Fire alarm NAC power supply

fireryan

Senior Member
Location
Minnesota
Last year on a project we had to change out a old NAC power supply panel. The old panel required 2.2k ohm resistors for EOL out in the field. The new panel states it requires 4.7k ohm resistors for EOL. When we changed these out the company who does the programming said you can leave the resistors as the 2.2 k ohm and it will work just fine. For the past 2 months we now get random troubles that show up, sometimes for 3 seconds sometimes for a few minutes. Of course whenever I go to troubleshoot everything is back in normal. Do you think having these 2.2k ohm resistors out in the field instead of the 4.7k ohm could be causing this or should that not be an issue?
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Last year on a project we had to change out a old NAC power supply panel. The old panel required 2.2k ohm resistors for EOL out in the field. The new panel states it requires 4.7k ohm resistors for EOL. When we changed these out the company who does the programming said you can leave the resistors as the 2.2 k ohm and it will work just fine. For the past 2 months we now get random troubles that show up, sometimes for 3 seconds sometimes for a few minutes. Of course whenever I go to troubleshoot everything is back in normal. Do you think having these 2.2k ohm resistors out in the field instead of the 4.7k ohm could be causing this or should that not be an issue?
Yes, they are very likely the issue. A booster power supply usually has two types of resistors. The first type are the ones that go at the end of line for each circuit in the field. Your new panel uses 4.7k ohm for this. The second resistor type is the one that lands on the trigger input from the fire alarm control unit (FACU) NAC circuit. This trigger treats the NAC booster like a field device, and that resistor has to match the FACU NAC resistors. The FACU monitors the trigger input via the resistor on the trigger, and this is where the booster indicates a trouble on the booster back to the FACU. The booster panel is probably seeing the hairy edge of a short circuit on the output NAC's, and when environmental conditions are just right it goes into trouble. The cheapest thing you can do is replace the field end of line resistors.
 

MichaelGP3

Senior Member
Location
San Francisco bay area
Occupation
Fire Alarm Technician
Wondering if being open-minded regarding the deviation from the factory spec has any corollaries in the program code, because, you know, it was easier. Could be interesting to proofread.

It is news to me that there is even one person who programs these systems without ever having installed or serviced the hardware.
 
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