VEHICLE LOOP DETECTOR INSTALLATION & USER GUIDE

INSTALLATION & USER GUIDE

The detector must be installed in a convenient weatherproof location as close to the loop as possible.

WARNING:

1) Installation and operation by service personnel only,

2) Disconnect power before working on this unit.

A correct loop configuration and detector installation will ensure a successful inductive loop detection system.

OPERATIONAL CONSTRAINTS

Cross talk

When two loop configurations are in close proximity, the magnetic field of one can overlap and disturb the field of the other. This phenomenon, known as crosstalk, can cause false detects and detector lock-up.  Crosstalk can be eliminated by:

a. Careful choice of operating frequency. The closer together the two loops, the further apart the frequencies of operation must be.

b. Where possible, a minimum spacing of 7 feet between loops should be adhered to.

c. Careful shielding of feeder cables if they are routed together with other electrical cables.  The shield must be earthed at the detector end only.

Reinforcing

The existence of reinforced steel below the road surface has the effect of reducing the inductance, and therefore the sensitivity of the loop detection system.  In this case, two additional turns of wire should be added to the loop.

The ideal spacing between the loop cable and steel reinforcing is 6 inch, although this is not always practically possible.  The slot depth should be kept as shallow as possible, taking care that no part of the loop or feeder remains exposed after the sealing compound has been applied.

INSTALLATION INFORMATION

Loop and Feeder Specification

The loop must consist of insulated wire with a minimum copper cross-sectional area equivalent to 1.5 square millimeters. The feeder should be of the same material, but twisted, a minimum of 20 twists per meter.

Joints in the loop or feeder are not recommended.  Where this is not possible, joints are to be soldered and terminated in a waterproof junction box. This is extremely important for reliable detector performance.

Where long loop feeders are used, or feeders are routed together with other electrical wiring, the use of a screened cable is suggested for the feeder.  The screen must be earthed at the detector end only.

Sensing Loop Geometry

Sensing loops should, unless site conditions prohibit, be rectangular in shape and should normally be installed with the longest sides at right angles to the direction of traffic movement.  These sides should ideally be 3 feet apart.

The length of the loop will be determined by the width of the roadway to be monitored. The loop should reach to within 1 foot of each edge of the roadway.

In general, loops having a circumference measurement in excess of 30 feet should be installed using 2 turns of wire, while loops of less than 30 feet in circumference should have three or more turns.  Loops with a circumference measurement of less than 18 feet should have four turns.  It is good practice at time of installation to construct adjacent loops with alternate three and four turn windings to minimize cross talk.

Loop Installation

All permanent loops should be installed in the roadway by cutting slots with a masonry cutting disc or similar device.  A 45º crosscut should be made across the loop corners to reduce the chance of damage that can be caused to the loop cable at right angle corners. A slot must also be cut from the loop circumference at one corner of the loop to the roadway edge to accommodate the feeder. Please refer to the loop documentation for the slot width and depth.

A continuous loop and feeder is obtained by leaving a tail long enough to reach the detector before inserting the cable into the loop slot. Once the required number of turns of wire is wound into the slot around the loop circumference, the wire is routed again via the feeder slot to the roadway edge.  A similar length is allowed to reach the detector and these two free ends are twisted together to ensure they remain in close proximity to one another.  (Minimum 10 turns per foot).  Maximum recommended loop feeder length is 300 feet.  It should be noted that the loop sensitivity decreases as the feeder length increases, so ideally, the feeder cable should be kept as short as possible. The loops are seated using a “quick-set” black epoxy compound or hot bitumen mastic to blend with the roadway surface. 

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