How to find the cause of your false alarms.
COMMENTS Homer SUGGESTIONS

How To Cure False Alarms

MORE PRACTICAL GUIDES

More
Practical Guides
Ron's Alarm
Circuits
Write To Ron More Free-to-Use Circuits Circuit Exchange International


This is not a step-by-step guide. You should begin by reading it all the way through. Then - based on what you've learned - decide on the best place to start looking for your fault.

False alarms are rarely caused by a faulty alarm panel. They are almost always caused by faulty sensors or bad connections.

If your building is divided into zones - the alarm's display panel may point you in the right direction. But there are other useful indicators.

If your false alarms only occur While The Alarm Is Set - then they're almost certainly caused by a fault in one of the Alarm loops.

However - if they also occur While The Alarm Is Switched Off - then they're almost certainly caused by a fault in a Tamper or Personal Attack loop. These loops are active all the time - even while the alarm is off.

Problems begin when contacts, terminals, sensors, junction boxes and tamper switches etc. become corroded. Joins in the wires can also give trouble - especially if they haven't been soldered.

Not surprisingly - corrosion happens most often in places where there's moisture in the air. So - in the absence of any other information to the contrary - it's worth starting your search in the kitchen or bathroom.


Measure The Resistance Of The Suspect Loop

Disconnect the loop from the alarm panel terminals. Then measure the resistance of the loop with your multimeter. The exact figure you get will depend on the length of the loop. But - unless the building is very large - it should normally be under about 20 ohms.

If there are PIR motion detectors in the loop - the reading will be a bit higher. Typically - each PIR will have a 10 to 20 ohm resistor in series with its normally-closed contacts. So the total resistance will - depend on the number of PIRs in the loop.

Some alarm panels also require an "end-of-line" resistor in the loop. It's typically 1k. As the name suggests - "end-of-line" resistors should be fitted into the loop at its remotest point. That is - at the point furthest from the panel.

The Resistance Of The Loop Should Be Steady

Whatever the total resistance of your loop - the meter reading should be steady. If - as you watch your meter - the reading is varying by several ohms - there's a fault in the loop.

Finding that fault is easier if you have a helper. You need one person to watch the meter - and another to visit the various sensors, terminal blocks etc. A sharp tap with the handle of a small screwdriver - will often reveal the culprit.

You can identify a faulty inertia sensor - with a tap. The meter reading will change while you tap. But when you stop tapping - it should return to its original value. If the loop resistance has changed - up or down - the sensor contacts are corroded.

You can repair some inertia sensors by drilling a small hole in the back of the housing - and spraying a little contact cleaner inside. The hole only needs to be large enough to take the switch cleaner's tube. On occasions - I've successfully used WD40.

Give the sensor a good shake - to help scrub off the oxidation. Keep checking it with your meter. When the resistance across its terminals - always returns to a steady reading of about one ohm or less - seal the hole with a little tape - and return the sensor to the loop.

You can test magnetic reed-swich contacts by opening and closing the doors and windows. The meter reading will change when you open the switch. But when you close it again - the reading should return to its original value. If the loop resistance has changed - up or down - the switch needs replacing.

While the loop is disconnected from the panel - you can test individual sensors and switches by measuring the device's resistance directly - across its terminals. The reading shouldn't be more than about one ohm. It should be steady - and it should return to the same value - every time the device is activated.

Another approach is to remove the individual devices from the loop - one at a time. Disconnect the two wires - and twist them together temporarily. If this produces a significant fall in loop resistance - or the meter reading becomes steady - then the device is faulty.

PIRs can become unreliable. Sometimes - they lose their ability to see an intruder. This won't cause false alarms - but it means that the area is no longer protected. Test your PIRs regularly.

Faulty PIRs will also activate - for no apparent reason. This will cause false alarms - but it won't show up in your resistance test.

PIRs can be activated by spiders or insects crawling over the lens of the sensor. They can also be activated by mice or rats - if they can pass close to the lens. Before you condemn a PIR - have a good clean and tidy in and around it. And remove any spider webs in the vicinity.

Then - if you're still getting false alarms - take the PIR out of the loop. Twist the wires together temporarily - and see if the false alarms stop. Be careful to twist the right pair of wires together. Make sure you're joining the alarm loop - and NOT the PIR's power leads.


Make A Note Of The Resistance Readings.

When you have all your loops displaying a steady resistance - and returning to that resistance every time they are opened and closed - make a careful note of the value of each. And write the resistance of each loop on the inside of the alarm panel.

This Is Good Advice. It will help you to locate future faults more quickly - because you'll already know what the resistance readings should be.

Indeed - if you check the resistance readings routinely - say once or twice a year - you should be able to spot a developing fault - long before it becomes a problem.


What If I'm Still Getting False Alarms?

If your satisfied that the loops and sensors are all sound. And you suspect that - after all - the alarm panel may be faulty. Disconnect all of the loops - and replace each with a short wire link. If your panel requires "end-of-line" resistors you'll have to use the correct value resistors - instead of the wire links. If that solves the problem - re-introduce the loops one at a time.

The switching of inductive loads - such as pumps, refrigerators and fluorescent lights - can induce electrical interference into your mains supply. There is also the possibility that the false alarms are caused by interference on the incoming mains - especially if they always occur around the same time of day or night.

Some panels have suppressor capacitors that remove mains interference. They are connected directly across the incoming mains terminals. It may be worth trying a replacement suppressor capacitor - before going to the trouble and expense of replacing your entire alarm panel.


More
Practical Guides
Ron's Alarm
Circuits
Write To Ron More Free-to-Use Circuits Circuit Exchange International