Mixing Tables and Calcs

I am sure I read somewhere that you should not mix the usage of tables and calculations, but cant seem to find it. Anyone know where this is?

I guess one could interpret the verbage in 70E to "mean" don't mix them but I have not seen what I would term "specific language" to say that is the case. In fact, I can envision installations where equipment close to the service might preclude the use of the tables either from fault duty/ clearing time/or both, but downstream equipment might fall into the range of the tables. Would seem logical that they could be mixed in that case.

Sure would not mix methods on the same piece of equipment!

If both methods are to be used, I would think there must be clearly defined and documented logic for the decision.

Alan

Someone is asking me about mixing them on the same peice of equipment, basically using the one that "works best" for them.

I think I read a letter of interpretation somewhere that said not to mix them, but maybe it was discussed at a conference.

Wow! I'd hate to be in a position of liability if something went wrong there!
How would you defend yourself if you had the calculations that said if was a high hazard, but you chose to go with the tables instead because they were lower and something went wrong?
Very, very bad idea.

Bingo! Alan

Thats what I am trying to explain to them.

It should not have to be said that if the calcs look bad, you don't just go back and use the tables. If you do the calcs and they are worse than the tables, then be glad that you did the calcs!

If they are only worried about the money and procedures, then they will probably end up paying more money anyway..... the problem is that it will be at someone else's expense unfortunately.

Good Luck,
Alan

.... misconstrued entirely

I happen to be the subject of this thread - the one who mixes tables and calculations. But my approach is anything but using the PPE calculation that "works best".

My approach is to perform the calculations for the energized bus, and label the enclosure accordingly.

Then, when approaching the equipment with "covers on", I advise going to 70E Table 130.7(C)(9)(a) to determine the appropriate PPE.

I'm a registered engineer, and I would never compromise safety in the determination of PPE requirements. I am charged with interpretation of the Codes, and the safe application thereof.

This thread originated at the Mike Holt forum. But now that I've found this Arc Flash forum, I will direct those type inquiries here.

I stumbled on this forum while searching ways to calculate incident energy buildup downstream of a UPS battery bank. I intend to search the forum for answers to that topic, but when I saw the incorrect reference to my mixing of calculations and tables as a way to lessen PPE requirements, I had to address it immediately.

John F. Mayan, P.E.

John, I wasnt talking about the Mike Holt Forum discussion, this is from a customer of mine, we are doing some mitigation projects for them. This is a common topic and point of confusion.

correction in my PPE advisement

In an earlier post I wrote that I use calcs for open bus PPE determination, and 70E table for closed-door operation of equipment. I no longer subscribe to that philosophy.

The current state of Arc Flash Hazard calculations is such that once a Hazard Risk Category has been determined by calculation, that's the level of PPE required anywhere within the calculated Arc Flash boundary, doors on or off. The rationale is that if the bus has a given Incident Energy (IE), then closing the doors on it won't shield you from that energy, and in some cases could exacerbate the problem - i.e. if pressure builds up in an enclosure until it explodes.

Apparently the IEEE 1584 Committee is still working on improving the analysis, but for now I can't advise a facility owner that it's safe to operate a panel meter on an MCC while wearing PPE level zero (per tables), if I've calculated a higher level for the exposed bus.

Perhaps the ideal approach is to limit the IE from the start, such that the calculations mandate a reasonable PPE requirement. For coordinated systems, that's not realistic in Low Voltage, (<600V), selectively coordinated, high power applications.

This is all the more reason for MV distriubution.

John M

Distribution study done partly IEEE partly NFPA Table

To keep costs reasonable for the client, we typically do the MV and main LV distribution (LV time adjustable protective devices and their direct loads) per IEEE detailed study, and the remainder of the system, where the clearing times and currents are within the required limits, the client uses NFPA Table methods.

Using the Chart and Study Results

We are currently implementing an Arc Flash Program. We have done the study on most of our equipment. We too felt it seemed to make sense to refer to the chart for special tasks because the study does not take into consideration what job you are performing. Performing Infrared Thermagraphy for example. The chart requires less protection because of distance and also there is no contact. Any thoughts on this approach?

Another question:

We performed our study of MCC buckets based on the main bus running through the back of the bus. Therefore every bucket in the MCC is labeled the same. Question is when a worker goes into a specific bucket to perform work with the bucket switched off, does he/she have to dress for arc flash because the main bus is still live?

If you have the calculations for a specific piece of equipment then my understanding is that the results are the results. But when doing IR testing there is an open panel but no one is "interacting" with the equipment, as you said no contact, seems to me only shock protection is required, if within the appropriate shock boundary and outside the arc flash boundary.

If someone is tightening lugs, excersising breakers, or anything else than the arc flash PPE would be required, if within the arc flash boundary.

If there is no interaction I would agree, but usually there is interaction in the sense that panel covers have to be removed off circuit breaker panels, and MCC doors have to be open.