Hello everyone

I have a question about arc flash PPE requirements for performing electrical work in manholes. Specifically performing work in manholes when energized medium voltage cables are present, but not necessarily being worked on. I have not been able to find any specific info on such a case. So my question is what arc flash PPE would be required when working in a manhole around energized MV cables?

Thanks

This is a very tricky subject. The conditions in a manhole are very different from arc flash tests that have been published. You have to do simulations of your configuration to determine actual conditions (quantify the hazard). Depending on the task, this is where arc flash blankets become useful. The other consideration is the likelihood. Look at the task and how it might cause an arc flash in the first place. Going in to work on telecom cables or doing inspections would not disturb the MV cables so little to no risk and thus no PPE requirement. Once you start moving cables, this is no longer true. Also most UD cables are shielded. So the single line to ground fault is what matters, not a 3 phase bolted fault unless the cables are close together. In my case I use a high resistance ground on all MV installations with ground fault detection and tripping. So risk is very small. The risk occurs where terminations into traditional style open lugs (not separable elbow connectors) are present.

Would disagree, since we conducted tests of Single Line to Ground (SLG) faults on MV splices w/n a manhole (MH) at the KEMA Labs, with an available 10KA (small for our centralized area – KSC, mind you utilities can be much greater), and the Incident Energies were significant, even with instantaneous ground fault protection engaged. This question seems to becoming age old, with little data/requirements available. I will agree to perform the best arc flash analysis, and consider your job tasks into an overall job hazards assessment, then good luck picking your PPE. With our studies (at KEMA labs as mentioned above), our linemen are using CAT 2 PPE for visible inspection only, and as mentioned - no handling of cables what so ever. Upon installation of grounding resistors at our HV-MV Substation yards (and of course upsizing of out MV voltage suppressors), will we allow our folks to enter into manholes to handle cabling (demo, install, repair). Unless a greater hazard can be incurred in the meantime – no electrical work is permitted above and beyond visual inspection for manholes with MV distribution (splices). Application of grounding resistors might be difficult for utilities serving municipalities (unbalanced loads). One neat option I ran into includes the following (speaking of current limiters):http://www.gwelec.com/current-limiting-protector-clip-p-102-l-en.html
NOTE: The arc flash blankets make a difference, but again you should consider the appropriate PPE upon installation of such blanketing on surrounding energized MV cabling within a MH. Since the arc blast circulates the hot gases (arc blast exhaust) within an enclosed – confined work space (MH), face shields are not recommended since they tend to capture these hot gases and may exacerbate entrance into the victim’s lungs. For the same reason all leg and arm protection (Arc rated jeans and sleeves) shall be tucked into their boots and gloves to prevent hot gas intrusion and possibly burning the victim’s skin. Hope this info helps….

The best manhole to enter is one that you have to pump out. If its filled with rainwater, and cables are still energized, the insulation is likely good. After pumping out, we suit up in Level 4, and enter with a tic tracer examining all cables for live parts. Although depending on size and entry, the level 4 suit may be with balaclava and face shield instead of hood. If no live parts are found, we dress down to level 2 and re-enter to perform work. Gloves might be optional depending on task. You'd never be able to do a hand taped splice in gloves - and remember at this point, all arcflash PPE is optional as you proved no live parts.

You met the safety requirement of being suited to perform the voltage check. Once no voltage is detected, Arc flash clothing is not mandated since there are no live parts. But level 2 provides a level of added assurance. I would not attempt any live splicing work in a manhole. Working in clothing greater than 8 cal would be near impossible for the type of work typically employed in manholes.

I have seen pulling Live Break elbows from wall mounted distribution blocks. This was before Art 130 became popular. I think if I was to authorize this type of work today I would first see what are the cal levels required and see if it could be met with 12 cal or less PPE and balaclava/faceshield. I'd be reluctant to put a person in PPE 3 or 4, and definitely not a hood - as I think that would increase the risk of an accident.

This may all sound somewhat outside the text book response - but one thing I learned from working in manholes is to guarantee your own safety first, and never trust anyone - including desk jockeys.

Going into manholes means you have Confined Space training, harness, retrieval winch, and an Attendant.

Disagree about the load break elbow comment there. There have been cases in cold weather (Northern states) where load break elbows exploded. This was found by the manufacturers to be caused by the way the rubber deforms as it pops off. They have since fixed this issue. Load break elbows are no different than other types of disconnects.

I don't like the approach of simply dressing in H/RC 4 (now "level 4") and hoping for the best, either. Doan collected data out of Dupont plants that took three approaches:
1. Plants that followed IEEE 1584.
2. Plants that followed the tables in 70E.
3. Plants that did none of the above.

The "practical" comments by the way sound like a mix of option 2 and 3.

Results:
1. Following IEEE 1584: no injuries.
2. Following tables in 70E: 50% chance of major injury.
3. Following nothing at all: 90% chance of major injury.

Keep in mind these are industrial plants, not manholes or outdoor, overhead distribution equipment. That stuff acts differently and you need to follow the appropriate guidance. NESC is for overhead distribution. I was pointing out that neither IEEE 1584 nor NESC does a great job for predicting what happens in a manhole. If for no other reason than this. All theoretical models of arc flash are usually off by a factor of between 3 and 6. The fundamental reason for this is that they all have to make the assumption that 100% of the energy in the arc is converted into heat and is radiated out uniformly in all directions, or perhaps focussed (arc-in-box effect). BUT, the energy conversion is not 100% efficient. And there are losses in reflecting heat from a steel surface. So the theoretical models overpredict by a wide margin.

Manholes represent an extreme case of where the "arc-in-a-box" model should be applied with the victim inside the box, surrounded by lossy reflecting walls. Small arc flashes will be fine since the walls have no effect but as the arc flsah gets bigger, the walls matter.

Unfortunately to date very little has been published about this. That's why the arc flash blankets are only slowly making inroads because we can't simply take IEEE 1584 and add a multiplier to determine "with arc flash blanket" results.

thanks for sharing. good luck