Is Protecting Relay Coils Essential or Not?

1. "I recently moved to another company".

2. "they said its not important."

3. "My ex employer used to claim that doing so would present a short circuit potential across the coil and is not acceptable, but my new employer seem to find it insignificant."

funny guy :))

What wire size are you using for the control wiring to these relay coils and indicator lights? They better support 25A. I'm not aware of any code exceptions, you have to always protect the wire by a properly sized fuse or CB.

Me thinks your employer needs to be enlightened. Or not, as Lyn suggests.

Use the code book as justification for what is correct in your industry.

But fault current at 240V through a 25 amp MCB, probably will exceed the fusing current of the control wiring. If you've ever dealt with the trip time of circuit breakers, while sustaining fault current, you learn not to violate the wire size requirement.

My experience was running power through printed wiring boards in avionics. As a kid engineer, I found no one could give me guidance on this. The drafting department would ask you what the steady state current was, and design circuit traces to support that. I had to learn about fault current from my product during flight test. It took an electrolytic capacitor failing short in the power supply. Which smoked the circuit trace, and never tripped the breaker. So it glowed converting FR4 board material to carbon, then ash, which released major amounts of smoke. What made is worse was these traces burn like fire cracker fuses, and if they cross paths with other power traces, or something as innocent as +28/open discrete inputs (which is powered from a +28 CB), it started them on new burning paths. The current level will never trip the breaker. At least I learned something from it.

Typically there are so many ways component can fail and even catch on fire, without tripping a breaker or blowing a fuse. In the O.P.s case, a 4A fuse on a 220V line would still allow a downstream fault to consume 880 watts for a period of time without blowing the fuse. A kW-scale fault for a minute easily has enough power to start a fire. Yes, 4A is a safer limit than 25A, but neither is perfect. Instead one has to use reliably-built relays (thankfully, fully-shorted relay coils are rare, shorts across a few turns are more likely), and mount them overall in metal boxes to contain any high-power fault fires. We've often heard of TVs or computer monitors catching fire, and causing serious trouble, burning down houses etc. Those devices are internally fused, at a fairly low level, but that didn't prevent the fires. Too bad they were made in plastic rather than metal boxes. At least their power supplies could have been in metal boxes (closed-frame) inside the unit. Damn, every plastic-boxed wall wort, rug wort power supply is a walking time bomb, to mix metaphors.

At least approach/notify your Boss once. Trice will be an insult to superiority. Then carry on. If something bad happens, it's his call.

What's the probability of getting a short circuit? How much money and possible collateral damage would result for a short circuit event? If the odds are little, may be by experience your Boss is on the edge of it from experience.

May be the main concerns of the OP is the interruption cause by a short circuit. If the equipment interrupts a mass production line which will eventually has a consequent down time and losses due to it. Definitely no effect on the load equipment but to supply panel there is or wherever a short circuit fault device is located.

If, "the main concerns of the OP is the interruption <of a mass production line>cause by a short circuit" why didn't they say so?

To what mass production line are you referring?

I got the impression that the OP is looking at random drawings.

To further the above opinions, you must also disavow yourself of the idea that a circuit protective device, be it fuse or breaker, will "present" a short circuit potential across the coil. That potential failure mode is ALWAYS there, whether the protective device is in the circuit or not, and regardless of it's size.The circuit protective device simply prevents that risk from causing a fire if it happens.

Now as to what size protective device to use, that depends upon two things: the size and rating of the conductors, and to some extent the ratings of the ratings of any switching devices in the circuit. 25A is probably too high even if your conductors are rated for 25A, because I don't know of any standard control circuit devices rated for moe than 16A and most are rated for 10A or less. The use of a 4A fuse may have been based on some small reed relay type devices in your circuits, often rated less than 5A.

Not sure what you are faced with but in most cases the individual control transformer located in each section of the MCC has it's own fuse based upon the transformer size and the wiring size downstream of the transformer.

As there are usually more than one device in control circuits there usually is not separate fuses for each device in each control circuit.

If one of the devices suffers any failure that presents a current overload that exceeds the control transformer fuse rating, the fuse will open.

A fault of an individual device such as a 200mA relay coil normally does not cause enough fault current in the circuit to interrupt (blow) the fuse.

I have seldom seen individual protective fuses for each device in any circuit.

Also; In many cases the control transformer is properly fused on the primary side and not on the secondary side to take advantage of the transformer impedance during fault conditions to limit the fault current.

The location of the fuse is up to the designer and the size is limited/dictated by the control power transformer and control wiring size which is dependent on the number of devices in the control circuit, the total control circuit current load requirement(s), and the physical restrictions of the switchgear.

If the wiring is of the correct size and the fuses are properly sized, it does not really matter where the fuse is located (primary vs secondary) as long as the fuse interrupts current flow before the wiring and/or transformer is damaged.

My experience with relay, solenoid and contactor coils is that of motion of the armature or moving part is restricted or complete closing is prevented, the impedance climbs. This can take place over a period of time. Determine what effect a rise in impedance has on the circuit.

The draw current of the energized relay coil is very small. Consequently, the wire is also very small. If the coil manages to draw excessive current (long before it becomes a short circuit) it will become an Open circuit and it will no longer be operational. Consequently, it is likely to be more of a nuisance than a significant problem. I am curious as to how the ex employer would prevent a short circuited coil. Perhaps he meant to protect the wire to the relay with a fused power connection? That seems to be excessive concern over a relay unless it drives an auto-destruct circuit.

The usual concern is about the potential voltage spike caused by magnetic energy stored in the coil when the power is interrupted. V = L di/dt so when the current is interrupted the voltage can become very large. Zero cross over circuits help to keep this to a minimum. For DC coils, a simple reverse biased diode protects other electronics upstream.

By the way, a boss with a little bit of information is a dangerous thing. Frequently anything that goes wrong with an electrical circuit is called a "short" by people with only a little bit of information. But a short circuit and an open circuit are two very different conditions. Short circuits usually generate a lot of smoke and occasionally some fire. Short circuits are rare while open circuits are quite common.