compared a few videos, cos I hadn’t noticed that sequence before either.

Potentially, but I don't see why that couldn't equally be done by having them flash one after the other!

 

[MotCO]

Could it be simply a mechanical/thermal thing? Do the red lights come on, and heat up the switch, and when hot, it switches that light off and switches the other light on. When the lights are initially activated, both mechanisms would be cold so both lights come on until the switch is hot, whereupon it swiches one light off and the other light on, until it gets into sequence.

(Apologies for the badly worded response - as you can tell, I am not an electrician or engineer )

 

[superjohn]

I concur with @Ianno87, it looks like an initial self test phase to prove both lamps are working. Quite common in safety critical equipment. It also looks as though they go straight into the alternating sequence but as the right lamp lights first it just stays on briefly after the test. This gives the initial illusion of them being out of sync. They are LED lamps as well and they can create strange frequency effects on digital video recordings.

The switching will be transistor based. Bimetallic strip operated flashing lights of the type described by @MotCO are very much old technology and not common these days, even funfairs have moved on.

 

[jupiter]

If they didn’t both come on together initially and one red wasn’t working (the first one to flash), the sequence would be: amber, nothing, red. Which probably isn’t ideal.

 

[Annetts key]

The way these would usually light up is C would light for a few seconds, then A and B would flash alternately. However on all light-controlled crossings that I've come across, for the first few seconds that A and B are supposed to be flashing, they actually start off flashing together, then go out of sync over a period of a second or two until they are flashing alternately.

The amber lights come on at 11 seconds, the red lights start at 13 seconds, but it takes until about 15 seconds for them to be flashing "properly"

I've wondered for a long time why these they do this - does anyone know?

It’s the design of the ‘modern’ electronic version of the flasher unit. The flasher unit takes in a DC supply voltage and produces two ‘flasher’ outputs, one feed goes to all the ‘A’ lights (as you call them), the other output goes to all the ‘B’ lights. Older designs of flasher (based on relay circuits) did not do this. I don’t however know if this is an intended feature or it just happens due to the design of the flasher unit.

One thing that you may not be aware of, that may also be a consideration in the design, is that all the red lights (apart from any auxiliary wig-wag lights) have current sensing relays to detect that they should be lit. For normal operation, at least one red light must be working in each wig-wag on each side of the crossing. The current sensing relays continuously monitor the lights, but are designed to cope with the on/off current as the lights flash.

The operation and timings of level crossing equipment is tested as part of maintenance and there are both railway requirements/regulations and road/highway requirements/regulations.