Resistors in series with tripping coils in 125VDC power circuit breaker control schemes. WHY ? 1

[bdn2004]

Have been asked to make modifications to the existing 125VDC hard-wired control system at an industrial plant. Below is a portion of that control. It looks like the Amber light drops the voltage to less than the threshold voltage of the control relay that's in series with it. Should an alarm occur (486-2, 486-F, 486-P), full voltage would be applied and it would energize the control relay that trips power circuit breaker. This strategy is used very often. Do they still design 125VDC systems like this today? What is the advantage? This is an 50+ year old plant.

 

[Sn00ze]

Resistors are voltage dividers for the lights. lights are usually rated for 110V and the control voltage is usually 125-130V. So without it you'd risk burning out the coil/light/etc.

(Apologies if I misunderstood what you are asking)

 

[stevenal]

The A light is monitoring the position of the lockout relay as well as the circuity to it. The resistor prevents tripping due to a shorted filament in the bulb and the cool running bulb should last longer. The R light is similar.

Today, I use inputs into a microprocessor relay so that I can monitor all remotely with SCADA.

 

[bdn2004]

Circuit theory 101 I think...the dc coil has no resistance, therefore it's seen as a short-circuit and that Amber light is ON in normal operation. But when full voltage is applied in an alarm situation, the voltage will be the same on both sides of the Amber light - therefore no current and the light goes out, the lockout relay coil reacts, and the lockout contacts open, making the Amber go out until the lockout relay is reset.

 

[cranky108]

In modern systems we use a resistor and an LED. The light is important to tell you that you have a circuit, not just that you have an open or closed breaker.
A relay input can be used, if you want to automate for a failed circuit, but the local indication should also exist.

 

[waross]

The lights are for status and monitoring.
The circuit operation should not change if the lamp is burned out or removed.

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Ohm's law
Not just a good idea;
It's the LAW!