Lonely wrote:

PaulEngr wrote:

I've worked with true DC generators. The tables are waaay off.

I noticed this too. Why is this?

It's all in the input parameters. And it is probably the tail wagging the dog.

If we were developing a table from scratch then we have a couple choices for nominal voltages. Motors and generators are typically 150, 300, and 600 V. Batteries are typically 60, 125, and 250 V. These are nominal voltages.

Now to encompass the greatest number of potential loads we want to pick a "ridiculous" kA rating, such as 20 kA+. We might even optimize this one by plugging in different kA valeus and observing the resulting incident energy, "optimizing" the table value to encompass the greatest number of loads.

70E has thrown out "PPE 0" so the tables all now start at 4 cal/cm^2 or PPE is not even needed. So with this as a target we can mix and match the above inputs to derive the fewest number of entries that covers the greatest number of loads.

But there's an obvious fallacy here. DC equipment that doesn't even reach 1.2 cal/cm^2 is the greatest amount of equipment. And there's no longer a "PPE 0" in the table. In fact the overall approach that has pretty much always existed in 70E, which is a huge fallacy, is that for any given task and any given equipment, there are 4 possible states:

1. Low risk, low hazard

2. Low risk, high hazard

3. High risk, low hazard

4. High risk, high hazard

Note that all versions of 70E from at least the 2000 edition address #4 with a PPE requirement and as of 2012 (and more strongly in the 2015 edition) point to the ANSI Z10 hierarchy of controls as the correct approach while supplying specific PPE requirements as a last resort. In the 2012 edition and earlier item #3 (1.2 cal/cm^2 or less incident energy) was addressed as either "H/RC 0" in the tables or simply ignored (ala arc flash hazard boundary). Items #1 and #2 were "addressed" extremely poorly by the definition of an arc flash hazard only. As of the 2015 edition with regards to tables items #1 and #2 are now explicitly addressed while with the expunging of H/RC 0 there is no longer any room in the tables for item #3 (no hazard). The net effect is that when using the tables case #3 no longer exists and only cases 1, 2, and 4 are addressed adequately. Effectively the tables start at 4 cal/cm^2. The same condition exists in the tables in NESC but arises out of the fact that OSHA is adamant that utility workers must wear arc flash PPE for effectively all work almost without regard for the magnitude of the hazard because the concern is over hazards other than arc flash (with copious hand waving going on).