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.
- When connecting more than one loop to a detector, always connect loops in series, and not in parallel. This is done so that if the wires of the loop break at any point, the whole circuit will become open, resulting in a loop fault and the detector going into its "Fail Safe" mode and holding the gate open.
- A pre-formed loop can be used in place of a saw-cut loop. A low-temperature pre-formed loop can be used when hot material is not placed directly on the loop. Pre-formed loops are used when a driveway is composed of gravel or other crushed material.
- A high-temperature pre-formed loop is used when hot tar or asphalt is poured directly over the loops. The pre-formed loop requires at least one inch of sand or dirt fill to act as a thermal blanket when installed under asphalt.
- The loop location must be selected to avoid placing wires directly on reinforcing steel, electrical cables, conduits, or water pipes. Loop wires must be installed a minimum of four inches above metallic objects. Never install loops below metallic objects.
- Never install loops within ten feet of underground electrical de-icing systems.
- Adjacent loops connected to different loop detectors should operate on different frequencies to avoid interference.
- Loops use XLP (Cross-Link Polyethylene) insulated wire. This insulation is rated for use on loops. Use of non-rated wire will result in a loop that will fail, usually with a short circuit to earth ground. This will cause false detections.
- These loops are designed for automobile and light truck traffic. Other types of vehicles passing over the loops (Such as motorcycles, or semi truck trailers) may not be reliably detected. It is recommended that the gate be manually held open in these situations.