October-November 2023 Q&A
Q: With distribution URD cable and other circuits in the same duct bank, should there be any concern about inductive current/voltage on the conductor if there is a fault on the opposing circuit? Most of our circuits are in plastic conduit and encased in concrete. We have a neutral running through from the substation, and the vaults are grounded to earth with it all tied together. We also still have some lead cable (PILC) in our system.
A: There are several conditions to your answer.
For the poly-dielectric insulated, if the cables are not jacketed, the concentric in a common conduit will be in contact, so they are not truly isolated. If the cables are jacketed, they may be considered isolated. The reasoning is that since the concentric is grounded, there is not enough voltage – even with fault current – to penetrate the PVC jacket.
Since polyethylene (PE) solid insulated cable and paper-insulated lead-covered (PILC) cable are shielded, the phase conductors do not affect each other in a common conduit or trench as far as induction is concerned. If bare concentrics are in contact in the conduit, they will share current. Any resistance among the three pathways at the terminations could result in potential differences between them.
On unjacketed cables, a ground fault will couple current onto other concentric or PILC shields they are in contact with.
Q: In distribution URD, what is the correct way to ground/isolate the conductor that we are going to work on? Where is the best place to ground, or how do we isolate, and which is best? For the most part, the concern is splicing and having the neutrals at different potentials. We are trying to get away from removing the jacket on the conductor and bonding across. This gives us another area to fix in the conductor, and sometimes space to do that can be an issue.
A: As part of confirming that a cable is de-energized, we ground them on a grounding bushing or feed through with a grounding cap at each end of the cable we are going to work on. Using either a feed-through and cap or grounded bushing, we take the ground lead and connect it to the twisted concentric for the cable we just grounded. To isolate, we cut the twisted concentric from the ring-bond ground in the pad or enclosure. You would also have to be sure to isolate the mount for the bushing from the transformer bracket it is usually mounted to.
Isolating via this method through the grounding bushing (or cap) tied back to the cable concentric eliminates any capacitance from the cable since the phase conductor is now in series with the concentric instead of parallel to it.
There is another safety method, and that’s to simply park the cable after you have grounded it to verify it. If the cable is parked at each end, the phase conductor is isolated. It is not grounded, but there is no rule that says you must ground the cable (note: most people think there is, but isolation is a legitimate safe work mode). If the elbow is parked, the phase conductor is isolated, but the concentric is still continuous.
Since the concentric is bonded to the pad, the concentric and enclosure are at the same potential.
A fault on the system could still create a high potential between the concentric/pad mount and the surrounding earth.
IEEE has a great compendium of preferred practices for protecting workers in de-energized URD. It’s titled “Worker protection while working de-energized underground distribution systems,” and you can access a copy at https://ieeexplore.ieee.org/document/1256391.
Q: We are in our windy season and a question has come up: Can our lineworkers climb in high winds? Is there a certain wind speed at which we cannot climb or is it a judgment call? Any help is appreciated.
A: There was a section in OSHA regarding climbing in dangerous conditions, but it was related to free climbing, which is no longer allowed. There is no statutory guidance that would suggest wind speed limits. So, the employer must make the judgment call and communicate that to their employees. Now that all workers are in 100% fall protection, wind is not much of a concern. However, the employer still must decide if related conditions – such as flying debris, losing conductor or fixtures off the pole, flying cows and so on – create untenable hazards.
Q: Our manual of safe practices requires all cutouts/fuses to be operated using a hot stick. We did a little outreach on our end, and a few utilities allow hand operation to open/close a cutout door during fuse changes when an employee jumpers out a cutout. In that way, they are only paralleling or breaking a parallel, not necessarily dropping or picking up load. A handful of others only allow cutout/fuse operation with a switch stick regardless of the scenario. I also read OSHA 29 CFR 1910.269(l)(9), which states, “When an employee must install or remove fuses with one or both terminals energized at more than 300 volts, or with exposed parts energized at more than 50 volts, the employer shall ensure that the employee uses tools or gloves rated for the voltage. When an employee installs or removes expulsion-type fuses with one or both terminals energized at more than 300 volts, the employer shall ensure that the employee wears eye protection meeting the requirements of subpart I of this part, uses a tool rated for the voltage, and is clear of the exhaust path of the fuse barrel.” What are your thoughts?
A: The OSHA standard does not prohibit closing expulsion fuses by hand in rated rubber gloves with face and eye protection. The first part of 1910.269(l)(9) is about open-type cartridge-enclosed fuses that do not rupture. The second part is about expulsion fuses. The reason is that expulsion fuses produce shotgun-velocity debris at close distances, such as when a worker is holding it. If the switch is jumpered out, as you say, you are not picking up a load, but that’s not the issue. A fuse can blow even if it is jumped out under certain conditions. The value of the hot stick is distance. The issue has always been to keep a distance away from the fuse and the faulted equipment being fed by the fuse by using a hot stick.
Q: Someone at our company said our tool room is not certified to build ground cables. So, how do we certify? I was wondering if you knew of any training material out there for building your own temporary grounds. We are just trying to prevent re-creating the wheel.
A: The good news is that there is no certification required to build grounds. However, there is a universal standard on the ratings and combinations of components required to build grounds that will work properly. The same standard also includes testing guidance. Many utilities and contractors use tool companies and labs for building their grounds simply for convenience. Most labs will certify that they build and test to the standard. Search online for ASTM F855-20. The PDF download is currently $76. This is the standard to which all temporary grounds are built.
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