Technology Report 2022 - Peoples Gas & North Shore Gas

Ultra-High Frequency EM Safety Sweep and its Effect on Damage Prevention


This case study describes an on-site utility sweeping technique to find difficult-to-locate facilities such as cast or ductile iron with electrically resistive joints, damaged tracer wire or tape, short side services, and old or abandoned lines. It can also be used to verify existing locate marks prior to excavation. The technique uses locating equipment that is currently available and easy to learn and use. The technique led to significant reductions in damages by in-house crews.

  • Following a non-injury strike to a high-voltage power line in 2010, Peoples Gas sought a solution that would reduce damages to all lines, with an emphasis on unmarked lines, as well as verify existing locate marks prior to excavation.
  • The resulting solution has three parts: the technology (ultra-high frequency EM locator), the technique (the safety sweep, highly effective in finding unmarked lines), and the training (both in the classroom and the field).
  • Peoples Gas realized a decrease in damages of 67% from its in-house crews using this new solution, and a 77% decrease in damages with the root cause of “Facility Not Marked” between 2016-2019.




Background, Challenge, and Opportunity

For more than 170 years, Peoples Gas has delivered safe and reliable natural gas to residents and businesses of Chicago. Over this time Peoples Gas has installed over 342 miles of transmission main, 4,620 miles of distribution main, and 507,897 services. This vast network of gas delivery serves over 879,000 customers and is supported by 2,700 employees.

Over the last few decades, Peoples Gas has worked diligently to improve our Damage Prevention efforts. In 2010, a non-injury strike to a high-voltage power line on private property in downtown Chicago challenged us to come up with further safe practices. Thankful that there were no injuries during the strike, we viewed this near-miss incident as an opportunity to learn and improve.


Investigation and Understanding

Peoples Gas Damage Prevention group investigated this incident and others like it to understand the major causes. After organizing historical data of damages by root cause, it was apparent that unmarked lines were the biggest issue.

Looking deeper into each individual unmarked utility damage of the previous decade, there was a noticeable theme of these struck utilities being on private property. Facilities on private property have always posed a threat to excavators nationwide, as the owners are responsible for keeping accurate records of what is underground and where. In the case of what happened on S. Wacker Dr., an area of the downtown Loop with already-crowded infrastructure, the building engineers had maintained inaccurate records of their facilities, thus leading to the incident.

With each new year, underground infrastructure on private properties continues to grow, posing a need for enhanced safety protocol. Further support for this Damage Prevention effort was seen through difficult-to-locate utilities such as cast or ductile iron with electrically resistive joints, damaged tracer wire/tape, short side services, and old/abandoned lines.

The mission was to reduce damages to all lines, with an emphasis on unmarked lines, as well as verify existing locate marks prior to excavation.


Possible Solution

The beginnings of a solution emerged almost immediately after the incident. After the strike occurred, while still onsite, two members of the Damage Prevention team used a Pipehorn 800-HL ultra-high frequency line locator to perform a quick sweep of the area. The previously unmarked power line was easily identified, suggesting utility strikes of this type can be avoided.

Criteria and Objectives

The goal was to find technology and techniques that could help find unmarked lines as well as verify existing locate marks. It was essential that the equipment and techniques be adaptable to existing procedures.

The Damage Prevention team collected and evaluated multiple pieces of EM locating equipment as well as tested techniques for using the equipment to achieve the objectives most effectively. The resulting solution had three parts: the Technology (ultra-high-frequency EM locator), the Technique (the Safety Sweep), and the Training (both classroom and field) – The Three T’s.

The Technology

The evaluation team believed that an EM line locator would be best for sweeping. The optimal active frequency and best equipment for the task required further investigation. The criteria were the following:

  • Induction capability – the ability to easily energize lines without directly connecting
  • Induction frequency – best signal to “light-up” or energize the most underground conductors
  • Simplicity – equipment that is easily adoptable by crews at the jobsite

Additionally, the EM locator should be able to trace both good conductors and poor conductors – essentially verifying marks as needed. Good conductors will conduct signal well, making them easier to energize (with the transmitter) and to detect/pick-up (with the receiver). Poor conductors can be challenging to energize and to detect.

It became clear that part of the best technology for finding everything, including unmarked lines, is application of higher frequencies. Similarly, the ability to connect with a low frequency can best verify, isolate, and identify lines at the dig site. 

The EM locator had to be simple to learn and easy to use. Locating and sweeping is a small, yet incredibly important part of the job. If crews found the equipment or procedure too difficult and time-consuming, the damage prevention efforts could be diminished.

The evaluation team tested a number of EM locators to assess performance against the objectives. Because ultra-high frequency is critical to success, the team looked further to the manufacturers’ experience and understanding in this area. Pipehorn and its dual-frequency Model 800-HL best met our selection criteria for frequency, induction, and simplicity. The ultra-high 480 kHz frequency for induction and sweeping for difficult conductors, plus a low frequency for connecting and isolating a line, was a good match. This particular locator also came with a handle to hold the transmitter closer to the ground when sweeping without having to bend over.

The Technique

The evaluation team found the process to perform a Safety Sweep (the 2nd “T”) relatively simple, yet highly effective in finding unmarked lines. With modest instruction, it was proven that a two-person crew could implement the technique with strong results. The basic procedure includes:

  • One person holds the receiver. Another person, with the transmitter, stands directly across from the receiver (see Figure 1).
  • A good rule of thumb is to stand apart at least 5 times the depth of any conductors. An ultra-high frequency, such as 480 kHz, may allow for a closer working distance.
  • Keeping the receiver and transmitter pointed at each other, the sweepers walk together across the dig site, maintaining separation and alignment, and keeping the equipment close to the ground.
  • When a line is crossed, the receiver’s signal will spike. At this point, the transmitter is placed on the ground.
  • Pinpoint with the receiver, then pinpoint with the transmitter. Double check the receiver’s pinpoint and mark it.
  • As already-marked utilities are located, verify with different colored paint
  • Continue this same process at different angles to locate conductors running in other directions in the dig area.

The Training

The third “T”, Training, was paramount to a successful implementation of such a procedural change. Without appropriate training both in class and out in the field, our efforts could fall short of the full potential . See Lessons Learned section below for further details.

Figure 1 – Crew Performing Safety Sweep


Implementation and Results

Peoples Gas first implemented the use of Pipehorn 800-HL locating technology and Safety Sweeps on a limited basis in 2013. Results were examined in two phases: the Unofficial Phase and Official Phase.

During the Unofficial Phase (2013-2015), total in-house damages decreased by 7% the first year and 12% the second year. These results showed promise that we were on the right track.

The Official Phase began in 2016, as this is when the Technology and Technique was formally written into Peoples Gas Damage Prevention procedures. Results from this phase unequivocally proved our application of the Three T’s. In the period of 2016-2019, Damages by In-House Crews fell by 54%. During that same timeframe, a subset of Damages by In-House Crews with the root cause being Facility Not Marked was tracked with a total 77% decrease (see Figure 2 Below).

When examining the results over the Unofficial and Official Phases (Figure 3), Peoples realized a decrease of 67% from our in-house crews. Given the primary objectives set for this study, this was a major success for reduced damages and increased safety.

Figure 2 – Official Phase In-House Damage Reduction

Figure 3 – Total In-House Damage Reduction


Lessons Learned

During this successful effort to reduce damages and increase safety, four lessons were learned that are worth noting:

  1. Experience and training matters
  2. Effective training requires both classroom and field instruction
  3. Communication is key
  4. Success generates buy-in

As can be seen in Figures 2 and 3 above, 2018 displays a small bump in damages. Many experienced employees retired that year. With this loss of institutional knowledge and a need for new supervisors, Peoples was able to spotlight Lessons 1 and 2. At first, new supervisors were trained at a mock-up neighborhood with live utilities at the state-of-the-art Peoples Energy Training Center. To further this education, field training was added to the curriculum. The success of this effort can be noted by the decrease in damages in the following year (2019) at an all-time low.

Figure 4 – In-class and Field Instruction

As for Lessons 3 and 4, the crews were interviewed in order to gather feedback on the implementation of the Safety Sweep. The resounding response was positive in that communication among crew members performing the two-person sweep was beneficial. Additionally, the concept of “seeing is believing,” and reduced damages over time gave the crews confidence in the Safety Sweep.



Peoples Gas set out to reduce damages and increase safety. Selecting the right technology, using the right technique and providing the right training yielded great success and the adoption of this practice at North Shore Gas.



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