Ghost A/C Voltage on Fire SLC circuit

C

CypherKeyJ

Member
Location
TN
Occupation
Low Voltage Tech
Hello, I have a very unique situation that in my 20+ years have never came across. AHU= Air Handler Unit


We have a NEW Fire System installed in a Mini Storage Building where everything is metal. On the SLC circuit which is "disconnected from the panel" Using my meter on Ohms, I get nothing shorted across the pair and neither leg has any short to ground. However, when metering A/C voltage I get 0.030-0.125 VAC on either leg to ground. I have isolated the issue down to the AHUs. Here is what I am doing.

All AHUs in the building have their breakers on at the electric panel. I have the disconnect at the AHU off on the AHUs creating the problem. In this scenario, I read NO A/C VOLTAGE to ground. BUT as soon as I turn on 1 or more of the problem AHUs I see the AC Voltage. Interestingly turning on several AHUs at the disconnect increases the voltage but then sometimes reduces the voltage. I'm not sure if it's because the inductance is getting dispersed or what.... Any thoughts on the cause of the Induced A/C Voltage? The Panel manufacturer says any A/C voltage on the SLC circuit for more than 30Sec can Blow the FACP.
 
What is your concern? Is their issues with the fire alarm system? is the SLC using flpr/flpr or MC/emt? seems like some type of coupling. Is there any interconnection between FA and HVAC? Alarm shut down? That's a really small voltage.
I installed a mircom FA system and ultra cheap Mircom intercom/door entry for apartment building and the SLC noise on the intercom wiring was so bad we had to rewire the SLC with shielded wire which reduced it to acceptable level.
 
My concern is that the voltage has already blown 1 panel. The Manufacturer says no A/C voltage can be on the SLC for longer than 30 Sec. Or it can blow the panel.

Its FPLP in conduit and the interconnect with the AHU is us tying into the Duct Detector for Monitoring & Shutdown.
 
CypherKeyJ said:
My concern is that the voltage has already blown 1 panel. The Manufacturer says no A/C voltage can be on the SLC for longer than 30 Sec. Or it can blow the panel.

Its FPLP in conduit and the interconnect with the AHU is us tying into the Duct Detector for Monitoring & Shutdown.
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Use a low impedance meter and see if the voltage goes away .
 
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