A recent discussion came up about whether or not there is an arc flash hazard when entering a transformer compartment. In some ways it's a great example of what pisses me off about the fact that the 70E Committee keeps insisting that first we need to implement the hierarchy of controls approach (eliminate or reduce the hazard first, and use PPE as the last resort). That's fine but then instead of acknowledging that the rubber glove method is only ONE method of avoiding shock hazards while working on equipment, they have been busy deleting all references to the other 3 techniques that are in the shock hazard standard that they reference. The other three techniques are insulating the equipment (cover up method), using insulated tools (hot stick working method), and finally the bare-hands/live-line technique. The latter probably has no purpose in the 70E standard since it is really best used at 69 kV and above and 70E is pretty much only a standard for 35 kV and below but insulating equipment directly eliminates the shock hazard and using insulated tools such as hot sticks removes the employee from the restricted approach boundary altogether, again eliminating the hazard. Hierarchy of controls pretty much mandates use of cover up and/or insulated tools as the first recommended method for avoiding shock hazards. Incidentally both techniques also severely reduce or eliminate the possibility of arc flash hazards due to accidental movement of employees, equipmnent, or materials by eliminating at least one conductor from the work space.

In years past a nice transformer termination looked like:
http://www.cablejoints.co.uk/blog/artic ... ansformers

However with respect to medium voltage there has been an improvement by leaps and bounds. Most padmount utility transformers now come with elbow connectors as the standard for termination. For those that don't work in medium voltage, this is essentially a pin-and-socket system except that the plug is shielded and the shielding carries through continuously into the socket, sort of like a coax connector for medium voltage power conductors. It has become so pervasive in a short span of time that open air termination chambers like the above are now a special order. I received two quotes around 2014 for a 1500 kVA, 4160:480, delta-wye, 1500 kVA transformer, with all copper coils. The elbow-terminated version was around $25K. The air termination version from the exact same manufacturer (one of the largest) was $40K. This type of transformer looks like what you see in this article about halfway down:
http://ibewlocal96.org/ibew-local-96-ap ... sachusetts

Notice what's missing...NO exposed conductors. In fact there are special capacitive taps that allow you to take measurements through the connectors that are both shielded and insulated WITHOUT exposing anything at all. There is really no difference when the doors are opened between this transformer and the outer door of a dead front lighting panel.

No reason that this doesn't apply to panelboards, either. For years the practice was to have open (live front) equipment. You saw all kinds of truly scary stuff like this:
http://forums.mikeholt.com/showthread.php?t=91496

Igor, throw the switch! So slowly we made them safe to be around to making it safe for unqualified personnel so that today once the covers are on, the equipment is dead front and safe to work on or around for unqualified personnel. But why did we feel that it's still OK to expose qualified personnel to the same risk as live front equipment? Just because they're qualified to work on it doesn't make it right. Even a typical 480 V panelboard if terminated properly doesn't have to look like that once you take the covers off:
http://forums.mikeholt.com/showthread.php?t=159563

But this is all well and good for brand spanking new installations with an ogre for an inspector that won't pass anything that is exposed, but what about equipment that is not available or doesn't meet this kind of requirement? You can buy premolded components to install directly over the exposed conductors to make the transformer example safe:
http://midsungroup.com/distribution/

Or you can buy sheets of lexan/lucite (because it's clear and has a high insulative value) or glastic (red FR fiberglass boards) and construct enclosures, or install metal cage covers, all of which serve the purpose of insulating (or isolating) exposed conductors.

And what about the equipment that has not been modified or installed new without exposed conductors? We can borrow a page from the utility industry that for years has required coverup in addition to or in lieu of insulating PPE. Simply apply rubber blankets with clothes pins and line hoses. Line hoses may not apply to industrial applications but rubber blankets certainly do and can achieve the same effect as the custom-made rubber boots and caps on an as-needed, temporary basis. Here is a picture of the technique including very obvious clothes pins in use:
http://www.nola.com/news/index.ssf/2015 ... audit.html

If we use coverup effectively, then we can work with NO exposed, energized conductors anywhere within the work space except for the ones that are being worked on. This is fantastic for shock protection. But in addition to that it also eliminates a very large fraction of arc flash incidents that are caused by human error.

By the way as a practical side note, there are two kinds of "rubber blankets" that fall under different codes as far as testing requirements go. The first type comes as a true "blanket" such as a 48" x 36" rectangle and has a test stamp. It has to be periodically retested/recertified just like rubber gloves, hot sticks, and other insulative equipment. The second type comes in a big roll and you just cut off whatever you need to do the job. It is factory certified...no need to retest periodically.

I must point out the Midsun page linked shows gear intended to reduce animal caused outages. It is not intended or tested to prevent shock hazards to humans.

The elbows you speak of have full load break capability when used/sized for 200 A. Can't do this with an open air termination.

I like the metal shopping cart parked in front of the knife blade switches. Nice touch.

True but then we get into "intended use" and "tested" which gets to be a very sticky morasse because what do you use for a standard for test/design criteria? I mostly used that web site for the pictures, not specifically for the product. There is another manufacturer that makes the reddish colored boots that seems to be very popular as well and isn't specifically targetting animal control. Ultimately the only "test" I know of and I forgot the standard it comes from when it comes to medium voltage cable is the "tin foil test" where you wrap all the supposed insulators in tin foil and then hi pot to ground.



It's even better than that. Elbow connectors are standardized under IEEE 386 so they are interchangeable from one manufacturer to another. There are about a half dozen standard voltages and two currents (200 A and 600 A). You can get either one in a nonl-loadbreak configuraiton but as mentioned the 200 A version is also available as a load break (disconnect) version. However I ran into a problem with using them. The nonloadbreak version has a maximum pull of I believe around 150 lbs. or less to remove one but the load break version goes all the way to 250 or 300 lbs. as I recall. If the nonloadbreak versions are stuffed into an awkward enclosure where you don't have direct access from the "back" of them, they can be somewhat difficult to remove after they have been in service for a few months but the loadbreak version is almost impossible to remove without assistance (comealong, vehicle tow, etc.). So I have been avoiding them for that reason.

They are incredibly tough connectors overall though...they can be used in underground vaults where they can be submerged indefinitely. They are immune to dust, dirt, and being submersed in water. And they are available in a fused (fused disconnect) version from Cooper, as "stackable" connectors (nest one inside the other), and even the transformer wells are interchangeable if it becomes necessary to change the female side.

Paul,

what you observe as advances for arc flash mitigation are coincidental and have nothing to do with arc flash.

The Load Break Elbows are a switching device used by Utilities at locations for at least 30-40 years. Any arc flash lowered risk (if any, that would need to be considered before assuming zero) is only a by-product of these thing's intended purpose of being able to park cables safely. They are fully insulated and work under water (but can't be switched of course until the vault is pumped down), but I would not say for certain there is no arc flash risk. The stinger is supposed to swipe clear any load current, but what if something breaks?? The Hazard would be lowered by using the intended hot stick for switching. The arc flash risk and hazard of these great devices would seem to still be higher than uninterrupted cables, and now NFPA 70E indicates that cables represent an arc flash risk if examined in a way that disturbs the cable.

The Stress cones you show as the older type installation are still utilized everywhere I am aware of that has primary air terminal chambers. Most industrial sites do not want to hot swap primary cables and do not specify Load Break Elbows. ( If better rodent and snake control is needed we will specify Stress cones with added anti-flash-over skirts intended for pole top installation, so the contamination path is longer and perhaps not continuous.)

The old slate open face switchboards are a historical item, encountered in buildings perhaps now 90 years old, now always in limited access areas. I have only encountered those in less than 300V systems, that would seem to be self extinguishing arcs if switched. An open face 480V system would seem to be doomed to failure as the voltage and current are generally high enough to sustain an arc.

What I HAVE encountered in 480V systems, that seems counter intuitive is small fuse holders meant to be withdrawn as a disconnect. I thought these were limited to Air conditioning installations where the risk might be slightly mitigated by an source side circuit breaker (though still seeming to be a questionable idea). The stabs on the rear of such fuse blocks do not casually appear to be as precise as 15kV stinger on load break elbows and I do not know if that is intended as an arc extinguishing device, but the fuse holders are rated as a switching device.

The worst situation seems to be utilization of 480V fuse blocks within a metering disconnect panel at multi-tenant commercial building service entrance. There might not be any upstream disconnect, there might not be any other method of turning off the power, and these devices with no easily identified arc- blow- out- system are the only way to turn off the power.

I am not sure of the point of your post, but would be very cautious about using a factory certified high voltage blanket that does not need to be retested periodically. Testing and dating rubber goods is a big part of utility safety programs, and bypassing this procedure by using a different blanket is suspicious. That product might not be intended as a tool.

Three PM's that are recommended by 70B and typically done for oil filled padmount transformers would be doing IR scans or UV scans of the connections, reading the temperature and pressure gauges, and sampling oil. The latter 3 should be done once a year. A very common padmount transformer is mounted over a cable pit and has the connections, gauges, and valve all located inside a double door arrangement with a pentagonal bolt and a lock to keep unauthorized personnel out.

All 3 PM's do not involve working on on (disturbing) energized equipment. Even with exposed terminations inside air termination compartments there is clearly a very low risk in performing these tasks but it does still depend on the worker being careful to avoid coming into contact with energized connections while stooping/kneeling right next to energized connections to reach the sample port, never mind whether or not opening the doors is a significant risk. With the elbow connector design all of these risks all but vanish.

Taking some other points:


There was at least one reported OSHA incident in the past 10 years where cables in a vault were being manipulated and a fault developed at an elbow connector that injured the employee in the vault from the arc flash. But this is anecdotal...it shows that that an arc flash injury while manipulating cables CAN happen, not that the likelihood is an issue. You can't really do statistics on a single incident...the confidence interval is just too large to be valid especially with regard to incidents with very low rates. The fact that an elbow was involved doesn't prove causality either. In a sand plant in New Jersey several years ago an arcing fault occurred while I was tracing cables by manipulating them (pull and see where it goes). It turned out that someone had previously spliced cables using split bolts and the splice failed. Is this an example of improper maintenance/installation or that cable manipulation is always a risk? Is it a "risk" if the equipment is properly designed/installed/maintained as was clearly not the case? Again these are anecdotal cases that show that it can happen but not proof positive that the likelihood is significant.



It can be stated right up front that there is always an inherent risk of hazards in all tasks...there is no such thing as "risk free". But what we can do is to establish a probability or likelihood of an injury based on making comparisons such as comparing the risk to the reported probability of an arc flash injury according to ESFI of 1 in 100,000 workers per year. If the risk from a specific, given procedure to perform a task the probability of injury is less than or greater than this criteria. If this is the case with either arc flash or shock then with the way that 70E defines things, this means that there is "no" risk. Technically we don't mean "zero" risk but that the probability is so low that the risk is less than other injuries.



Most sites use either fused disconnects or a circuit breaker on the primary side, even if the fuse is a cutout. Eliminating the disconnect (fuses only) makes it impossible to change fuses. So the load break elbow is at best redundant in an industrial environment. But a termination still exists and the non-load break variety is a good option, which was really my point.



There was an old power house in New Jersey. The site had been manufacturing cast iron water pipe for over 200 years and was the first iron pipe foundry in the United States. Before the plant was finally closed down yours truly was the last engineer reporting to the site. The bus in the power house was a 2300 V system. I was told that at one time they ran the boiler (still existing but unused) to run a generator which in turn produced power for the whole site. All that remained when I was there was the boiler (not used but still in place), some bus and cable, and some switches. The two disconnects were open units that had chain link fences around them so they were "isolated". One fed the power house and the other fed a nearby machine shop. There's really not a lot of difference between those switches and the live front 120/240 panelboards of the same time period. Arcing is clearly self-sustaining up until the point that the overcurrent devices (fuses) trip. It is no more "doomed" than enclosed equipment. In 2008 we were able to commission a new substation and refeed the single machine shop fed through the power house. We bypassed the disconnects (lockout was done from considerably safer upstream equipment) to the 2300:480 transformer which decommissioned this equipment for good. There was clearly evidence all over this equipment of previous incidents but as I said an arcing fault would not be any worse than it is in any other enclosed equipment and perhaps better because most of the energy in the arc is radiated out instead of contained.



Two different products altogether with different ASTM requirements. Rubber blankets fall under ASTM D1048 and must be tested at 12 month intervals (see OSHA 1910.137, Table I-5) just like sleeves and gloves (although gloves are tested every 6 months). Typical sizes would be for instance 3 feet x 3 feet or 3 feet x 4 feet. Rubber insulating sheeting falls under ASTM F2320. A typical size would be 3 feet x 30 feet. It is a bulk roll product with the intended use that pieces are cut and used as needed so it is rated identical to rubber line hose (ASTM D1050), covers (ASTM D1049) and matting (ASTM D178) and is only tested upon visual inspection that it is suspect. It's this disposable roll aspect of it that means it is designed, testing, and treated differently. And reputable manufacturers such as Salisbury make both products.

ASTM abstract for D1048: "This specification covers the acceptance testing of insulating rubber blankets that are used for the personal protection of workers from accidental contact with live electrical conductors, apparatus, or circuits.".
ASTM abstract for F2320: "This specification covers the acceptance testing of insulating rubber sheeting that are used as a covering for the personal protection of workers from accidental contact with live electrical conductors, apparatus, or circuits."

Looks like it's intended to be used as a tool for personal protection of workers from accidental contact to me. The only difference is that the intended storage and how it is used is different so the specifications are more stringent for rubber sheeting compared to rubber blankets.