Came across a job that's been driving me nuts. The unit is running fine since I repaired it, however the fault light on the site's display panel lights every time the unit runs.
No wiring diagrams of course, and the units have been retrofitted from heat pump to cooling only.
On being called out recently, I found the unit had no refrigerant, but also the LP switch was faulty.

Now, it was also wired such that, having traced out the wiring (which is a huge mess, and in a tight panel too!) it appeared that the switch was wired so that there would be a signal to run whether the LP was made or not. So I corrected the wiring (as I saw it) when I fitted the new switch, putting it across the normally closed contacts (Penn switch, 1 & 3 are the NC, 2 & 4 are the LP & HP fault contacts respectively). This is the only way I have ever seen (as far as I can remember) or been taught to wire LP/HP switches - you want a fault signal, you add a core to your cable spec. The signal then goes to power a relay that feeds the compressor contactor signal.
On being recalled to this 'fault light problem' today, I have racked my brains, and finally found legible copy of the original wiring diagram in another plant room (WTF isn't it with the unit it belongs to, or at least in a related plant room!!!!????? ) and I think the LP/H switch is not directly in the control circuit, but paralel to it.
From the original wiring diagram and a lot of lateral thought (remember that the unit has been significantly altered from HtPmp to Cooling only) I now think that the way the switch is meant to be connected in this system is so that with no fault, there is NO signal through the switch. What I thought was the compressor relay is most likely the fault relay. So that when either of the safety switches break, making the fault circuit, this relay pulls in, and the signal for the compressor should be on the normally closed connection so that on activation of the relay, the compressor loses power.

Sorry for the long winded explanation, I wanted to make sure I gave all the info I can recall.

Now, as I said, I have always been taught, and as far as I know, always seen the HP & LP swicthes in series with the compressor contactor circuit. If I'm right about this circuit, they are effectively in parallel. I believe this is not best practice as the relay is another thing that may fail (let's face it, most of us keep a few spare relays in the van). And in this case if it fails, it fails UNsafe, in that although the switches are trying to stop the compressor, it can keep running.

So, have I gone mad? Is it actually a common way of wiring the safety system? Given a choice, would you wire it so that the HP/LP is in series with the compressor control circuit and use the relay purely for fault signalling, or would you wire it so that with the LP/HP made, the relay is OFF, and the compressor circuit is made through the relay's NC connection?

I look forward to your opinions.