Today I had the pleasure of working on a Saturday. It was a service call to track down and locate a ground fault on a Fire Lite FCPS24 NAC power supply. This ground fault is the reason another company felt it was necessary to disconnect the NAC circuit from the main fire control panel to the NAC circuit power supply. Read about here.
We traced out the ground fault on the negative power wire (constant power used for global HVAC shut down) to the rooftop air conditioning units. Upon a close examination of the unit with the ground fault we found that the 120VAC power leg (black) wire on the PAM-1 relay had been pinched in between the 4 S box and cover.
In this case our fire alarm control panel or FCPS24 NAC power supply was powering up the PAM-1 relays. This mean we connected our constant power to the Red (positive) and White (common) wires on the PAM-1 relay. Therefore the black wire is not used. However, the 120VAC lead (black wire) can still transfer a ground fault through the coil.
Always make sure to cap off the black lead and ensure it is not grounded out when using the PAM-1 relay for HVAC shutdown, elevator recall, door holder, etc.
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Interesting! Your example of the PAM-1 Relay lends credence to my diligence in always isolating all unused conductors whenever I connect up ANYTHING with multiple flying leads. Others have said that I'm probably just wasting my time, but as an example, I've come across situations where 3-pole dry-contact leads had one of the leads left unconnected, which ended up causing either an unintended output, a short circuit, or an earth-fault immediately or even years later. I am surprised that the PAM-1 Relay doesn't have complete isolation between its high-power AC coil and its low-power DC coil. Thanks for the tip!
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