Security & Fire Protection Systems/Optical beam detectors
QUESTION: hello dave ,
i am pretty new to fire and safety field so it would be great if you could shed some light on this problem. well technically speaking an optical beam detector should detect a possible firm scenario under 10secs. however, the beam detector is giving an alarm only in 20 secs .so i was wondering if dis problem arises due to low sensitivity of the detector or other factors like it gradual degradation of IR light due to time etc... i hope you can point out de posible causes. thanking you
ANSWER: Hello Alan, and thanks for your question.
It's hard to say whether the test you describe shows a problem or just normal operation. For example, how did you conduct the test? Did you use an obscuration tool to test, completely block the beam with a piece of cardboard, use artificial smoke, or some other method?
In a typical installation in the United States, a beam would have to be blocked for 8 to 10 seconds at the cutoff threshold, which is usually 35% but can be as high as 50%. A correctly operating beam detector will initiate an alarm signal to the control panel.
But there may be a difference in when the beam detector trips, and when the control panel annunciates an alarm; for example the zone for the beam detector may be equipped with a "retard" as a means to prevent false alarms.
So it's important to figure out if the beam detector relay itself is tripping at ten seconds. There may be an alarm indicating LED on the detector itself that you can see.
The second part of your question concerned degradation of the IR light source. This would not cause an alarm to be delayed, but rather would tend to create false alarms. Remember, the detector works on the principle of light obscuration; less light reaching the receiver causes the alarm. A weak IR source would produce less light to begin with.
I hope this helps, and please let me know what you find. Also, if you have follow-up questions, please write again, and include the make and model of the beam detector and that of the fire alarm control panel.
---------- FOLLOW-UP ----------
QUESTION: hello dave
your reply was very satisfactory . the beam detector used at site is of FIRE RAY make having a model number 50IR . The beam detector is 2 years old and the trigger time is 5- 10min when we did the fault test by covering the reflector completely and fire test by covering it half. so is it advisable to replace the detector ?? Also , it would be great if you could point out the reasons for such a long trigger time. In the mean wile i will see if a retard is placed in the control panel as you have mentioned in your reply.
note: the triggering time is 5 - 10mins ....when ideally it has to be in 10 to 15secs..
Thanks for the follow-up, Alan!
A delay of 5-10 minutes to me indicates a serious issue, and the likely course would be to replace the detector. The good news is that in checking some material on the device, it has a three-year warranty, so since yours is two years old, you should get a free replacement.
Before doing that, though, I'd recommend first verifying that the power requirements are being met (10.2 - 30 VDC, 20 MA). I don't think it's a power issue, but it's always a good idea to eliminate that possibility, since electronics that are under powered sometimes do unexpected things.
It would also be helpful to meter the alarm relay on the device directly during a fire test to prove beyond doubt that the device is not tripping with 8-10 seconds. If it's taking minutes to trip during a fire test, the unit has definitely failed. But if the alarm relay trips in 8-10 seconds as it should, and the fire alarm panel does not immediately follow, then there is a problem elsewhere.
After these tests, I'd contact technical support for Fire Fighting Enterprises. Here's where you can see the contact information: http://www.ffeuk.com/tech-support.html
I did not see a process for a warranty return, but I'm sure technical support can point you to the proper person to handle that.