Q: I keep getting intermittent tamper false alarms for no apparent reason but can’t identify why or which tamper is reacting because the panel has a global tamper circuit. How can I identify and fix this difficult, time consuming problem?
A: Global tamper circuits are the scourge of the industry. Apart from someone actually tampering with one of the circuit devices, a tamper fault can be caused by a loose cable joint, faulty tamper switch, cable short, unstable resistance, moisture ingression or induced mains spike – and up to ten tamper circuits can be connected in series to make a global circuit.
Process of elimination: First open the panel and disconnect the global tamper circuit from the panel. Separate each tamper circuit so they can be tested individually. Next, switch your multimeter to the lowest available resistance range and measure the resistance of each tamper circuit.
A 10m length of alarm cable connected to a good tamper switch should measure approx 1ohm and the reading should be stable. Knowing this will enable you to test tamper circuits and identify high or varying resistances.
If one of the tamper circuits is ‘open’ then the fault has been identified. But if tamper circuits are all ‘closed’ with stable resistance readings and intermittent tamper faults keep occurring, troubleshooting is harder.
Visually identifying tamper faults with a Multimeter: Important … The following tests must be carried out with the tamper circuits removed from the control panel terminals.
(1) Switch your meter to the lowest ohms resistance range (e.g. 200 ohms) and connect to the circuit under test. Visually note the resistance reading obtained. Using a terminal screwdriver, tap each alarm detector or joint box in turn and check the meter reading to see if the resistance has changed. Repair or replace any tamper joint or switch which fluctuates up or down in resistance by more than 5 ohms.
(2) Next, switch to the 20VAC range and look for induced AC voltage between DC positive and individual tamper circuits. Any induced voltage above 1.2VAC can cause tamper faults and should be eliminated by fitting any 12V spike suppressor across the 12VDC supply and mains Earth in the control panel.
To finish this month, here are some general tips to prevent false alarms.
Control Equipment: Take the mains supply from the consumer board; Charge batteries overnight before using them; On larger systems, power PIRs from a separate PSU; Make sure PSUs come off the same phase / Earth mains supply; Eliminate induced AC by fitting a 12V spike suppressor.
Alarm Cables: Don’t run alarm cables next to mains or any other service cables; Connect unused spare wires down to Earth; Always use screened cable for keypads, LIMs and iD circuits; Never run data and sounder circuits in the same cable; Use insulated staples in staple guns.
PIR Detectors: Choose PIRs fitted with genuine white light filters; Don’t fit PIRs within 50% of their range facing glass; Avoid placing PIRs close to radiators, fireplaces and boilers etc; Make sure all windows and doors are closed; Seal PIR entry holes, especially where cables come through walls; Use dual technology where insects and rodents are a problem; Make sure the DC supply is above 13V and stable.