Investigation of an Energized Light Pole

Background

This particular case was developed generically from four separate investigations where different problems with light poles have resulted in shocking complaints. The basic investigations all involved a complaint about a street-level metal light pole shocking people who had been walking by the particular intersection where the light pole was located. To better emphasize the procedural aspects of such investigations, this case study is presented in the format described in Section 7. This will aid as an example of how to apply the recommended evaluation protocol.

Planning the Investigation

After receiving the complaint and identifying it as an “energized object concern,” it was determined that a qualified investigator should be immediately dispatched to the location in question. There was no need for discussion or to have a question-and-answer session because the objective was to secure the area and determine the cause of the shocking concern.

Pre-Site Visit Checklist and Equipment List

Before visiting the location, the following determinations and/or plans were put in place:

1. A determination was made through a complaint database search that no prior shocking complaints had been logged for that particular location.
2. The one-line circuit diagrams for the area in question and basic circuit information for the complaint location was identified (primary voltages, voltage configurations, and location of the transformer serving the load).
3. Based on this complaint type (for an energized object), the power company had an existing checklist of instrumentation and hardware suitable for the type of investigation to be undertaken along with pre-defined procedure and data forms.

Site Investigation

Visual Inspection

The investigators started the investigation by locating the subject street light and blocked off the areas with yellow plastic tape and portable stands. The light pole was clearly identified by the complaining party, who met the power company investigators at the location. In many cases, it is not that simple, and a group of lights may need to be quickly spot checked to find the energized one.

Caution: It should be noted that depending on the energization mechanism, the voltage may be present only at night, when the light is on or on y in the daytime when the light is off.

Next, one investigator stayed at the shock location, while the other located the serving power source and looked for possible disconnect points. The second investigator observed that there was a 12.47-kV power circuit running overhead above the light poles and made a note to evaluate induced voltage possibilities if a faulted 60-Hz source was not located during the investigation.

Field Measurements

The following measurement equipment and support hardware were brought along for this investigation:

• Capacitive voltage tester
• High-impedance ac voltmeter
• Screwdriver
• Paint scraper
• Harmonics analyzer
• dc voltmeter
• Recording forms
• Watch
• Metal detector

The procedure used during the investigation was:

1. Using an operational non-contact voltage indicator, which was accurate to less than 15 Vac, positioned within 6 inches of the light pole, a voltage was indicated.
2. Using a second contact voltage indicator, which was accurate to approximately 90 Vac, a voltage above that level was not indicated. This gave the investigators confidence that the source was not a “high voltage” circuit that might compromise their safety while using the 600 V-maximum-rated test equipment that they had with them.
3. After pushing a metal screwdriver into the ground approximately 4 feet away from the light pole, the investigators measured the voltage between the pole and the remote earth at 60 Vac. No dc was measured, and the harmonic spectrum indicated primarily 60-Hz content.
4. This 60-Vac measurement and the time of day was recorded along with notes for the dc and spectral content.
5. The screwdriver was moved 45 degrees around the pole from its initial location, and a second voltage measurement verified the 60 Vac. Neutrals, unpainted surfaces on curb protectors, fire hydrants, and other grounded metal objects were used for the verification as well.
6. After confirming the traffic lighting at the intersection would not be interrupted, the serving power source was disconnected, and the voltage reading went to zero.

Analysis of Field Measurements

In this particular case, the source was clearly identified as the serving 60-Hz ac power circuit. The measurements confirmed the fact that the light pole had a fault that was not tripping the upstream circuit protection. An investigation of the wiring inside the light housing indicated that an uninsulated part of the phase (hot) wire was touching the metallic part of the pole.

Evaluation of Solutions Options

The analysis and measurements yielded a clear path to resolution (by repairing the faulted circuit). More importantly, this case demonstrates that written procedures and personnel trained in use of those procedures are extremely important for the investigating energized objects.

Further, it is important to note that on an electric utility-by-utility basis, there is a need to better understand the range of ways that the metallic objects in their individual service territory might become energized. It is also important that the trained investigators have knowledge of the specific types of lighting and control circuits used in their service territory, as well as the installation, wiring, and grounding practices used. The range of shocking concern sources for energized light poles is diverse, and the investigation plan should consider as a minimum:

• When a "dig in” to the lighting circuit occurs, the wires can be inadvertently reconnected in reverse (neutral becomes hot and hot becomes neutral).
• When a phase conductor faults to the metal case, the voltage reading will be some percentage of the 60-Hz ac source. That percentage will depend upon the ground impedance.
• Overhead transmission or distribution circuits can induce voltages onto light poles, cross arms, traffic lights, and other metallic objects.
• If the light uses a photo eye or a timed relay for turning on or off, the voltage may be present only at certain times during the day or night.
• Just because the design specification of the pole may have called for a ground rod – that does not necessarily mean that the installer did the job correctly or to the specification.