User's Manual

301UM0031_01 TD136 Enhanced Vehicle Detector User Manual Page: 16 of 29
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. Other forms of joins such as those available in kits, where the joint is properly
sealed against moisture, are also permitted.
5.4 Sensing Loop Geometry
NOTE: 1) The circumference of the loop must not exceed 30 m.
2) The area of the loop must not exceed 30 m² and must be not less than 1 m².
3) The loop must be constructed as detailed below.
Sensing loops should, unless site conditions prohibit, be rectangular in shape and should normally be
installed with the longest sides at right angle to the direction of traffic movement. These sides should
ideally be 1 metre apart.
The length of the loop will be determined by the width of the roadway to be monitored. The loop should
reach to within 300mm of each edge of the roadway.
In general, loops having a circumference measurement in excess of 10 metres should be installed
using two turns of wire, while loops of less than 10 metres in circumference, should have three turns
or more. Loops having a circumference measurement less than 6 metres should have four turns. It is
good practice at time of installation to construct adjacent loops with alternate three and four turn
windings.
For additional Information on loop geometry refer to the following documents:
“INDUCTIVE LOOP VEHICLE DETECTION” – Nortech Doc. No. MKT01.
“TRAFFIC DETECTION CONCEPTS” – Nortech Doc. No. MKT02.
“TRAFFIC APPLICATION MANUAL” – Nortech Doc. No. MKT04
5.5 Loop Installation
All permanent loop installations should be installed in the roadway by cutting slots with a masonary
cutting disc or similar devise. A 45° crosscut shou ld be made across the loop corners to reduce the
chance of damage that can be caused to the loop at right angle corners.
NOMINAL SLOT WIDTH: 4 mm
NOMINAL SLOT DEPTH : 30 mm TO 50 mm
A slot must also be cut from the loop circumference at one corner of the loop to the roadway edge to
accommodate the feeder.
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 are 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 20 turns per metre) Maximum recommended
feeder length is 100 metres. 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 sealed using a “quick-set” black epoxy compound or hot bitumen mastic to blend with
the roadway surface.