Fence-mounted and buried perimeter sensors

Fence-mounted sensors detect the vibration and movement of someone climbing, cutting or lifting a fence. Buried sensors detect someone crossing the ground above them. Both work well when the fence or ground suits them and the installation is careful, and both produce a steady stream of nuisance alarms when it is not.

Fence-mounted sensors

A fence sensor turns the fence itself into a detector. The common types are:

• Vibration sensor cable, attached along the fence fabric, which responds to the flexing and vibration of climbing or cutting.
• Fibre optic cable, which detects disturbance through changes in light passing through the fibre. It carries no electrical power along the fence and is unaffected by electrical interference or lightning.
• Point sensors, small devices mounted at intervals on the fence, each reporting its own disturbance, which can locate an intrusion more precisely.
• Taut-wire systems, tensioned wires that detect being pulled, cut or pushed, often on top of or in front of a fence.

The fence is divided into zones, each fed to a processor channel that analyses the signal and decides whether it looks like an intrusion or like wind and rain. Gates move, so they are treated as their own zones with a separate sensor, such as a contact, a beam or a dedicated gate sensor, that can be masked while the gate is in use.

The fence comes first

A fence sensor can only be as good as the fence. Before installing one, the fence should be checked and repaired:

• Fabric tensioned evenly, with no loose or flapping sections.
• Ties, clips and tension bars secure, so they do not rattle in the wind.
• Posts plumb and solid, with no lean or movement at the base.
• Gates hung so they latch firmly and do not bang against stops or posts.
• The gap at the bottom of the fabric small enough that nobody can crawl under without disturbing it, or a bottom rail or buried skirt added.
• Signs, banners and attachments removed or secured, since they act like sails.
• Vegetation cleared from the fabric on both sides.
• Places where the fence could be bridged, such as nearby trees, buildings, poles or stacked material, identified and dealt with.

Installation and zoning

Cable is attached at the height and spacing the manufacturer specifies, usually with UV-resistant ties, and protected where it crosses gates, posts or ground. Zones are kept to lengths that one camera view can assess, and zone ends fall at logical points such as corners and gates. Where the fence passes over a culvert, ditch or other opening, that gap needs its own barrier and detection, because sensors on the fence above will not detect someone crawling through the culvert below.

Buried sensors

Buried sensors are invisible and follow the terrain, so they suit sites where a visible fence sensor is undesirable or where there is no fence. The main types are:

• Ported coaxial cable, usually a pair of "leaky" cables that create a radio field in the ground and the air just above it. A person crossing disturbs the field, and the processor can often locate the disturbance along the cable.
• Buried fibre optic cable, which detects the pressure and vibration of footsteps or vehicles.
• Pressure and seismic sensors, such as fluid-filled tubes or geophones, which detect the ground disturbance of someone walking above.

Buried sensors ask a lot of the ground. The soil should be stable, uniform and well drained. Standing water, tree roots, buried metal, nearby utilities and heavy vehicle traffic all affect performance. Frost and deep snow change how the ground transmits pressure and how radio fields behave, so performance should be verified through the winter, not only at commissioning in summer. Burial depth and cable spacing come from the manufacturer and must be held consistently along the run. Before any digging, locate existing utilities through your provincial one-call service and any private locates the site needs.

Tuning and testing

Sensors are tuned zone by zone during commissioning, then proven by walk tests that imitate real intrusions. A sound procedure records:

1. The zone, the date, the weather and wind, and the ground conditions.
2. Each test method used, such as climbing at mid-span and at a post, simulated cutting, lifting the fabric, or walking, running and crawling across a buried zone.
3. Whether each attempt alarmed, and in which zone.
4. The processor settings in use.
5. Nuisance alarms observed during the test period, with their likely causes.

Testing should be repeated periodically, commonly on a schedule set in the maintenance plan and after any repair to the fence or ground, and at least once in winter conditions.

A typical case

A fenced utility compound reports several alarms a night from one fence zone, and operators have started acknowledging them without looking. A review of the logs shows the alarms cluster on windy nights. On site, the fabric in that zone has loosened where a vehicle once struck a post, and a sign is tied to the fence with a single cable tie. The post is reset, the fabric re-tensioned, the sign removed and the zone retuned to its original settings. A walk test confirms that climbing at mid-span and at the repaired post is detected, and the nightly alarms stop. The fix was the fence, not the sensor.

Keeping them working

Common mistakes

• Installing fence sensors on a loose or damaged fence and then turning the sensitivity down until it stops alarming, which also stops it detecting.
• Making zones too long for a camera to assess.
• Forgetting culverts, drainage openings and gaps under gates.
• Burying cable in poorly drained ground or close to trees.
• Commissioning in summer and never testing in winter.
• Logging nuisance alarms without investigating them. The record is only useful if someone acts on it, as described in security alarm monitoring and response.

Fence and buried sensors are often paired with cameras so alarms can be assessed; see perimeter detection with video analytics, and for the wider choice of technologies, perimeter detection technologies compared.

How ehZee can help

We assess fences and ground conditions, choose and lay out sensors, define zones, and oversee installation, tuning and walk testing. Design drawings are sealed by a licensed professional engineer where required, and installation is by licensed electrical contractors and qualified security installers. See our perimeter intrusion detection service.

Common questions