A capacitance sensor, also known as a proximity detector, operates by creating an electrostatic field around a specific metallic object it is designed to protect, such as a safe, filing cabinet, or art piece. The sensor and the object form a capacitor with the surrounding environment. When a person or another conductive object approaches and enters this field, their body adds capacitance to the circuit. This change in the electrical balance is detected by the sensor's control unit, which then triggers an alarm.

A. Ultrasonic motion: This sensor detects motion by emitting high-frequency sound waves and analyzing changes in the reflected waves caused by a moving object, not by creating an electrical field.

B. Grid wire electrolytic: This is not a standard sensor classification. Grid wires use a simple electrical circuit on a surface, and electrolytic sensors detect chemical changes; the combination is nonsensical.

D. Balance magnetic: This sensor uses a carefully balanced magnetic field between a magnet and a switch, typically on a door or window, to detect opening, not an electrical field around an object.

1. Fennelly

L. J. (2012). Effective Physical Security (4th ed.). Butterworth-Heinemann. In Chapter 11

"Intrusion Detection Systems

" it is explained that "Capacitance

or proximity

detectors are used to protect metal containers such as safes and filing cabinets. The detector creates an invisible electrostatic field around the object... When an intruder approaches the object

the additional capacitance of the intruder’s body causes the detector to alarm." (p. 201).

2. Garcia

M. L. (2008). The Design and Evaluation of Physical Protection Systems (2nd ed.). Butterworth-Heinemann. Chapter 5

"Detection and Assessment

" describes proximity sensors (capacitance) as devices that "establish an electrostatic field around a metallic object... An alarm is initiated when an intruder disturbs this field." (p. 91).

3. ASIS International. (2021). Protection of Assets (POA)

Physical Security. In the section on Intrusion Detection Systems

capacitance proximity sensors are detailed as creating an electromagnetic field around a conductive object

with an alarm triggered by a change in capacitance caused by an intruder. (Reference to specific page varies by edition

but the concept is consistently covered in the sensor technology chapter).