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.???