Reasons why sensor generates alarm signal

[Guest Modestas]

Hello,

I am not instaler, so sorry for disturbing. I need help from profesionals...monday I want to know why sensors go wrong? maybe there is statistics of these couses...I am writing my bachelour qualification work and I have problem to find statistics...

Regards,

Modestas

[Guest Peter James]

Good branded detectors do not go wrong. (cheap nasty ones pick up all sorts of problems) Your question is a little ambiguos what exactly are you asking ?

Peter

[integrafire]

I am sure everyone has an interesting story as to why detectors have false alarmed. I got called to one site where a Dual tec pir kept going into alarm when no one was in. The device had already been checked and changed by other engineers and cabling belled out to no avail. I noticed on inspection a radiator in the hall on the wall behind the detector. I checked the log and noticed tha detector went off between 1705 ann 1715 evertime. I said to the clien i bet your central heating comes on at 1700hrs. Yes was the suprised answer. The only thing I could think that could have caused it was the water moving in the pipes buried in the wall activated the microwave element, then the heating of the air dicectly around the device triggered the infra red. Moved device a few feet to the left and all ok. Does this sound viable. Also, how the hell does a spider over an inch in diameter get into a pir that only has had the knockout for the cable for entry. Nearly fell off me ladder when that bugger jumped at me.

[CompostCORNER]

I fitted a detector once facing inwards towards the dining room wall which was also the property's dividing wall. Within hours it was falsing. I presumed it was a faulty unit and replaced it. That night, the customer couldn't set the alarm as it just kept going off. I arrived the next day and started fault finding. After spending about half an hour with no viable reason for the activations, I looked up and could see the detector lighting up like hell as if there was a crowd in the room.

I decided to inspect the dining wall the pir was facing just out of curiosity and smack bang in the centre was a large area of hot wall. Their neighbours house, being slightly different in design to the end terrace I had fitted the alarm in, had their coal fire up against this wall. A simple resite of the detector solved this. Ironically, when the alarm was fitted and tested, it was clear as the neighbour must have been out, hence no fire.

Strange but true.

Tony

[Guest Gimmick]

An interesting question, and I have to agree with Peter on this. A good branded (or well known manufacturer) product should not give any false alarms. Like any electronic device you will get a percentage of what is known as "component mortality". Something in the electronics that fails a short time after installation (and power up). If you eliminate these from statistics you generally find poor design and inconsiderate installation are the main causes of false alarms.

Almost every document available on any model (or manufacturer) or detector provides advice on "things to avoid when installing a detector". They often seem rather obvious, but can regularly be missed by installers who lack the experience (and who haven't had their fingers burnt with high repeat call-outs).

PIR detectors operate on temperature changes across focal points in a room (depending on lens type and quality), so in general avoid pointing the detector at heat sources (radiators, pipework, kitchen appliances). Of course the Sun radiates IR energy too so in as many instances as possible, avoid pointing the detector at large windowed areas of a room. Can't be avoided in some cases, so try and present as little of the window as possible to the detector (or see where the sunlight scans arcoss the room during a day, and mount in the opposite corner).

Microwave detectors operate on movement alone and as can be seen in one of the example above, can react to water moving around in pipes (unless they are metal). Always enqure that a room is well protected against drafts, since air movement can displace objects such as blinds or curtains. Enough movement there can trigger a microwave detector.

If you put the two technologies together you get a Dual Tech. sensor, and a common myth is that these products are instantly more reliable than a standard PIR. Unfortunately in a high number of cases, where there are changes in temperature that can be processed as movement (from a PIR), there is associated movement of objects within the room. You would then get a false alarm. Also microwaves can (depending on frequency band) penetrate solid surfaces, giving detection outside the required area. This means that your microwave could be regularly generating false alarms, just waiting on the first IR signal to produce a full alarm condition.

A good quality (not mentioning names of course - can't bias), carefully and correctly mounted PIR can often be more reliable that any other combined technology devices.

One more thing. Seal the cable entry holes (and mounting holes) if possible. Again through the example above . . things get in and move around. A spider in a room is a tiny little insect. Unfortunately when it crawls over the top of the PIR component or the lens . . . . relatively . . it's 15ft high !! I use a silicon based industrial sealant (it's an RS part number if anyone wants it), because it never really sets properly. Since it retains a lot of it's "tacky" feel, harvest flies get stuck even if there are blow-holes and ways in through the cable access hole.

Oooh look another rant.

Regards

Gimmick.

[Guest]

a pir that does'nt fa.... thats why all alarm co's use this same kit is it??

you will always get fa's for nar,if i knew the answer id be very very rich....

here's hoping!!!!!!!!!!!!!!!!

[Guest luckybob]

having worked for a PIR manufacturer for 12 years here are some of my pointers,

Reflective surfaces, mirrors, ceramic hobs in kitchens can be a pain . as the sunlight moves around and falls unpon the pyro it will be like looking directly at the sun!!!!!

The main culprit in my experiance is induced AC, any AC voltage above 2v is enough to make most detectors unstable. Most engineers do not have a clue how to check for it, once identified it is easy to remove using something like an ACT 1313 filter. Its more prevalant on polycarb end stations. Have been out to several jobs where several detectors have been changed and then changed again and again. One simple measurment on the first visit, or on the install! would have been enough.

[CompostCORNER]

My thoughts exactly. Induced AC can be the cause of all the problems. We've taken over dozens of alarms that were persistently troublesome. The problems of falses all but vanish upon fitting both an ACT1313 and ACT230MSF.

Even top end detectors can false. Obviously, you get what you pay for but even good detectors false. As shown in my post above, installation of the detector could cause troublesome operation. How many here fill all the holes around the cables that come through the cieling? How many here fill the cable entry holes in the detector once the cable has been fed through?

I remember Gardiner Tech telling me once that the most likely cause of false alarms is incorrect installation. If you have dodgy detectors and you send them back to Gardiner Tech, they vigourously test the offending detector and will bill you and return the product back to you if no fault is found. Alternatively though, 5 years ago whilst installing Oriel Norwich Union alarms, we sent 60 out of 60 detectors back to Gardiner Tech all with the same batch number on the units. Every one went faulty approximately 6 hours after installation. Not good when our average installed NU alarm was 50 miles away. Gardiner Tech replaced the lot but never conceeded a faulty batch.

Tony

[CompostCORNER]

a pir that does'nt fa....   thats why all alarm co's use this same kit is it??

you will always get fa's for nar,if i knew the answer id be very very rich....

here's hoping!!!!!!!!!!!!!!!!

False alarms don't exist though. Theres no such thing. Something caused the alarm. It's up to us, the installers to use our experience to figure out what caused it. Ring David grant at ACT meters and he'll tell you theres no such thing as a false alarm.

Theres too many people in our trade who turn up on a call-out when an alarm has supposedly falsed and come away from that job with the same phrase on their job sheet.....

'Cause of alarm:- Environmental'

If a draught caused the alarm, remedy it then. Charging the punter £70 just to reset their alarm is scandallous. Move the detector if need be. Fit a quad or dualtec instead of a standard PIR. Get the punter to draught proof the door your detector is facing. Tell the punters to secure the doors inside the building whilst the alarm is in use.

'Cause of alarm:- Spider'

Bung the bloody holes up around and in the detector then. Inform the punter that it won't hurt the detector if they brush away the cobbwebs.

'Cause of alarm:- UNKNOWN'

You need sacking if you put that on your sheet. lol

Then theres those who add 'Double Knock' to the panel programming for an offending detector. The cheaters way out!

Theres always a cause. We have to find it. If you install decent gear, install it correctly, add all the filters and gadgets AND instruct the punter on how to operate the alarm in a fool proof manner that even a small child can grasp, and properly maintain the alarm on a regular basis, you can virtually guarantee that the alarm will never ever false.

Anyone else here who agrees??

Tony

[morph]

There is always a reason and generally it depends on the investigating engineer to have the time/interest to find the cause.

And a good customer interogation technique helps too

But with customers always wanting to drive the price down, as security is a grudge purchase, how do we get them to understand that there are lots of factors to consider that affect the detector.

Salesman wants the job(commision calling) Engineer has to sort the C$%P out, same old story.

Colin.