OF BONDS AND BOUNDARIES
Let's begin with the practicality of trying to create an equipotential zone (EPZ) in an underground pad mount transformer. The configuration of an EPZ is to bond all conductors and conductive components together and then work within the EPZ boundary. That is easy in overhead. The phase conductors and neutral are all exposed and easily bonded. On a 10-inch diameter pole it's hard to accidentally step below the ground chain and out of the EPZ. There is nothing within reach of the worker that is not at the same potential, except sky.
In a pad mount transformer there is no effective way to bond the concrete pad, the earth in the bottom of the pad cutout and, most important, the phase conductor being worked to the same potential as a ground mat, the transformer case and the ring bonded neutral. In other words, there are too many things within reach of the worker that cannot be bonded to the EPZ system. The biggest problem is the cable to be worked on. If it is grounded remotely, there is no way that it can be at the same potential as your EPZ.
The only way to bring the cable to equal potential is to totally isolate it from all other sources at the other ends. Consider the possibility of adding another step to your switching orders when sectionalizing cables. The added step is to disconnect the neutral for the isolated cable sections. After grounding the cable on a grounding bushing for safety, park it on a parking stand and disconnect its neutral from the system. This removes all possible electrical connections from a fault-induced source. This may not be as effective in a common ditch if the other phases running with the isolated phase remain energized. Untapping a neutral usually requires a design change as many utilities squeeze on the connections. Installing a terminal pin connector on the cable neutrals will make handling easier and protect the stranding at the twisted end. Connect the neutrals to the ring buss with split bolts to facilitate easy disconnect and reconnect.
To bring the totally isolated cable section to the equal potential of your zone requires another step. Use a grounding bushing on the disconnected phase at the remote end and connect it to its own concentric neutral to short them together. Take care to keep the shorted cable end from being in contact with any grounds in the pad at the remote end. If the work location is a ditch, drive a sufficient temporary ground and attach the tail of the neutral, effectively grounding the neutral and the phase conductor together. If driving an isolated ground, take care not to drive a rod too close to a system ground rod. A separate ground driven too close to a system ground rod may make it possible for ground gradients to energize your isolated cable during a fault on the energized system. If you are working in a single-phase pad and you have isolated the cable and shorted it, you can now effectively bond the cable into your EPZ using the transformer's ground rod.
IS IT WORTH THE EFFORT?
As I worked out this scheme I thought, "This is a lot of work to make EPZ happen in a pad." But the possibility of an accident grows as we increase voltages and customer density, adding more and more cables into the ground.
As we have learned more about effective temporary grounding practices, we have become aware that some of our past methods have been ineffective. Whether you are working in a single-phase pad or a ditch where cables have been sectionalized and grounded, you are still exposed if a fault occurs on the system—impressing voltage on the ground. It could be a car plowing through a pothead pole or lightning striking a switch pad—the result is the same. There will be fault current on all neutrals connected to the system. If your system underground cables have bare concentric, you have the advantage of much of the current and voltage bleeding into the earth, such as pole bonds do in overhead (but that cannot be predicted with enough certainty to protect employees).
Consider what happened in the past when we parked an elbow on a grounding bushing. We essentially made two parallel connections to the system neutral. Current follows all available paths. The worker's body is in series with the cable conductor or the neutral and the earth. Even without a fault there is a real hazard created when workers repair cable in a ditch if the cable neutrals are still connected at the remote ends. When we switch out a section and back feed up to the pads at each end of the faulted cable section, there is still the possibility of current on the neutral. Consider this: if an underground feeder splits into three single phase loops, a single cut cable neutral in a ditch is in parallel with the other two phases associated with the feeder and is carrying feeder neutral current. That creates not only the feeder neutral current during normal operation, but also fault current and high voltage appearing on the neutral if a dig-in or fault occurs on one of the other phases in one of the other loops. ip
Author's Note: Please keep in mind that though I believe the concepts presented in this article are accurate and based on sound principles, you the employer are required to evaluate your system and establish effective procedures to protect your employees. I will be sending a request to OSHA for some interpretive guidance on the procedures discussed in this article. OSHA's interpretations carry the force of the regulations. Their response will be published in a future issue of IP. In the mean time, log onto www.incident-prevention.com and go to the new discussion board. Post your ideas and questions. There are many knowledgeable engineers and subject matter experts that can offer guidance on this issue and we would like to hear your thoughts. Your participation will help us all toward a safer workplace.