I’m working on an unusual installation where 3 prox readers are connected to a door controller with a positive and negative wire between each of the readers and power. There’s a problem with something in the circuit but it’s not easy to know where the problem is. The readers themselves are fine and I can’t find a problem in the cabling. The system has been undergoing a head end upgrade and has been working perfectly until now.

A: We tend to think that what you’ve got here is a parallel circuit with each of the readers connected to the positive and negative terminals of your power source. Whoever installed these readers was probably trained as a domestic electrician and/or a lighting technician.

The advantage of a parallel circuit is that if there’s a problem with a reader (or any device on such a parallel loop) it will only impact on one door/device. The drag with parallel circuits are that they need loads of cabling and are harder to troubleshoot in that each loop must be checked separately.

The 2 most common faults in a parallel circuit are open circuits or short circuits and in the case of an open circuit the current at the reader will be zero while the other readers will show normal current. If a short circuit occurs, total current measurement across the loop under test will show the full current of the power supply, minus the resistance of the loop.

Typically, with an AC short circuit, the mains fuse blows and this is because all outlets in a home are wired in parallel with mains for greater safety. In any case, it sounds to us like your problem is most likely to be with the controller and/or the management software.