Most of time I select "report as category 0 if transformer size <125kVA" from the PTW32. However, yesterday I tried to select "report calculated values from equations", I was shocked by the result since one bus rated at category 0 had been changed to category 3. Anyone who is familiar with PTW32 can give me a explaination? Thanks in advance!

Do you have > 2 second clearing time?

I assume that the time and current results in a Category 3 if calculated using the IEEE 1584 equations. PTW32 gives you the option of not reporting this high hazard because NFPA 70E and IEEE 1584 recognize that arcs on 240 volt systems with small transformers (high source impedance) are not sustained long enough to create energy above 4 cal/cm².

I'm not familiar with using that particular program but what may be happening is that with the <125 kva option selected the program automatically ignores the actual results and defaults to Category 0. With the actual results selected, it is giving the actual AFH calculated from the equations, using your parameters, such as time limits that are set.

Thanks a lot for the inputs. I did select 2s clear time. The result makes me very nervous since we might end up with high risk if we select the option <125kVA. Do we have to run both to make sure which is the worst? Or IEEE1584/NFPA doesn't care about it?

NFPA 70E does not require an arc flash hazard analysis where:

1) The circuit is rated 240 V or less.
2) The circuit is supplied by one transformer.
AND
3) The transformer supplying the circuit is rated less than 125 kVA.

There is a bit of difference between IEEE and NFPA 70E.

IEEE 1584 states below 240 Volts which implies include 240 Volts in the calculations and omit 208 Volts from the calculations

NFPA says 240 Volts and below which implies omit everything 240 Volts and below.

In either case, the logic from IEEE was that the lower short circuit currents from the smaller transformers combined with the lower voltage would make it hard to sustain an arc so that language was put in the 1584 guide to avoid calculating down to every small dry type lighting transformer.

Same kind of concept with short circuit studies. You don't necesarily have to calculate on the 208 secondary of small transformers because you reach a point where the fault current will be below 10 kA which is the lowest breaker rating (assuming it's marked - very old unmarked breakers default to 5 kA) so taking it further doesn't gain anything.

There is some test data to support that low voltage, low current arcs don't sustain very long but there is also more research going on.

The 2 second rule is based on reaction time - when the standard was written, the question about long clearing times and unusually large results came up and the 2 second rule was about the only resposne that could be given until we learn more about arc extinction and sustainability.

The high results you saw were likely due to the longer clearing time of the primary device of the transformer.

Jim you said:

"IEEE 1584 states below 240 Volts which implies include 240 Volts and omit 208 Volts"

I think you meant that reversed, it implies that you should include 208V, but that 240V may not be included. The wording says below 240, so the next nominal below 240V is 208V. NFPA says 240V and below, so 240V is included.

I guess I should have used more complete sentences. I did mean to say omit but I should have said "omit from the calculations"

The IEEE language suggests you include everything 240 volts and above but you can omit 208 volt circuits as long as the circuit is served by a small transformer that limits the short circuit current i.e. smaller than 125 kVA according the standard. I added "from the calculations to my previous post" Thanks for the input!

Did you ever get an answer on this? I have the same issue. SKM will default to Cat 0 1.2 Cal/cm2 when you select this option but is Cat 3 without.

Is is safe to mark all transformers <=125KVA as Catagory 0?

Thanks