Safety Loop Installation
Safety Loops are a term used to describe a vehicle detection system that uses one of more "loops" of wire buried underground, in combination with a "Loop Detector", to sense the presence of a vehicle.
It does this by generating a "field" around the buried wires. When a metal mass comes into close proximity of this field, it changes the electrical characteristics of the field by causing a frequency shift. The attached detector senses this change, and triggers a relay output which tells a gate operator that there is a vehicle in the loop.
On a standard slide gate or lift type gate, the safety loop installation will consist of two loops, one on either side of the gate. The minimum distance for the loop to the gate itself is three feet, and the maximum is five feet. If the loops are too close to the gate, the metal of the gate itself can be detected as the gate is moving and cause a false signal. If the loops are too far apart, it's possible for a small vehicle to stop in-between them and allow the gate to close.
Below is an example of how a safety loop is wired. It also shows how the detection field is generated.
Note: The two loops are connected so that the current flow in the wires closest to the gate is opposite of each other. This is done to reduce the chances of the loop picking up the gate itself.