Great response last week!

This weeks’ “Question of the Week� is a continuation in our series about arc flash labels. This time it is about the size of disconnect switches that should be labeled. By size, we are talking about the current rating.

What size disconnect switches should have an arc flash warning label? (Select ALL that apply)

• Fusible disconnects 100 Amps or less
• Fusible disconnects greater than 100 Amps
• Unfused disconnects 100 Amps or less
• Unfused disconnects greater than 100 Amps
• It depends - Please elaborate!

It depends on the available fault current and voltage. a 15 kV disconnect is going to be a much different story at 100 A, even if fused, than a 240 V HVAC disconnect. I just brought four 5 kV metal enclosed disconnects online yesterday. The incident energy was pathetically low (<1 cal/cm^2) so no real need for anything other than "basic PPE" but currents were capable of getting as high as around 300 A. High speed relays and breakers coupled with relatively small transformers are a good thing. On the other hand if you have a fused disconnect fed from a 2500 kVA, 480 V secondary transformer with a reasonably stuff upstream bus, I challenge anyone to find a way to get down much below 40 cal/cm^2 (this is a real case).

Second, failure rates for disconnects, whether fused or not, are very low, but the failures that do occur happen almost always when they are operated. It can be argued that the switch failed at some other point and is only revealed during switching but the result is the same. I have not heard of arcing faults occurring on switches without being operated except unprotected switches hit by lightning and under severe contamination. Regardless, failure rates are well below accepted safety thresholds so these should categorically qualify as safe regardless of incident energy while in normal use.

Third, when undergoing maintenance, everything changes and you'd better know what the incident energy, shock potential, etc., is. This is true regardless of whether there is a fuse or not (and the type...boric acid vented fuses bring their own hazards to the party).

This is similar to the question of a "low cutoff". At around 28 volts or below, simple physics even for DC makes it impossible to sustain an arc no matter what the fault current is. Up to 50 volts, the required current even at DC in order to sustain an arc is enormous (thousands of amperes). OSHA has never recorded a shock fatality, and MSHA has only 1 at 48 V but it was under very extenuating circumstances that are unlikely in the first place. The massive amount of telecom equipment (much of which is DC) operating at 48 V without shock fatalities is a testament to whether or not the 50 volt cutoff is reasonable. Above that point it requires some amount of current to sustain an arc but the amount required decreases as the voltage increases. These values should be setting the thresholds to replace the "150 kVA, 208 V or less" rule.

I think that all three phase disconnects should have some type of label. It is required to at least have a shock hazard label and generic labels can be purchased or made for the low voltage equipment but the requirement in 70E is that equipment being worked on or potentially worked on energized is to be labeled. I don't think that a disconnect should be left unlabeled just because it doen't have a protective device since there is no reason for a person to be in there while it is energized. At some point someone will be in there taking voltage or current measurements while troubleshoooting the connected equipment.

For me it comes down to the liability. A disconnect which doesn't have a label doesn't require PPE. Yes, if a person is in there they should know the minimum PPE for opening the door but I didn't know the danger even after the first appearances of arc flash safety years ago. If the equipment is labeled the liability is reduced and the worker can choose the proper PPE.

I don't work on medium or high voltage equipment so this is limited to 208-600v systems.

I think this may be at the heart of some of the labeling problems. Many consider it as you stated. No Label = No PPE Required. A bit of a mis-interpretation.

You are right, but perhaps it goes back to general unawareness. We are steeped in tradition - "we have always done it this way" or "you are just trying to scare us" or "you are just being over-cautious" or "we just can't afford to do that"

For all of the statistics, given so many incidents do occur each year, many people just have not seen an arc flash or the aftermath of an incident. Or even when they do see a starter can with the side melted off, they just don't get that that could have happened with them standing right next to it. Is that unawareness, apathy or just an air of invincibility.

I think any disconnect that can be opened should have a label. If it is unfused, print a label with the information of the upstream protective device and affix it to the disconnect. If it is fused then it needs to be assessed. We have some fused disconnects that the Incident Energy is higher than the upstream device because of clearing time. They need to be labeled.