Equipotential or Total Isolation?
System grounding is one of the topics that people ask me about most, which is great because I have always found temporary system grounding for employee protection to be a fascinating subject.
I performed bracket grounding all throughout my years spent working on line crews in the 1970s and ’80s. I was taught that it was the best method for protection while working on de-energized lines and equipment. There was no other option that I was aware of, so I never questioned the training. For the most part, we never left my former employer’s system until mutual assistance became an issue later in my career. The company I worked for began assisting other companies with storm work, and it was then that I began to see different methods of system grounding for employee protection. Questions were asked of our employer, and we were told that due to our construction standards of building our system with pole grounds on every pole, the equipotential method would not benefit employees nor make work any safer than bracket grounding. Tests were performed that documented these results. The method of bracket grounding within a mile of the work location was accepted, and I believed that the testing was accurate – until much later in my career when incidents occurred with bracket grounding.
As time passed, I began to hear about some incidents that had resulted in injuries and fatalities; they were due to the use of different types of system grounding, especially grounding of delta systems and single-conductor wye systems without a neutral. These incidents really raised questions as to the best method to ground for the safety of the employee. Suddenly, equipotential grounding was the hot topic of the industry. Today, I believe much of the industry remains convinced that bracket grounding is the best method of protecting employees.
Since 1994, OSHA 29 CFR 1910.269(n)(2) has included a sentence that states an exception to traditional bracket grounding, but the true meaning has never been explained by the agency. Read the full text of (n)(2) below; the exception is bolded.
“For any employee to work transmission and distribution lines or equipment as deenergized, the employer shall ensure that the lines or equipment are deenergized under the provisions of paragraph (m) of this section and shall ensure proper grounding of the lines or equipment as specified in paragraphs (n)(3) through (n)(8) of this section. However, if the employer can demonstrate that installation of a ground is impracticable or that the conditions resulting from the installation of a ground would present greater hazards to employees than working without grounds, the lines and equipment may be treated as deenergized provided that the employer establishes that all of the following conditions apply: The employer ensures that the lines and equipment are deenergized under the provisions of paragraph (m) of this section; there is no possibility of contact with another energized source; and the hazard of induced voltage is not present.”
A Review of Incidents
A few years ago, several reported electrical contact incidents occurred on systems where bracket grounding was installed. At least two of these incidents resulted in near fatalities.
More recently, two different incidents occurred in Georgia on a single-phase, direct-buried UD primary that was being spliced. In both cases, the loop had been switched out and grounded via the traditional method with grounding elbows in nearby transformers. No other cables were in the ditch. The primary cable had faulted and was being spliced to find and repair the fault. A splice pit was opened, and a lineman was in the process of splicing the primary conductor when the contact occurred. One was a nearby lightning strike, and the second was a tree being cut down one span of overhead from the riser pole. In each case, the voltage rise on the neutral was high enough due to the location of the fault that it was dangerous to the employee. There were no severe burns, but there was enough voltage for a powerful shock that could have caused atrial fibrillation and been fatal; thankfully, it was not. The question here is, why did this happen when the system was switched out and grounded? The fact is that the primary conductor was now just a neutral conductor since it was grounded to a system neutral at each termination in adjoining transformers. The fault caused a rise in the neutral conductors in both cases.
There was another event that happened during storm work after Hurricane Katrina that resulted in a severe shock. It occurred on an overhead, single-phase pull-off that had been isolated and grounded. The pull-off was only five spans, the end was visible, and all the transformers had been opened to prevent any backfeed from generators. There was no chance of induced voltage. The #2 ACSR primary conductor had been isolated with a cut jumper and checked with a voltage detector. A system safety ground had been installed on the primary conductor to the neutral at the pole where the jumper was cut and secured. The two meters off the transformer pole being worked were removed to prevent backfeed from the two houses served at that location. Everything was standard operating procedure to isolate and ground so that the crew could replace a broken pole and transformer. All other work was complete, and connections were being made for a 1/0 secondary feeding the two houses across the road from the pole that had been replaced. As the lineman was in series with the service neutral and the main line neutral, connecting the #2 ACSR service neutral to the low-side neutral on the transformer, a squirrel stepped off a completely self-protected transformer to a lightning arrestor and caused a fault near the pull-off pole. The nearby fault caused a voltage rise on the neutral conductor that was still intact at the pull-off pole, and the lineman received a severe shock. At first, no one knew what had happened since the line was totally isolated from primary and secondary. It was then brought to their attention that the effect of nearby faults on the system had caused an immediate voltage to rise on the neutral in all directions. A multi-grounded wye system will dissipate this voltage through pole grounds, but a shock will certainly be felt if a person is in series with the neutral and another difference of potential near enough to fault.
In addition to the dangers of nearby faults on neutral conductors, the possibility of return neutral currents is always there. No properly trained lineworker would open a neutral on an energized system without first jumping out of neutral. The current flow is always on the neutral, but the voltage will be present if the neutral is opened under load, especially if there is a lot of motor load as found in industrial areas. I have seen upward of 100 amps on neutrals in many locations, including near substations. Never open neutrals under load without following the appropriate procedures, even in loop feed systems. But what happens on the underground when splicing a primary? We must open the neutral. And now, the primary conductor is also a neutral due to bracket grounding.
Understanding the OSHA Regulations
Lastly, I’d like you to read 1910.269(n)(2) again (see above). “Total isolation” must be better understood. Once isolated from all known sources of energy, and once it’s been checked for the absence of voltage and induced voltage, cut the neutral in the clear and maintain total isolation through additional cover if needed. Many of my customers and other organizations throughout the industry are adopting this policy for certain situations and systems.
And now we know.
About the Author: Danny Raines, CUSP, is an author, an OSHA-authorized trainer, and a transmission and distribution safety consultant who retired from Georgia Power after 40 years of service and now operates Raines Utility Safety Solutions LLC.
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