RD4000 Locating System User Manual
Page 36
Using CD
CD reset
Why CD reset is necessary
As you trace a very long line the signal on the transmitter gradually ‘leaks’ into the ground by capacitance, so
the phase angle of the remaining signal gradually changes. This is referred to as phase-shift and can occur
whenever an alternating current signal is flowing in a system of conductors that have a significant capacitance
or inductance. The relative phase angle between the two frequencies will alter, but only over extended
distances. This can be best illustrated by reference to the illustration below.
This is a diagram of a buried line with significant capacitance to ground. It illustrates the gradual phase-shift that
occurs along the line. The diagram shows a reference point on each waveform. At first, the reference point occurs
right on the peak of a positive half cycle (A). It gradually moves towards the zero crossing point (D) and ends up
on the peak of a negative half cycle (F). The direction has shifted from forwards to backwards. At the intermediate
point (D) the current flow cannot be determined. At this point the CD arrows on the RD4000 receiver will flash on
and off. In the example shown in the illustration, by returning to (C) which is the last point at which a CD reading
was achieved, and performing a CD reset the response at (C) will be equivalent to that of (A).
In most cases the phase angle shift does not occur until many miles/kilometres from the transmitter and in some
cases a CD rest may only be required every 20 kilometres (14 miles)
When to perform a CD reset
After connecting the transmitter signal, trace the target line about 15 metres/16 yards away from the transmitter
and with the transmitter behind you perform a CD reset.
If the CD arrow flashes on and off, return to the last point at which a good CD reading was achieved and
perform a CD reset. Check the phase angle reading on the receiver. If it is near 0
0
and the arrow has stopped
flashing the CD reset has been successful. If the arrow is still flashing it could be a sign of a weak transmitter
signal, possibly caused by the transmitter signal leaking onto another line. In any case the cause of the weak
signal should be investigated.
To perform a CD reset proceed as follows:
•
Pinpoint the line by holding the receiver facing forward with its foot on the ground and the transmitter behind you.
•
Press menu, the phase angle in degrees is displayed. In the bottom left corner of the screen the word
‘RESET’ will also be displayed.
•
Press any key other than menu key to return to CD locate without performing a reset, or
Press menu again to perform a CD reset.
