Odd reading with my Fluke 902

[Retread]

I was working with the building automation guy today trouble shooting some noise in the comm circuit of the EMS system. To find which of the two comm wires goes from the lighting panel to the EMS panel I shorted the two wires in the pair together at one end and used my ohm meter on the other.

This is where it got weird. When I first checked the circuit checked open. When I checked again 0 ohms a dead short. by changing the which lead went to each wire my reading changed from open to short.

Now the meter is a fluke 902 the only thing i came up with is that some circuit in the system is activated by the voltage my meter used to check continuity. reversing the leads changes polarity therefor not activating it. Or a diode or something was blocking it.

The building automation guy had no answers so I thought I would throw it out here for ideas.???

[jburchstead]

I would agree with what you came up with. My Fluke when set to Ohms defaults to impedence so when it detects voltage it will put 2 ohms of resistance on the reading and display the voltage, not resistance. Since it auto-detects either AC or DC a diode makes sense. That feature is great for trouble shooting DDC systems because it eliminates a lot of false readings. You can turn that feature off after you have turned your meter on, at least on mine.

[Retread]

I will look at mine and see is that is the case with the 902 good to know thanks.

I finally found that yes the comm wire went to the roof like I thought but the Building Automation guy said now way it does that he has never seen that. I was getting readings off the daisy chain of control boards on the roof.