Buried Cables; Surface Objects; Blast Furnace Slag; Loop Tails - Siemens BH17 7ER Installation Manual

However, in the case of SDE/SA loops, the presence of an isolated heavy
metal object can distort the electromagnetic effect of one of the pair of loops
so that a false speed assessment is given. In these cases it is necessary to
re-site the loops 1 metre away from such objects.
Buried tramlines may present a problem as the metal tie rods sometimes
make an intermittent electrical connection with the running rails. Any
intermittent connection of this type placed under the loop can cause spurious
detector signals and it is advisable to remove the track from the area of the
loop leaving 305 mm clearance. (One metre clearance for SDE/SA loops).

2.2.3 Buried Cables

Buried Electrical Supply cables do not cause interference with Detectors.
However, SDE/SA Loops should be kept 1 metre away from heavier
armoured type of cable as the metal content could cause the distortion
mentioned in section 2.2.2.

2.2.4 Surface Objects

Loop slots should be kept one metre clear of manhole covers, drain covers,
expansion joints and similar disturbances in the road surface. Failure to
observe this can weaken the road surface and may cause subsequent
breaking up.

2.2.5 Blast Furnace Slag

Occasionally road surfaces incorporating blast furnace slag in their
construction will be encountered. This has a de-sensitising effect on detector
loops. It is not possible to detect the presence of slag from the road surface
appearance and it will usually be discovered only during commissioning in the
form of insensitive loops.
Commercial should make it clear that if this material is encountered the
customer will be responsible for work involved in overcoming the problem.
Avoiding multiple loops on a channel may increase working sensitivity. This
will involve some re-cutting and probably additional detector cards. However it
may be necessary to replace the material in the vicinity of the affected loops.

2.2.6 Loop Tails

It is advisable that loop tails are twisted with at least 10 twists per metre,
outside of the slot. See Figure 14.
However loop tails may be untwisted loop tails over a short distance where
they are taken across or along the carriageway to a connection point at the
roadside. The length of untwisted loop tails must not exceed 30 metres. No
more than 15 metres of tails from any other loop should be laid in a common
slot, and then only if the loops are driven from the same detector unit.
667/HE/20663/000 Issue 13
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OOP ETECTOR ANDBOOK
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