PaulEngr wrote:

Can you perform an ANSI or IEC short circuit calculation in detail with no assumptions (no infinite bus transformer primaries, no zero impedance conductors) correctly? Can you perform a TCC study based on the short circuit study? If the answer is yes then you have the prerequisites to use Arcad's web software. He has a lot of stuff on there. Some of it is based on somewhat more theoretical calculations and some of it is based on more empirical ones and standards. All are fairly well documented.

The short circuit and coordination studies are required by NEC Code which unlike 70E is state law in all 50 states and not just "required" indirectly. Most licensed electricians should be fully capable of doing an NEC grade "study" in a matter of minutes. But the nature of these studies is that there are some shortcuts which result in a "conservative" (over-estimate) result which is generally good enough to meet NEC requirements. But these same results cause the arc flash result to be underestimated.

ARCAD's SCA V1.0 software program[/url] for short circuit fault current calculations is based on the comprehensive methodfor short circuit analysis, factors in both active and reactive parts of equipment impedance and takes into account contributions from multiple sources including motors and generators. The disparity between calculation results from ANSI, IEC comprehensive method etc. can be explained by different approximations and assumptions adopted by different methodologies. For example, some techniques ignore equipment X/R ratios in the analysis and apply different correction factors to equipment ratings. Disregarding the equipment X/R ratio introduces up to 15% uncertainty in a single step of adding two impedances alone while applying different correction factors does not quantifies and does not corrects the error but only amplifies it.

PaulEngr wrote:

In the case of >15 kV OSHA has more or less "blessed" ArcPro software with the revised 30 CFR 1910.269 rules, so you are probably best following their guidance. Read 1910.269 in the appendices for more information on this. From 250 V-15 kV currently the best documented/most reliable calculation is the IEEE 1584 empirical calculation. Below 250 V you are generally stuck using a table or research results since the calculation breaks down at that point. Since at least some of Arcad's calculations are based on IEEE 1584 then you can use those but again, you need a lot more information than just a one line.

ARCAD's AFA V5.0 software program and mobile apps[/url] for arc flash hazard analysis and labeling are based on IEEE 1584 Guide for Performing Arc-Flash Hazard Calculations. The IEEE 1584 empirically derived model was chosen for arc flash analysis due to the model's capability to accurately account for a wide variety of setup parameters including:

* open and box equipment configurations

* grounding of all types and ungrounded

* gap between conductors of 1/8 to 6 inches

* bolted fault currents in the range of 700A to 106kA

* system voltages in the range of 208V to 15kV

* working distances from 10 to 80 inches

For cases where voltage is over 15kV (up to 46kV max) or gap is outside the range of the empirical model (more than 6 inches gap), IEEE 1584 recommends applying the theoretically derived Lee method and the method is also included in Arc-Flash-Analytic.

You may also consider ARCAD's for arc flash analysis in DC power system. Check for DC arc flash modeling implemented in DCAFA V3.0. The program calculation procedure[/url] is very similar to the procedure proposed by Jim Phillips in

Know Your Arc: DC arc flash calculations. It also takes into account circuit time constant when determining arc duration as a function of predicted arcing current, upstream protection device type and rating and the fault time constant.

ARCAD has recently released new primary aimed for single phase arc flash analysis. Single phase AC tests on systems with an open-circuit voltage of 146V described in L. E. Fisher "Resistance of Low-Voltage AC Arcs" showed the arcing can continuously sustain even at this low value of voltage. ArcMaster's shows the arc voltage and arc sustainability depends on the circuit power factor. Note that both in theory and practice the arcing currents are dependent on system power factor when even a small change in the power factor may result in a significant change of arcing current. Namely, the tests tend to indicate weak unstable arcing at 100% PF but spectacular sustained arcing at low power factors at the same current and voltage.

PaulEngr wrote:

If the answer is no then I offer you three solutions. The first one is to use the table method in 70E which means that you need to know the task being performed, the equipment being worked on, and third you need to know the short circuit and opening time of the equipment, in theory. If you've already done these calculations then you might as well do the engineering study approach since the PPE requirement will be much less among other reasons.

This is where Arcad and I have a difference of opinion. In the case that you don't have the engineering calculations, and as I'm hinting at there's a lot more than just arc flash, then using the tables without benefit of the calculations is better than nothing at all. In fact a study has been done that shows that "dumb luck" (not doing anything about arc flash) done by some folks at Dupont based on about 30-40 cases has a serious injury rate of 100%. If you just use the tables without the engineering calculations then based on an older edition of the tables (the 2015 version is much improved) there is a 50% chance of avoiding serious injury. And if you do the engineering calculations although there is a theoretical 5-10% chance of injury in practice it drops to 0%. So as much as I'd argue for the engineering study approach whether you use Arcad's web site or not, using the tables at least initially is better than doing nothing at all by a wide margin.

I would not recommend using the NFPA 70E table method as I don't believe that encouraging your personnel working on energized equipment when there is 50% chance of serious injury is right. There are two methods to select the appropriate PPE.

1. Incident Energy Analysis OR

2. PPE Tables Method [130.7(c)(15)(A)(b) or 130.7(C)(15)(B) and 130.7(C)(16)]

An incident energy analysis shall be required in accordance with NFPA 70E 130.5 for the following:

1. Tasks not listed in Table 130.7(C)(15)(A)(a)

2. Power systems with greater than the estimated maximum available short circuit current

3. Power systems with longer than the maximum fault clearing times

4. Tasks with less than the minimum working distance

Hence, NFPA 70E prohibits using the PPE Table Method if short circuit fault current (2) and protection device operating time (3) is not know. Arcing current need be calculated in order to determine the upstream protection device operating time. Once, short circuit fault current, arcing current and arc duration have been calculated, incident energy and arc flash boundary for any tasks (1) and at any working distance (4) can easily be calculated using IEEE 1584 equations.