Larry Stutts wrote:
Thanks Paul.
Much of this I knew. I was really looking for guidelines on what does not exist in a well-defined form yet. This seems to be a recurring theme in many of my questions such as previous questions like 'What constitutes a suitable barrier' and 'How large an opening is acceptable'
Being a manufacturer, we are of course just as interested in the wiring standards for the inside of the enclosure. We do have wiring standards. Of course they were developed before the coming of the great arc-flash focus. We want to insure everything we build is compliant with UL, CE, OSHA - basically all European and North American standards.
Sorry...missed it and when it said production and maintenance I was thinking of an end user, not a vendor. As you are probably aware the closest thing to a "generic" UL requirement is 809A for industrial control panels. Manufacturers are slowly getting on board with other directions though. There are multiple "arc resistant" standards, although a large portion of "arc resistant" gear is in fact not actually "arc resistant" as such. An example is low voltage MCC's that are tested following the ANSI C37 protocols for medium voltage switchgear. There is no such thing as ANSI-compliant "arc resistant" gear because the relevant (UL) standards don't cover this. It comes from either misuse of the ANSI standards or else use of the IEC standards.
A huge body of research including enough details to write code and/or do calculations on arc resistant gear was recently released by CIGRE. As an end user I found it fascinating and lacking at the same time. There are two issues with this body of research. The first is that essentially the most basic model considers a heat source producing a relatively constant amount of heat and the pressure rise due to that heat source. The model then arbitrarily uses a cutoff (Pmax) where beyond this value, the enclosure ruptures. The first issue is that although a lot of testing and modelling was done, they failed to be able to provide a way to calculate Pmax so this value must be measured through testing. Similar issues exist in explosion proof enclosures though so I can't really fault them for this. Second issue is that the arc input energy was modelled using the Lee model which is not a great start in the first place, and then using somewhat arbitrary constants to correct the results. All published test work used artificial heat sources instead of voltage and current as inputs. It stands to reason that substituting a better model (say IEEE 1584 differentiated with respect to time) would result in a better fit to the data but there's not enough published data to go down this path.
What I got out of it without using the model for its intended purpose (as an end user I don't particularly care about modelling arc resistant gear) is that the enclosure can be treated almost like a sealed box if the total surface area of openings is less than 10%. Otherwise, the full model applies.
Second item that was discussed in that work is alternative atmospheres such as SF6 and what a dramatic difference it can make. And the third interesting item is using "absorbent" materials that can significantly retard arc flash. Rockwell Automation has done some proprietary research on this subject as well and it almost looks like all they are talking about is what color the enclosure is painted but they aren't sharing details. With respect to paint, I can easily understand this. The emissivity of aluminum is down around less than 0.1 while it is around 0.65 for rusty steel, around 0.80 for painted, and close to 0.9 for black painted steel. Thus we can significantly alter the absorpotion or reflection of heat of the enclosure merely by altering the material and/or color. A black colored absorber with a high heat capacity within the enclosure is likely to significantly draw heat away from the arc flash and retard the time before it ruptures.
At this point in time there are 4 basic ways to attack arc flash as a general rule:
1. Reduce the arcing fault time. This is the realm of relay manufacturers. I haven't seen an "AFCI" for 480 V yet but I have no idea why it can't be done. Right now we have light detectors, di/dt detectors, differential relaying zone selective relaying (allows increased use of instantaneous tripping without consequences to coordination), and standard relaying as our means of detection/tripping.
2. Arc elimination. This consists of either high resistance grounding which reduces the likelihood of arcing in most cases, or various devices that suffocate the arc by providing a lower impedance path temporarily in conjunction with a main breaker.
3. Reducing the current. This is a two edged sword though. Current reduction can also mean increases in opening time and hence increased incident energy if not coupled with changes to trip functions. This causes odd effects such as switching to a current limiting fuse can actually increase rather than decrease incident energy.
4. Various schemes such as some items in the new IEEE MCC guide document that result in a reduction of hazards during certain tasks. For instance racking out buckets is normally pretty hazardous. Changing the mechanism to incorporate a disconnect or shorter stab arms or adding "closed door" racking in combination with arc resistant gear eliminates the hazard altogether from this particular task. Various means of verifying voltage with the door closed also significantly reduce the hazard during testing for absence of voltage. And finally as I already mentioned, eliminating any and all exposed wiring with the doors open is a big step forward. In combination with upcoming IEC rules for test meter tip designs, there will no longer be any exposed wiring during testing functions.
In the medium voltage world, the IEEE 386 "elbow connectors" are finding more and more uses outside of their original intended use for underground equipment. What you start with is a molded rubber connector that can be load break or non-load break with standardized fittings that extends shielding continuity all the way through the connector to both ends so that there are no exposures to any of the conductors. The connector itself is designed for burial so it is waterproof and dust/dirt proof. With no external exposures and shielding all the way around, arc flash is virtually eliminated. A similar design exists with SF6, rubber, or solid epoxy insulated switchgear. The gear is frequently sealed for life and maintenance schedules are very low (lubrication/inspection 3-4 times over the entire lifespan of the equipment). This is a far cry from drawout gear where every racking operation is questionable in terms of safety and the gear requires extensive annual PM's that were generally not being followed. As an example just last month I had a Westinghouse breaker where the gear was stripped out so when I went to rack it out, the breaker just stopped halfway in/out. Even if we cut off power and accessed it from the back, the entire racking mechanism was built into the breaker and if we had not managed to coax it out, there was no practical way to remove the breaker short of disassembling the entire switchgear or going in with demolition equipment like torches and cutting the breaker apart. This is as opposed to the UL rated equipment such as switchboards and panelboards where everything is bolted so it can't be maintained "online" and must out of necessity have much longer PM schedules. Today the designs have converged. Switchgear manufacturers have standardized on a small number of breakers that are used in both drawout and bolted designs for cost reasons, so we now have very long maintenance schedules even on drawout switchgear, but the breakers are not the issue. It's the drawout mechanism itself. As an example the ABB AMVAC breakers that have been out for close to 20 years have proven themselves in use. They were originally advertised as "zero maintenance". Far from it...they require some maintenance, but not very much. The big problems I've seen with them are where the low voltage signal connector doesn't engage properly, or the cell fingers don't engage properly, or the cell arms misalign for one reason or another, leaving it in various states of not quite truly closed/open or engaged/disengaged. ABB themselves report that 80% of arc flashes in drawout gear is in the drawout mechanism, not the breaker. This all points to one clear conclusion: perhaps it is time to retire drawout gear. Panelboards are a lot less expensive and much smaller. And gas insulated gear is the ultimate in reliability and small size, but also can't be removed without tearing the equipment apart. Adding spares is relatively inexpensive for both types of equipment. A final option that provides the ability to take breakers on/offline without a drawout mechanism is to use the elbow connector to tie breakers together. I've actually seen equipment meant for underground vaults built this way but there's no reason to restrict use to underground only.
Taking this all together I believe the trend is going to be towards defining what specific tasks are required for maintaining equipment followed by what the hazards of those tasks are, and designing equipment around those hazards so that they no longer exist in the future. Medium voltage gear is becoming more standardized at least from a single manufacturer and becoming more of a modularized affair with bolt-together sections that are stacked to produce a particular function. Similar trends are occurring in 600 V class gear. Many overload relays are now microprocessor driven electronic relays and the inputs/outputs are designed to bolt directly onto the lugs of the contactor without any jumper wires. It would not surprise me to see the same trend in breaker or fuse clip marriages to the contactors. There are already some small (10 HP) starters on the market from Rockwell Automation that don't even have enclosures anymore. The entire "MCC" is now a set of tightly integrated modules with no user serviceable parts and in some cases, even the cables to the motors are premolded assemblies, and even field disconnects are available in the same system. Most of the maintenance tasks that would have an arc flash potential are elminated as a result. If this design extends upwards into higher currents, although arc flash is still a very real threat, it could become a very limited and specialized problem.