307UM0200_01 May 2016
PD27x User Manual
Page 16 of 30
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 m
2
and must not be less than 1 m
2
.
3) The loop must be constructed as detailed below.
Sensing loops should, unless site condi
�
ons prohibit, be rectangular in shape and should normally be installed with the
longest sides at right angles to the direc
�
on of traffic movement. These sides should ideally be 1 metre apart.
Loops opera
�
ng from the same detector module can share a common slot along one of the longer sides, if so required.
This type of configuration could be applied in a direc
�
on logic applica
�
on. The maximum separa
�
on permi
�
ed for this
applica
�
on is 1 metre, ensuring that a vehicle can straddle both loops simultaneously in the required direc
�
on of travel.
The only factor which governs maximum separation between loops in all other applications is the feeder length, with
100 metres being the maximum recommended length.
The length of the loop will be determined by the width of the roadway to be monitored. The loop should reach to within
300 mm 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. Loops having a circumference
measurement less than 6 metres should have four turns.
It is good prac
�
ce at
�
me of installation to construct adjacent loops with alternate three and four turn windings.
For addi
�
onal Informa
�
on on loop geometry refer to the following documents:
•
“INDUCTIVE LOOP VEHICLE DETECTION” - Nortech Doc. No. MKT0001.
•
“TRAFFIC DETECTION” - Nortech Doc. No. MKT0002.
•
“PARKING APPLICATIONS MANUAL” - Nortech Doc. No. MKT0003.
•
“LOOPS and LOOP INSTALLATION” – Nortech Doc. No. MKT05
5.5 Loop Installation
All permanent loop installa
�
ons should be installed in the roadway by cu
�
ng slots with a masonry cu
�
ng disc or similar
device. A 45° crosscut should 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, leading to the roadway edge to
accommodate the feeder.
A con
�
nuous loop and feeder is obtained by leaving a tail long enough to reach the detector before inser
�
ng 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.
