Radiodetection rd7000+, Руководство по эксплуатации

Страница: 33 / 53

© 2014 Radiodetection Ltd 26

Avoid using existing structures for ground
connections; other buried lines may be
bonded to them
A bad ground connection or just laying the ground lead
on the surface at right angles to the line may result in
less coupling than a good ground provided long
distance tracing is not required.
7.2 Signal grounding
7.2.1 Manhole covers
Sometimes when locating, it is not possible to insert the
ground stake into the earth, for example, when locating
on hard ground such as roads. In this case, the ground
return can be made by attaching the ground lead to the
metal frame of a manhole.
7.2.2 Using lighting columns
Direct connecting to a metal, street-lamp column is
almost as effective as connecting to the cable sheath
itself. Normally the cable sheath is bonded to the metal
column, therefore a simple connection onto the column
enables the user to locate the street lighting quickly and
safely without having to call out a technician from the
lighting company.
If the lighting column is made from concrete make the
transmitter connection to the cable sheath unless the
cable is earthed to the inspection doorframe.
Connection to the cable sheath applies the transmitter
signal for a considerable distance enabling the locator
to trace cables feeding illuminated street furniture as
well as other street lights.
WARNING! The live cable connector is for use only by
operators licensed or permitted to work on live cables.
The use of a street light column as a means of applying
a signal to other power cables on the same electric
circuit is a possibility. The signal may be weak using this
method because it may have travelled some distance
back to the sub-station and out again on the other
system. With the locator used on a high sensitivity
setting it is often possible to locate a cable, which would
otherwise have been difficult or inconvenient to energize
with the transmitter signal.
7.2.3 Finding a good ground point
When using a direct connection, it is important to get the
best possible grounding for the transmitter. This
provides the lowest resistance ground path and the best
output signal. If it is not possible to use the ground stake
the following are examples of good alternative ground
points:

Metal manhole covers

Metal drainage grates

Metal railings

Metal fence posts
7.3 Double-ended connections
Large diameter water pipes and gas distribution pipes
that are laid in sections sometimes have insulated joints
between the sections and can be difficult to locate using
a single ended connection. This is because when using
a single ended connection ground return, signals can
often cause confusion by returning to the transmitter
along other lines. The problem sometimes occurs when
return signals appear stronger than on the target line,
usually because the target line is deeper than the line
carrying the return signal, or the return path may be a
better electrical conductor than the target line.
Applying a double-ended transmitter connection is a
useful technique for positively tracing and identifying a
target line in a situation such as a heavily congested
industrial site, provided there are access points at each
end of the line.
7.3.1 Making a double-ended
connection
Connect the transmitter to an access point on the target
line. The transmitter ground is connected with a long
cable to another access point further along the line. A
complete circuit is achieved without using ground as a
current return path. The long cable should be kept as far
away as possible from the expected route of the line.
Radiodetection supplies 50 meter and 200 meter
extension cables for this purpose.
This method of applying the transmitter signal is ideal
for positive identification of a target line. When a
connection has been made to two points on the same
line, the same level of current should be detectable
around the circuit. The locator display should remain
constant if the depth of the line does not change.
Figure 7.5: Making double-ended connections