Fire Alarm Testing for shorts and open circuits

[bs1224]

I am installing a fire alarm system and have finished running the wire and devicing. I am wondering what i should be reading for when i test between both the supply and return. I have no shorts to ground, but i read resistance between the conductors.

 

[iwire]

I am installing a fire alarm system and have finished running the wire and devicing. I am wondering what i should be reading for when i test between both the supply and return. I have no shorts to ground, but i read resistance between the conductors.

Take a look at the panels manual, they usually tell you the maximum loop resistance.

 

[nhfire77]

I am installing a fire alarm system and have finished running the wire and devicing. I am wondering what i should be reading for when i test between both the supply and return. I have no shorts to ground, but i read resistance between the conductors.

I agree with Iwire read the manual.

What is the reading exactly? Are we talking Mohm?

 

[LarryFine]

I am wondering what i should be reading for when i test between both the supply and return. I have no shorts to ground, but i read resistance between the conductors.

You could be reading the end-of-line resistors, or the devices themselves if they're already terminated. Otherwise, you should be reading open.

 

[iwire]

You could be reading the end-of-line resistors, or the devices themselves if they're already terminated. Otherwise, you should be reading open.

Larry he said 'supply and return' that strongly suggests a class A loop, no EOLs.

 

[G._S._Ohm]

I have no shorts to ground, but i read resistance between the conductors.

If you have a diode test on your continuity checker it will tell you if you are seeing semiconductor devices hooked to the line.
The thing with semiconductors is that the resistance they show depends on how much current or voltage your tester puts out. And it is a current-dependent resistance, like incandescent bulbs
and unlike resistors or insulation resistance or conductor resistance.

 

[LarryFine]

Larry he said 'supply and return' that strongly suggests a class A loop, no EOLs.

Ah, now I see. He's measuring end-to-end loop resistance, not conductor-to-conductor. So, he should be measuring wire DC resistance and nothing else. I'd also check each wire to ground.

 

[gadfly56]

I am installing a fire alarm system and have finished running the wire and devicing. I am wondering what i should be reading for when i test between both the supply and return. I have no shorts to ground, but i read resistance between the conductors.

You will naturally have some resistance from one end of the wire to the other! The typical limit is 100 ohms, divided by the number of conductors. Class B, with two wires would be 50 out and 50 back. Class A is 25 out, 25 back, 25 out, 25 back.

If you have to megger the conductors, don't do this with the devices in place! You'll almost certainly fry any conventional smoke detectors or addressable devices of any kind. Conventional heats and pulls might be OK.