Over the past few days I have been wiring a Siemens SXL-EX panel. I was going through the wiring check-out procedures, and testing the Zones and NACs for correct resistance, but when I got to the NACs, a strange phenomenon occurred. Both read ~5.15K, instead of the required 10K. When the resistor was removed, the NAC terminals would read 10K, and alone the resistor was 10K as well. As soon it was connected back to the system, the resistance reading returned to 5K. For reference as to what is on the system, I have a Wheelock MT installed on NAC 1, and NAC 2 does not have a load, other than the resistor. Could someone explain what is going on, and is it safe to use the system like this, or will it malfunction?
1. Was the panel powered up at the time and does the manual say it should be when you do these tests?
2. Both NAC1 and NAC2 read the same reading but both have different loads?
3. Did you try flipping the polarity of your meter?
4. Does the panel have any troubles when all the resistors and batteries are installed, with no devices connected?
5. You are positive (har har) the resistors you are using are indeed 10K – Brown Black Orange Gold – right?
6. Is your method of reading the resistance correct? You should say exactly how you are doing it.
7. What kind of wire are you using?
Here’s why for each one:
- Whether the panel is powered or not would have an effect. If the manual says that it has to be powered on when you do these checks, make sure you do that.
- See #3
- At first I thought you might have been reading the MT’s resistance but you mentioned NAC 2 is doing the same thing.
- If there’s anything wrong with the panel’s internal circuits then game over.
- This does matter… by your description it sounded a little like the ohm value of your resistors is too low.
- Just checking.
- This matters a lot, as too thin of a gauge can produce a lot of resistance on its own.
Thanks for the quick response.
- The panel was not powered, the manual does not explicitly state whether it should be powered or not, but the process involves connecting and removing wiring from the panel, so I assume it is supposed to be off.
- NAC 1 reads exactly 5.16K, NAC 2 reads exactly 5.15K.
- Switching the polarity did not change the reading, and it is mentioned in the manual that polarity does not matter.
- When I first powered it up when I got it, it showed no troubles and functioned correctly when I hit the drill button, silenced, and reset it. It also successfully entered programming mode.
- Yes, and they read 10K when tested individually.
- I measured resistance by placing the probes onto the tops of the screws on the terminal blocks, and holding them there for a few seconds. The multimeter was set to measure resistance, and displayed the ohm symbol on the screen. I measured the zones in the same fashion and got the correct readings.
- I am using FPLP 14 AWG fire alarm cable (the kind with 2 conductors in the one shielding).
OK so you physically disconnected the wires from the panel before taking the reading? Because the circuit in the panel would screw up the reading…
Not initially, the wires were still connected when I did the first test that came up with 5K. Trying to figure out what was going on, I disconnected them and they read 10K. I’m pretty sure the manual says to take the reading on the actual terminals with the wires connected, though. Is that the problem then?
If you measured the resistance of the nac circuit with the field wires on, you are reading the NAC circuit AND the panels resistance. Wire a 10k ohm resistor to TB-7 NAC-1 and NAC-2 outputs. Power on, the NAC-1 & NAC-2 outputs should be normal, they will reverse polarity in alarm, you will measure 24vdc.