70E "Minus One Rule"

[Electrobe]

I have seen some references to the "Minus One Rule" for operating disconnect switches with the door closed and fully latched. Basically it states: If the PPE level has been determined by an actual calculation and the door is properly closed/latched then the PPE level may be reduced by one.

Is this a valid rule and where did it come from??

Thanks

 

[zog]

I have seen some references to the "Minus One Rule" for operating disconnect switches with the door closed and fully latched. Basically it states: If the PPE level has been determined by an actual calculation and the door is properly closed/latched then the PPE level may be reduced by one.

Is this a valid rule and where did it come from??

Thanks

No it is not a valid rule. Someone is misunderstanding the standard.

 

[davidr43229]

It is my understanding for the NFPA-70E 2009, in article 100 under Arc Flash Hazard, FPN No. 1
An arc flash hazzard may exist when energized electrical conductors or circuit parts are exposed OR when they are within equipment in a guarded or enclosed condition, provided a person is interacting with the equipment in such a manner that would cause an electric arc. Under normal operating conditions, enclosed energized equipment that has been properly installed and maintained is not likely to pose an arc flash hazard.
My question:
If a Disconnect, Switchboard or Panelboard has not been properly maintained, do we assume worst case #4 per the task tables?

 

[zog]

My question:
If a Disconnect, Switchboard or Panelboard has not been properly maintained, do we assume worst case #4 per the task tables?

You could but that may not be enough, if the breaker wont open to clear the fault your Ei values may be higher than that. An arc flash study is only valid if the OCPD's operate in the times they are assumed to operate. That is what chapter 2 of the 70E is all about.

 

[wtucker]

The minus one rule had to do with the 2004 footnote 6 to Table 130.7(C)(9)(a), which said, "For < 25 kA short circuit current available, the hazard/risk category required may be reduced by one number."

That's been eliminated.

However, new Note (f) to that same table says, "For equipment protected by upstream current limiting fuses with arcing fault current in their current limiting range (1/2 cycle fault clearing time of less), the hazard/risk category required may be reduced by one number." Generally this has to do with motors fed from MCC's with the proper fuses. Frankly, I find this confusing. Can anyone elaborate?

 

[zog]

However, new Note (f) to that same table says, "For equipment protected by upstream current limiting fuses with arcing fault current in their current limiting range (1/2 cycle fault clearing time of less), the hazard/risk category required may be reduced by one number." Generally this has to do with motors fed from MCC's with the proper fuses. Frankly, I find this confusing. Can anyone elaborate?

No, not really. This is a silly note, how is someone using the tables supposed to know if the arcing fault current is in the current limiting range. If they knew that info they had to have done an analysis and wouldnt be using the tables anyways.

I bet this was a proposal from someone at a fuse manufacturer as an attempt to sell fuses.

IMO this will lead to some dangerous assumptions being made by people not fully understanding what this note really means.

 

[jim dungar]

IMO this will lead to some dangerous assumptions being made by people not fully understanding what this note really means.

I agree that this note will be misapplied.
I doubt there are many locations which will generate an arcing fault with enough current to cause any large fuse (i.e. >100A) to enter its current limiting range and 1/2 cycle clearing time. But, I can picture many people saying they have a 2000A current limiting fuse in the service equipment and then using the footnote everywhere in their facility.