All troubleshooting is a "Guess-and-Verify" process: guess where you think where the problem is located, and verify that's really where the problem is located.
You also have to think global: a system is made up of many parts. Often, a problem seen in one place is caused by something happening somewhere else. The relay you mention is just part of the whole system that is being troubleshoot.
In this case, the problems are that all the door holders are no longer working, and at the same time, the circuit breaker on the main panel board is tripping.
System Separated into Parts
A door holder circuit system has four major parts:
- The Power Supply with its Breaker and the Main Distribution Board Breaker
- The Wiring in the Building along with other equipment in the building attached to the power supply
- The Door Holder Electromagnets
- The Door Holder Relay
Door Holder Relay
The relay coil could be shorting out and causing the power supply, with the circuit breaker, to overload. The method of testing this is to disconnect relay and see if the circuit breaker trips. If the breaker still trips, the problem can't be with the relay because the wires are disconnected.
Door Holder Electromagnet
Something in one of the door holder magnets could be shorting out and causing the power supply, with the circuit breaker, to overload. The method of testing this is to disconnect the door holder electromagnet that you think might be the problem and see if the breaker still trips. If the breaker still trips, the door holder electromagnet could not be the cause because the wires are disconnected.
There are many door holders, so rather than checking each one one-at-a-time, the door holder electromagnets might be something to try as a last resort.
The Wiring in the Building along with any other equipment in the building attached to the power supply
This could be an intermittent problem with the wiring. There could be an intermittent short or an overload that is starting to cause problems with the power supply. There could be added devices that are starting to overload the power supply, Etc. The method of testing this is to disconnect the building wiring from the power supply and wait. If the circuit breaker trips even though the building wiring is disconnected, then the problem is not the building wiring.
Power Supply with its Breaker
The power supply could be having heat problems, or other problems. Even the circuit breaker or the utility panel board breaker could be marginally bad, and being in a warmer box, the circuit breaker could be tripping on its own.
The power supply might seem to be normal, but when the box its in gets warmer, the power supply could fail. The power supply might be overloaded. (If the current being used is more than ½ of the rated current of the power supply, it is actually overloaded because the manufacturer ratings are only for ideal conditions.)
Remember that the power supply and its box get cool when the power supply is turned off. Once the door holder system is working to power the door holders, the box and power supply get warm again. This is also true of the breaker on the distribution board, when there is no current, it will get cooler. Once current is going through it, it gets warmer.
Feel the box itself. If the outside of the power supply box is quite warm, the inside is a lot warmer. Heat could be the issue. You have to find and the cause of the heat, whether it's the power supply itself, or too much stuff has been added, or it really was slightly overloaded in the first place and it's taken years to actually cause a problem.
Remember, also, the system may be bigger than it looks. Is there anything else using utility power in the building that is being powered through the breaker on the main distribution board? Has anything been added? If you know absolutely for certain that there's nothing else, just keep this idea in the back of your mind for later, when there's other things needing troubleshooting.
Troubleshooting is a Guess-and-Verify process. However, when troubleshooting, one has to look at the entire system and not just a specific component of the system.