Troubleshooting Normally Open Alarm, Automation Circuits
Finding a break in a normally open circuit can be a bit of a fiddle but there are procedures you can follow that will ensure fast trouble shooting.
Remember there should not normally be any current in a normally open circuit such as an alarm zone loop. To find a break, make sure all switches are on – if you leave a switch off you’ll build another break into the circuit. Check all the fuses in the circuit and ensure circuit breakers are no tripped. Now, with your multimeter set to DC volts, measure the load voltage by placing the red probe to the load’s positive and black to negative.
If the measured voltage is 12V any break is internal to the load. In such a case check the load’s internal fuses, disconnect wires from the load and make sure all the load’s internal switches are on before you measure load resistance with a multimeter set to ohms. Almost every load will have resistance (fluoro lights are an exception). If you find a reading of infinite then the open circuit is the load – try reversing the test probes and check the results. If your reading is 0 volts at the load – then there’s no current reaching the load and the trouble is some other place on the circuit. This means you’re going to need to check the entire circuit and all its components.
To do this, set the multimeter to DC and test the circuit components by putting your probes on either side of the component. If you find 12V then you have found the break. If you can’t find a break at any component then you’ll need to search for voltage between load and ground and load and the positive terminal of the power supply/battery.