Cable voltage drop refers to the lower potential available at the far end of a cable run compared to the potential available at the power supply end. Voltage drop in access control systems is often ignored until it presents a problem and can be difficult to diagnose.
The effect of voltage drop is most often encountered with high current devices such as mag locks at the end of a long cable run. The voltage drop will cause a weak bond and will often cause the bond sense circuit to fail.
A voltage drop fault can be diagnosed by checking the voltage at the lock under load and no load conditions. The no load condition should be very close to the voltage at the power supply. Under load a severe problem will cause a drop of 2 or more volts at the lock while the voltage at the power supply will appear relatively unaffected. The voltage is lost in the cable.
Very occasionally cable damage or a manufacturing fault will cause this problem even on short runs but in the vast majority of cases the long cable run cannot support the required voltage at that current.
This problem is not limited to electric locks; any device with an insufficient cable can be affected. Voltage drop faults are often seen at field door controllers, remote devices and external gates.
In part 2 we will talk about solutions to this problem and some related calculations.