Considerations While Line Locating

Dairyland decouplers play a crucial role in protecting personnel and infrastructure in various applications, including AC mitigation systems. It is important for users to understand how a decoupler will interact with external currents used during certain maintenance processes, like line locating or current surveys, to avoid inaccurate readings or more severe complications.  

Line locating is a key operation for the pipeline industry, used to locate buried pipelines before performing routine maintenance or construction activities nearby.  To locate buried pipelines, a transmitter is connected to the pipeline and sends various frequencies through it. A receiver, commonly referred to as the locator, is then used to read the signal from these frequencies above ground. If the locator is directly above the pipeline, the signal shows it’s exact depth and location. When performing pipeline locating, accuracy is imperative to ensure safe digging and/or accurate reads when performing other testing on the pipeline such as ACVG, DCVG, CIS, etc. 

When locating, anything that is in proximity—such as a water line—could cause the signal to jump off the pipeline and follow that neighboring structure. Because the signal travels through soil to reach the neighboring structure, some impedance occurs, resulting in a weaker signal. If the signal becomes weaker, the locator is no longer following the intended pipeline. 

Figure 1. Locator signal travels along a natural gas pipeline and is picked up by a water pipeline

The signal can jump to anything that is physically connected to the pipeline—such as the grounding conductor of a motor operated valve (MOV) or an AC mitigation system. This again can cause the line locater veer off of the intended path. In this instance the signal will not necessarily be weaker, adding difficulty in determining if the pipeline is being followed correctly.  

If a locator is following the signal being read from the grounding conductor of an MOV, the locator will discover this once they arrive at the electrical connection. It is more difficult to know if the locator is following the signal being read from the AC mitigation system. 

 

Figure 2. Locator signal passing through a decoupler onto the AC mitigation wire

AC mitigation systems will most likely run in parallel with the pipeline that is being located. These systems are designed to mitigate the steady-state current from transmission towers that typically operate at 60Hz in the United States, or 50Hz outside of the US.  Because Dairyland decouplers are designed to have a low impedance, any frequency above 7Hz will pass through the decoupler onto the AC mitigation system.  If the locator jumps off the pipeline and begins following the path of the AC mitigation system, it won’t be immediately obvious, as the signal will not appear weaker like it would in the case of it following a foreign water line. This can cause location markings to be off by several feet.

For activities, such as line locating that involve the use a signal varying in frequency, as well as current surveys, ACVG, and DCVG coating surveys-Dairyland recommends installing a disconnect switch to prevent this current from moving onto to the connected AC mitigation system. The use of a disconnect switch is a quick and safe solution in these circumstances. View all Dairyland switches on our product accessories page.

For decouplers that are in an electrical circuit for electrical equipment grounding isolation, we do not recommend the use of a disconnect switch since these circuits should not be open for electrical safety.

If you should have any questions regarding the information in this article, please feel free to contact us at techsupport@dairyland.com.