The word interlock from Wikipedia = “Interlocking is a method of preventing undesired states in a state machine, which in a general sense can include any electrical, electronic, or mechanical device or system”. In broadcasting that “undesired state” is electricity being on inside a cabinet my hand is in, or RF power still going to an overheated filter.
Transmitters have both internal and external interlocks, the internal ones are usually associated with electrical power or high voltage. The high voltage cabinet on my UHF transmitter makes me go through a whole routine before letting me open its high voltage compartment door. I have to turn a key, pull down a shorting handle,insert the key in another lock and turn it and only then can I pull out a key that lets me open the high voltage compartment. That’s a lot of steps to open one little door, and even then I use the shorting stick to be sure.
External interlocks are things like water pressure, coolant temperature in a dummy load, or an RF filter over temp, if any of these fault it takes the RF off. Transmitters always give you at least one external interlock connection you can use for something you think should shut the RF off. In my case, with a Thomson DCX, it’s for the C.I.F. (Constance Impedance Filter).
To run the transmitter the two external interlock terminals must be shorted together, opening the connection shuts off the RF power. The beauty of the normally closed interlock is that you can daisy-chain from one interlock switch to the next, any of them that opens the circuit will kill the RF power. And if a fault develops in a switch or the wiring the circuit opens and the RF goes off, this is called “fail-safe”. A failure causes a safe condition, the RF is off.
This forces you to acknowledge that there is a problem and to fix it to stay on the air. This is a good thing.
If it was wired the other way, so that a normally open circuit kept the transmitter on and only by shorting them would it take the transmitter off, then a failure in a switch or the wiring would keep the RF on, no matter what the interlocks condition. In this case a system failure would set up an unsafe condition which could cause damage to equipment or injury to an engineer.
I recently came across a transmitter that used a normally open external interlock, apparently their thinking was that they wanted to keep the RF on no matter what! If the interlock wiring breaks they wanted to keep the station’s signal on the air. This even came around to bite them in the rear when there was an interlock fault and in trying to get them back on the air, this engineer mistakenly tried to short their external interlock together. Their transmitter was in the other room so I could not hear it start up. I finally gave up and the source of the problem was fixed and they went on the air.
I think a normally open system is good in some situations but it goes against standard practice, confuses engineer (like me) and sets the station up for a major failure in the event that the interlock should open but the wires have busted and no one noticed.
Now the transmitter it’s self did not use a normally open interlock but some engineer had added it to the control interface. I have also heard of the remote control Fail-Safe being bypassed or wired in a similar fashion. Both of these situations are not safe or in the best interest of the station, or it’s engineers who could get hurt, badly.