I am being asked if a control cubicle that is part of the building's main switchgear could be labeled with a different label than the entire switchgear itself. The argument being that there is only 120V and 24V in the cabinet.

I see no definitive stance on whether or not an arc flash fault can propagate between switchgear sections. Are there any NFPA code prescriptions on this topic?

First off, there is a small problem with your statement. What is the voltage on a CT wire when an arcing fault occurs? Under normal circumstances the voltage present on a CT is low but if an open develops in the circuit, the open circuit voltage can be very, very high. For example in a 480 V circuit with a 100:1 CT, the open circuit voltage will be 48,000 volts since the open circuit voltage is increased by the same ratio that the current decreases. Hence the reason shorting blocks are so important to have That being said the amount of power available from a CT (the "KVA" rating although we express this in terms of burden) is very small. So you may want to rethink your stance with regards to shock protection but in terms of arc flash protection, you're on the right track.

There is no code or reference talking about propagation as such. I have seen the aftermath of several arc flashes. I haven't seen propagation within switchgear at all. Most practitioners treat the controls/low voltage compartment as separate and not an arc flash hazard. This is in contrast to panelboards where propagation definitely occurs. And I have seen more than one case where an arc started at one bucket in an MCC and travelled down the vertical bus to blow out the door at the bottom where the arc flash actually occurred. I haven't seen any evidence of horizontal propagation within MCC's. I really haven't even seen any propagation happening with metal enclosed gear as opposed to metal clad. My suspicion is that at least with medium voltage equipment it might be because the enclosures are so large but this is purely conjecture on my part. So many practitioners use this as a guideline (propagation within panelboards and vertical sections of MCC's absent evidence otherwise). Absent a Code the goal should be consistency in the assumptions.

I pulled together everything I could find and put together a summary paper on low voltage arc flash hazards not too long ago that I put in the articles section. The inherent problem here is that even if IEEE 1584 predicts a significant arcing fault, you can't necessarily go by that for three reasons. First quite often the fusing/breakers on the high voltage side are quite small (1E fuses and such) and even if there's no overcurrent protection, quite often the wire itself becomes a "fuse". IEEE 1584 testing in the past (lab tests used for the 2002 standard) used a #14 wire as a "fuse" to initiate arcs. Third and most importantly, somewhere in the neighborhood of 200-300 V, you get self-sustaining arcs. Below that voltage, arcs don't sustain and so it is nearly impossible to get an arc flash. The trouble is that so far, there seems to be no way to predict this. The summary I pulled together helps because I was able to document enough sources to show approximately where this division occurs. If nothing else referencing your voltages, Duke undertook a study for DC arcs in substation batteries and found that at 140 VDC, arcs would not self-sustain. I have not seen any evidence anywhere of an injury due to a 120 V arc flash outside of burned fingers and such that 70E doesn't do anything about anyways (its a survivability standard). There was a case documented by OSHA in Georgia involving a temporary construction panel (120/240) in 2009 where one worker was hospitalized and the other died. So it seems that at 240 VAC arc flash is possible not only in the lab but in the field.

However if you want a Code, IEEE C2 (the NESC) specifically deals with utility distribution systems and has it's own arc flash tables that include a variety of equipment, mostly "type tested" by EPRI. Unfortunately the table is developed for the utility industry and currently has a basic assumption of ATPV 4 PPE as minimum PPE but if you want a Code, at least it gives you one that is more reasonable than NFPA 70E. Wearing FR PPE as a minimum is a practice in several industries and if you can stand the cost of it, it's not necessarily a bad thing.

Thank you very much for your response.

There are two standards that touch on this topic of the arc flash propagating from one section of an enclosure to another section.

First is standard IEEE C37.20.7 - 2007: IEEE Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults. This standard certifies many things. One is that the arc flash energy will not propagate out of the enclosure, but it does NOT guarantee that the arc flash will not propagate to adjacent sections. If you add an additional test, in Suffix C, then you can guarantee that the arc flash energy will NOT propagate to the adjacent sections. This standard applies to low and medium voltages enclosures.

The fact that this standard allows for the additional test for proper segregation betweeen adjacent sections in MV and LV metal enclosed enclosures, opens the possibility that, if not tested, there could be propagation of arc flash energy to adjacent enclosures. The only way to guarantee it will not propagate, is to test it.

Second is standard IEEE 1584.1 2013. Here, in at least two sections, the standard mentions that the engineer performing the study should evaluate if there could be propagation (i.e. not having proper segregation) between the main breaker section and the adjacent sections. It also recommends the engineer performing this evaluation to consult with the equipment manufacturer for proper segregation.

I hope this gives you additional info that applies to your situation.

What's driving this whole discussion is that the switchgear has a Dangerous rating, meaning no work is allowed. So you cannot get into the gear to even do a deenergized confirmation test. Does NFPA 70E offer any exemption for Dangerous rated switchgear to do simple things like replace control relays and/or do deenergized tests?

I don't believe a "Dangerous Rating" is part of NFPA 70E.

"Dangerous" rating is made up crap from the software companies. There are two fundamental problems with it. First, it is a flagrant violation of ANSI Z535 which governs safety labelling. ANSI states that the signal word "DANGER" is to be applied when there is a hazard that poses an immediate threat to life and not just major injury. It applies for instance to doors covering exposed bus work. It is specifically forbidden to use it for hazards that are likely to cause major injury (such as arc flash) or where there isn't an immediate danger. Arc flash is rare, roughly half the likelihood of shock, and only around 1 out of every 10-20 cases is actually fatal. It is in no way appropriate to use the signal word DANGER. That's why 70E never mentions this or uses it.

Second issue is that where the software companies made up this crap is that at one time the best available PPE was around 40 ATPV. 100 ATPV arc flash suits are now routinely used in some industries and even higher ratings are available. There is no 40 cal/cm2 "limit". At this point you may as well set a practical limit for your own purposes...as in not supplying or purchasing above a particular ATPV and treating that stuff differently but there's no technological limit.

As to "exemptions", the current (2015) task tables recognize that there are a lot of tasks that would not have a potential to cause an arc flash. I suggest you look there for ideas. Also a similar (and much better in my opinion) list of examples is in the Annexes to 1910.269. If you went down the engineered approach which you obviously have if you are doing an arc flash hazard analysis then under 70E-2015 and later editions you have to also do a task analysis to look at which tasks have an arc flash risk. No risk = no PPE needed. Outside of what I've already described in medium voltage gear and similar designs where the low voltage compartment is isolated from the medium/high voltage compartment, there are typically low or no major arc flash risks present.

Moving on to testing for absence of voltage, you have really a small list of viable options. The first one is to remember that the hazard can only be controlled "upstream". For example a typical issue involves a 1500 kVA or larger 480 V transformer with a fused disconnect or circuit breaker on the high side. Typically the "main" breaker on the low side, whether it is an MCC or a panelboard, will have some pretty crazy high incident energy. You can approach this technologically. One approach that I've used successfully several times is to insert either a fuse panel (no need for a disconnect) or a small panelboard between the transformer and the "normal" panelboard/MCC. The settings are higher than the "normal" equipment but lower than the arcing current. This equipment should only be worked on by opening the disconnect (and testing for absence of voltage) on the high (primary) side. For this reason fuses work best due to their extremely low maintenance requirements and hence the reason that a disconnect is not really needed. Another alternative is to put CT's on the bushings of the low voltage side of the transformer and feed a protection relay (50/51) that trips a circuit breaker on the high side. For 12.5 kV systems there is also a "smart fuse" on the market that accepts a shunt trip input that you can use to essentially instantaneously trip the fuse using the signal from the low voltage side. Sometimes you can even do this within the transformer itself by using the right size Bay-O-Net fuses if it has them or mounting a molded case or fuse block directly on the termination compartment of the transformer if there is room. Again the goal is really to trip only in the event of an electrical fault in the line feeding the downstream switchgear, not to eliminate the need for that equipment.

Another option is that frequently the primary side protection has to be purposely jacked up to avoid the transformer inrush. As an alternative if the primary side protection is a circuit breaker, the settings can be dialed down to provide lower incident energy on the secondary side. This can either be done manually (a so-called "maintenance switch" makes this very convenient to do) or it can be done automatically...to use higher settings when the breaker is closed for a few seconds for instance giving the transformer time to magnetize, then dialing down to the "normal" settings after inrush has passed. Similarly if you can provide the signals or do it manually breaker settings can be raised/lowered to avoid inrush from other large inductive components too such as motors. Most ANSI C37-style breakers can be retrofitted with an aftermarket trip unit to add the capability and we do these in our shop.

Another option is to increase the distance at least in terms of the equipment. For example you can buy a Beier's PD50 meter which is a high voltage (up to 50 kV) meter and test that way. It is hot stick mounted so you can easily get 8-10 feet or more of distance away from the equipment when performing the test for absence of voltage. Don't use tape for this...it contaminates the hot stick. A similar solution can be achieved at low voltage using zip ties and a pair of hot sticks with an ordinary multimeter.

The next two options take another path in terms of testing by providing a safer location. The first option is to open the disconnect on the high side of the transformer and also perform the testing for absence of voltage at that location. The incident energy at that location will be much lower. The final option along the same lines is that my company sells a small box that can be mounted on the front of the enclosure that has current limited taps (jacks) that limit current below the current considered a shock hazard...simply remove the dust covers and stick your meter into the sockets safely outside the enclosure with the door closed.

Anyways...just some practical ideas on how to solve what is a very impractical issue. I made several suggested approaches because different situations call for different approaches. There is no "one size fits all".