Locating Service Drops

Anyone who has experience with cable locating will quickly point out a few valuable facts that will make the job easier. These may include:

  • Always try to use a low frequency
  • Make sure the line has good continuity
  • Make sure that both ends are well earthed
  • Make sure you make a good earth connection

These are all excellent suggestions, but we know that life is not always so simple. For instance, the third suggestion is “make sure both ends are well earthed.” 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 earthed, 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.

Pot Ended Cable

If there is no or poor earthing, making a good ground connection at the application point and checking for continuity will not help matters as there is nowhere for the signal to travel. We should use a different technique to get the signal to travel to the end of the cable. This is where a locator such as the VM-560 is helpful with its 480kHz for poor grounding situations. Typical locate frequencies tend to be in the range of 512Hz to 32kHz. These frequencies offer good conductive locates where good grounding is present.

So why 480kHz? Well the answer is at 480kHz, the signal will “bleed off” the cable due to the distributed capacitance of the cable to the ground. There’s no need to explain what this is but suffice to say that the higher the frequency, the greater these effects. This helps with ungrounded cables because the capacitance creates a pseudo ground that allows the signal to pass from the conductor into the ground and completes the circuit back to the transmitter. Using the 480kHz mode helps detect these short, unearthed cable drops.

Low Frequency (signal travels short distance on un-earthed cable)

High Frequency (Signal travels almost all the way to end of cable)

This is not the end of the story as 480kHz can also help detect older cast iron pipelines. Many of these pipelines will have insulated joints. This electrical insulation may result from 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 terrible 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 low-frequency pipe detecting techniques does not get a good signal path but switching to 480kHz allows the signal to use the capacitance of the joint to jump over the insulated section.

High Frequency — “Jumps” Joint

A word of warning: As we have seen, using high frequencies such as 480kHz can be beneficial in many circumstances, but there is a downside like all good things. Understanding these limitations can still allow the user to appreciate the benefits of high frequency locate tones. The two main downsides are:

  • If there are any utilities nearby, there is a danger that the “bleed off” effect can also result in a “bleed on” effect. So, returning 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.

Effect of High Frequency on Adsacent Cables

  • 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, for this reason, the VM-560FF also has 512Hz, 8kHz locate modes and can also detect 50/60Hz power signals.