RIDGID SeekTech SR-20 - Manual (Page 15)

Ridge Tool Company 
Tools For The Professional TM
seekTech sR-20
Passive Line Tracing
In passive mode, the SR-20 is looking for electromagnetic
“noise” that has found its way onto a buried utility line by any
available means.
Electromagnetic signals can get onto buried utility lines in a
variety of ways.
The most common reason is by means of direct connection
to some signal source. All operating electronic devices that
are connected to AC power will radiate a certain amount
of electronic “noise” back onto the power lines they are
connected to.
In some areas for example, buried utilities act as antennas for
high powered, low frequency radio transmissions (submarine
navigational and communication signals in the UK for
example) and will reradiate these signals. These reradiated
signals can be very useful for locating.
In short, frequencies can show up on buried conductors in
numerous ways, and these can be picked up passively, if the
fields are strong enough.
1. Select a Passive Line Trace Frequency ( or icon).
Figure 24: 60 9th Hz Passive Trace Frequency
2. The SR-20 has multiple Passive Line Trace
frequency settings. Power frequencies (identified
with the power icon ) are used to locate signals
generated as the result of power transmissions,
usually 50 or 60 Hz. To reduce the effects of inherent
noise from line-load or neighboring devices the SR-20
can be set to locate various multiples (or harmonics)
of the base 50/60 Hz frequency up to 4,000 Hz.
The 9x multiple is the setting most commonly used to
locate 50/60 Hz signal. In well-balanced high voltage
electric distribution systems, the 5x multiple may work
better. The 100 Hz (in 50 Hz countries) and 120 Hz (in
60 Hz countries) frequency settings are particularly
useful for pipelines that have been equipped with
cathodic protection using rectifiers.
As in Active Line Tracing, the Tracing Line will reflect distortion
in the detected field by appearing unfocused or cloudy in
proportion to the distortion. This “distortion response” is
useful in recognizing when the field being traced is being
distorted by other fields of metallic objects in the vicinity.
3. There are also two additional radio frequency
bands to help locate lines passively. They are:
• 4kHz to 15kHz (LF)
• > 15kHz (HF)
The Radio Frequency and <4 kHz bands can be useful in
discriminating when tracing in a noisy environment. They
are also very helpful in finding lines on blind searches.
When searching over a wide area where the location of
targets is unknown, one useful approach is to have multiple
frequencies selected for use and to check the area at a
number of frequencies in sequence looking for meaningful
signals.
In general, directly connected Active Line Tracing is more
reliable than Passive Line Tracing.
WARNING: In Passive Line tracing, or when signals are
extremely weak, the Measured Depth will generally read too
DEEP and the actual buried depth may be MUCH shallower.