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 4.1.1.3 Screening and Detection. Metallic screening or grating can be used
for window and other portal physical and intrusion detection protection.
Use of screening consisting of breakwire sensor type wire, functioning in
the same manner (electric circuit interruption) as a breakwire sensor, is an
effective application. To be effective, the entire area must be covered
(such as both halves of a double-hung window) and anchored to the building
to minimize the possibility of bypass. Recessed BMS can be employed to
detect movement of removable screening in separate frames. Larger grating
can be protected by fence sensors or by capacitance proximity sensors as
discussed later in this section. The considerations for application and
installation are the same as for the breakwire sensors discussed above and
for the applicable fence and capacitance proximity sensors discussed later
in this section.
4.1.2 Object Protection. Some applications require the protection of
individual objects within a space rather than protection of the entire
space. This is often dictated by operational procedures or cost
constraints. This section provides a discussion of both sensors designed
specifically for object protection and the application of other sensors for
that purpose.
4.1.2.1 Capacitance Proximity Sensors. This type of sensor is specifically
designed for protection of an individual object or series of individual
objects within a relatively small area. The only restrictions are that the
object be metal and insulated from an electrical ground. Successful
applications have ranged from safes and file cabinets to vehicles and
aircraft. Capacitance sensors establish an electrostatic field on an object
and monitor the capacity for electrical energy storage between a specific
metal object or a series of objects and an electrical ground. Any
disturbance in this capacitance, caused by someone or something touching the
object, causes an alarm. Sensitivity adjustment permits extension of the
capacitance field to the close proximity of the object surface or limitation
at the object surface depending upon the application. The better quality
capacitance proximity sensors automatically adjust to small changes in the
capacitance of the protected object(s). The following application and
installation considerations apply to capacitance proximity sensors:
a) Capacitance proximity sensors are relatively insensitive to
environmental factors which may adversely affect volumetric (space)
protection sensors.
b)
Objects to be protected must be isolated to preclude grounding.
c) Figure 18 depicts an application of a capacitance proximity
sensor to a series of objects. Note where the system is grounded and the
relatively large end of line resistance (minimum 10k ohms) required.
d) Capacitance proximity sensors may be used with grid wire or
screening (breakwire) sensors. However, this application is infrequent
since
13.02-38
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