Page 14 - Mississippi 811 Magazine 2020 Issue 4
P. 14

Can’t get a good ground? Now what?
Locating is not all short grass and pavement. Everything you hook to (if you can hook to it at all) doesn’t always locate like it says it will in the book. So, we’ve found an old locator who has been called on many times for help. He wasn’t involved in building the first locating device, but he probably used it once or twice before he upgraded! If he doesn’t cover your question, just contact him and let him rAespond to your specific situation. I know he’ll be more than glad to help you get it right. Publisher
nyone who has been cable that you need a device that has even a low frequency pipe detecting techniques locating for any time will be higher frequency that you typically use does not get a good signal path but quick to point out a few useful to help you on this one. Typical locate switching to 480kHz allows the signal facts that help to locate a frequencies tend to be in the range to use the capacitance of the joint to
cable or pipe. These may include: of 512Hz to 32kHz. These frequencies jump over the insulated section.
• Always try to use a low frequency
• Make sure the line has good continuity
• Make sure that both ends are well grounded
• Make sure you make a good ground connection
These are all good suggestions, but
we know that life is not always so simple. Take, for instance, the third suggestion “make sure both ends are well grounded.” Many cables will not be grounded at the far end (or the beginning for that matter). Applying
a low frequency signal locate tone in these circumstances can be fruitless. For example, short telephone drops to a premise will probably not be grounded, pot ended cables that have been terminated because they are no longer required or because they have been laid in anticipation of being used later are also not likely to be grounded.
If there is no ground, or the grounding is poor, making a good ground connection at the application point and checking for continuity is not going
to help matters as there is nowhere
for the signal to travel to if there is no ground at the far end. We should use a different technique to get the signal to travel to the end of the cable. It may be
offer good signal to noise locates where good grounding is present. Have you considered using something in the range of 480kHz for poor grounding situations?
So why 480kHz? Well the answer is that at 480kHz the signal will “bleed off” the cable due to the distributed capacitance of the cable to ground. There’s no need to go into explanations of what this is, but suffice it to say— the higher the frequency, the greater these effects. Now this helps with ungrounded cables because the capacitance creates
a sort of pseudo ground that allows the signal to pass from the conductor into the ground and so completes the circuit back to the transmitter. Using the 480kHz mode helps detect these short, unearthed cable drops.
This is not the end of the story as 480kHz can also be a great help in detecting older cast iron pipelines. Many of these pipelines will have insulated joints. This electrical insulation may be a result of corroding joints and nuts and bolts, or because the sealing material used to join the pipes can create an insulated joint. Either way, this is bad news for someone trying to detect the position of the pipeline. Capacitive effects help here also. The jointed ends, although possibly insulated, will have some capacitance across the joint. Using
A word of warning: As we have seen, the use of high frequencies such 480kHz can be beneficial in many circumstances, but like all good things there is a downside. Understanding these limitations can still allow the user to appreciate the benefits of high frequency locate tones. The two main downsides are:
1) If there are any utilities nearby, there is a danger that the “bleed off” effect can also result in a “bleed on” effect.
So retuning signal currents traveling through the ground may hitch a ride on other utilities, resulting in multiple signal paths. These tend to be smaller signals so careful locating techniques can help to identify the correct one.
2) As the signal will “bleed off” over the entire length of the cable, the distance the signal will travel along the cable
or pipe will be less than that of a low frequency signal. It should be noted that using low frequency locate tones
is always best where good grounds are present.
If you have questions or comments about this article or related topics, feel free to contact Steve at: steve.benzie@ vxmt.com.
By Steve Benzie
12 • Mississippi 811 2020, Issue 4


































































































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