Spike suppressors that elimate induced AC voltage and prevents false alarms Next to user error, the biggest cause of false alarms is EMI (Electromagnetic Interference), commonly known as induced AC. When intruder or fire alarm cables come near mains or telephone wires, high induced AC voltage passes onto the DC supply to detectors and control equipment. This induced AC voltage causes random and intermittent false alarms as well as system malfunctions. Very often, mains and other high voltage cables are run next to alarm circuits after the system has been installed. When false alarms occur caused by AC interference, the installer is responsible for getting rid of the problem. Eliminating induced AC is simple by fitting a ‘Spike Suppressor’ as standard in every control panel or PSU. The suppressor connects across the DC supply and to mains Earth, dumping induced AC noise and clamping the DC voltage against ‘spikes’
TESTING - ELIMINATING INDUCED AC: 1: Switch multimeter to the 20 - 50VAC range. 2: Connect probes across any DC+ and Earth. 3: Reading should not exceed 1.2VAC. 4: Fit a ‘Spike Suppressor’, (Red+ Black- across the DC supply and Green to mains Earth). 5: Retest to ensure reading falls to below 1VAC.
Important: i) Some meters can’t measure induced AC on a 200VAC range, ii) If there is no induced voltage showing on a Polycarbonate control panel, an alarm cable is shorted to Earth. iii) On some Metal control panels, the mains supply must be disconnected and both Neutral and Earth removed to test for induced AC. Measurement can then be made between any DC+ and the removed Earth wire
