4 Locating
Page 19 of 33
1.
There is a ground fault on the line which is shunting signal to ground.
2.
There is a “T” off from the mainline.
3.
The operator has migrated from the connected line to a line which has some signal that has bled off from
the main line.
Note that the current reading will only be displayed if dedicated 33 kHz or 131 kHz modes are selected.
4.4
When to use 33 kHz or 131 kHz
Apart from the induction mode, the transmitter transmits both 33 kHz and 131 kHz simultaneously, so no
adjustments are required on the transmitter.
As a general rule 33 kHz will provide the best compromise between clarity of signal and effects of “bleed off”
to other services. However, there times when the high 131 kHz frequency will be beneficial. These are:
1.
Locating pot ended cables: Pot ended cables are generally not grounded. This means that the signal will
not readily travel to the pot end. Using a higher frequency will encourage signal current to flow.
2.
Small diameter cables: Higher frequencies tend to flow better on small diameter cables, although the rule
“first try 33 kHz” still applies.
3.
Locating old cast iron pipes: These tend to have mechanical connections between sections which rust
over time and prevent an electrical connection between pipe sections. The 131 kHz signal will tend to
jump across these joints and continue down the line.
4.
Poorly grounded cables: Generally, higher frequencies will travel along a poorly grounded cable better
than lower frequencies.
4.5
Metal Locate Mode (vScanM Cover Detector)
The metal locate mode is used to detect buried manhole covers. For this feature, be sure to order vScanM.
The cover locator has been designed to respond to most metal covers.
4.5.1
Locating a Buried Cover
Before locating a buried cover it is necessary to perform a simple set up procedure.
Switch on the vScan and select the cover detection mode using the mode switch. The cover detection icon
should be shown on the screen.
Hold the vScan well away from any metallic objects.
Momentarily press the “On/Off” button. The bar graph will show close to zero deflection and there should be a
slow pulse sound from the speaker. The unit is now set up.
NOTE
The sensitivity control does not function when using the M mode.
Hold the locator just a small distance from the ground (approximately 2” or 50mm) Now sweep the area in a
grid action in a similar way to sweeping for a buried cable. Keep the locator vertical and avoid swinging it. As
a metallic object is approached, the bar graph will start to deflect more and the speaker tone will increase.
Find the peak signal by moving the locator forward and back and then side to side. Use the peak level
indicator to help pinpoint the position of largest signal. The largest peak signal will indicate the position of the
center of the buried cover.
