When Recessed Contacts Fail Closed; PIRs and Detection Through Glass

December 31, 2002 SSI Staff

Q: I have had trouble recently with recessed contacts going bad on me. When they do fail they seem to fail closed. Any suggestions?

A: Installers need to be very careful in installing recessed alarm contacts. It is very easy to compress the sensitive reed capsule inside an alarm contact. This can be done by applying too much pressure when inserting the contact in the mounting hole.

Avoid tapping in the enclosure with the back of your screwdriver. The problem can also be caused by placing a contact in a hole that has been drilled in green wood in new construction.

To be safe, I would suggest using floating-type recessed contacts such as the Sentrol #1275 “wing-fit” contacts, or the G.R.I. 2120-12 “ribbed” series whenever possible. Q: We have recently had some debate about PIRs detecting through glass. Do they detect body movement through glass?

A: No, PIRs do not see through glass. This concept can be confusing. Infrared energy covers a wide range of wavelengths, typically 700 nanometers (nm) to 20,000 nm. Since the lower end of the range is near the visible light spectrum, it is referred to as the “near” infrared spectrum and the other end being the “far” infrared spectrum.

Typically, window glass will not pass infrared light that is above 2,000 nm. PIRs detect in the far infrared spectrum and, therefore, glass appears opaque. Keep in mind though not to have PIRs look directly at exterior windows as the energy from sunlight can warm the windows and trip the PIR looking at it.

If you have any tips to share or have questions about installations or troubleshooting, E-mail Security Sales & Integration Contributing Technical Columnist Bob Dolph at [email protected]

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