protected objects will be lowest. If the sensitivity is not sufficient in the LO position, then the HI position should be tried.
After setting the sensitivity switch in the desired position, always check for proper sensitivity. Remember that the total
protected capacitance range is changed with the sensitivity switch and that capacitances exceeding the 5, 000 picofarad
or 15, 000 picofarad limits may result in an unbalanced system which will not alarm due to capacitance changes.
Notably, system sensitivity is somewhat affected by the size of the protected capacitance; that is, operating a system with
a 500 picofarad load will result in a higher sensitivity than will a 5, 000 picofarad load.
c. Intermediate Sensitivity Implementation. Intermediate sensitivities can be achieved by adding a stable capacitor
(silver-mica or equal) in series with the protected equipment. This must be done in such a manner that the DC path
through the center conductor of the sensor cable, the 470K ohm terminating resistor, and the outer conductor of the
sensor cable is intact; otherwise, the system will indicate a continuous tamper alarm. Figure 2-3 shows the method of
connecting the capacitor in series with the sensor cable and the protected object. The value of the series capacitor may
be determined by the following formula (all units must be expressed in pf):
CS is the value of the series capacitor in pf
CO is the capacitance of the protected object in pf
S is the degree by which it is desired to reduce sensitivity, always less than one.
For example, let the capacitance of the protected object be 2, 000 picofarads, and it is desired to reduce the sensitivity at
this object to ¼ th its former value.
CS = 0.25 x 2000
500 = 666 picofarads
1 - 0.25
However, it will often be difficult or impossible to define the degree by which it is desired to reduce sensitivity, and the
capacitance of the protected object may not be known. In many cases, therefore, it will be more practical to connect a
decade capacitance box, having settings available in increments of 100 picofarads or less, as the series capacitor. Then
determine a satisfactory sensitivity experimentally and wire in a fixed capacitor of the value indicated by the decade
Figure 2-3. Method of Installing Series Capacitor for Reduced Sensitivity