g/fire_alarm • Posted by u/panel_programming_paul • 1 days ago

Resistance reading on an initiating circuit — how far from spec before you start digging?

EST3 panel, Class B initiating circuit, spec resistance from the panel manual is 30Ω nominal. Current reading is 47Ω. No trouble condition, the panel hasn't generated a fault.

At what resistance deviation do you start investigating a Class B IDC? Is 47Ω vs 30Ω nominal significant enough to warrant pulling wiring, or do I document it and watch it?

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u/smoke_detector_steve • 1 days ago

Thermal imaging is useful for tracking down high-resistance connections on long initiating circuits. A connection that's 15-20Ω high will show as a hot spot when the circuit is loaded. Borrow a FLIR if you have one, or rent one for a day on a complex building.

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u/fire_code_fred • 1 days ago

47Ω vs 30Ω is a 57% deviation — that's worth investigating. Some panels tolerate this without generating a fault, but the elevated resistance is hiding a problem (high-resistance connection, moisture intrusion, damaged conductor). Class B circuits still need to function to 100% of their rated resistance limit. Document it, but also trace the circuit in the area where you suspect the fault.

u/suppression_sue • 1 days ago

Also check your connection points at every device on that circuit. EST3 circuits with SCI devices sometimes have loose connections at the device base that develop resistance over time. Reseat every connection and recheck — sometimes that gets you back to nominal without finding a specific fault location.