A customer has asked if we can protect a safe by detecting and alarming when a person touches the door – the issue is that the safe is located on the secure side of the access control system, but multiple people have access to the area.

A: Yes – you could use a capacitance proximity sensor to detect people approaching or touching the safe. Such a sensor would work in a similar way to electrical capacitors which consist of a pair of conductor plates separated by a dielectric – a change in the charge results in a change in capacitance between 2 plates.

With capacitance proximity sensors, one plate would be formed by the metal safe being protected, while the other would be formed by an electrical reference ground plane, which had been established around the safe. The safe will need to be from the ground plane using insulating blocks of non-conducting plastic, not wood, so that air surrounds the safe completely and forms the dielectric medium.

The way it would work is that the safe would be electrically charged to a potential sufficient to set up an electrostatic field between itself and the air that forms the direct electric medium. The strength of this field would be safe to touch, yet sufficient to cause a change in capacitance, that can be detected and used to generate an alarm event if a person closely approaches or touches the safe.

When that happens, the electrical conductivity of the person’s body changes the dielectric characteristics of proximity sensor, and these changes alter the capacitance between protected object and reference ground so that a threshold is crossed and an alarm event is activated. It’s possible to tweak sensitivity so such a proximity sensor detects a person approaching the safe, or sensitivity can be reduced, so the intruder must actually contact the safe before an alarm event is generated.

Proximity detection would be possible up to about 1-1.5 metres from the safe, but that level of sensitivity would be best in high security, low traffic applications. In busy environments, 15cm or direct contact is likely to be the best option if you want to avoid false alarms.

Something to bear in mind is that sensitivity of any capacitance sensor is impacted by changes in humidity, as well as the position of other metals in proximity to any protected object such as a safe. Humidity is that it changes the dielectric characteristics of the proximity sensor, and this causes increases in conductivity and a reduction in capacitance. At the same time, decreases in humidity will reduce conductivity.

Another issues can be caused by adjacent metal objects. If a filing cabinet is placed alongside a safe, they may become electrically coupled by the electrostatic field, which increases the size of the capacitor plate and reduces the overall capacitance monitored by the proximity sensor.

A capacitance sensor will have a signal processor which is essentially a balanced bridge circuit with the safe formed part of the circuit. If somebody touches the safe, and the capacitance changes, the circuit is unbalanced, and an alarm is activated. During commissioning this circuit needs to be tweaked to balance the bridge and rebalancing will be required as seasons change and/or metal objects are moved. It’s possible to install sensors with self-balancing circuitry which eliminate the need for manual adjustment.

Something to bear in mind with a capacitance proximity sensor is that it will only activate when the intruder has actually reached their objective. The immediate area needs to be protected by volumetric sensors, and by reed switches installed on entry points, and internal doors to protect the approaches. With an internal issue like this one an event recording camera will record every event that takes place near the safe, making investigations easier and providing significant deterrent.