we are having a discussion and we are sort of at an impasse. I'd love to know some educated opinions. We are just electricians turned data input guys, essentially.

We have a vendor submitting an incident energy analysis where he refuses to included any loads fed from subpanels. It's a typical 480 V system with 208 V subpanels. Many of the panels feed VFDs and various three phase motor loads. When we do these in house we include everything 208V three phase and above. He does not give any reason why he is leaving these out but we would like him to include them.

I am of the opinion that without having it specifically spelled out in the contract with him, it is mostly up to his discretion as to what is included. I know IEEE 1584 says that is can be used to model 208V and above but that does not mean it requires it, correct? NFPA 70e seems to leave it at whether the person performing the analysis believes their is a likelihood of an arc flash incident. I can see how this would require every piece of equipment to be "evaluated" even if its just in his head and determined to be unlikely to cause an incident.

Can you guide me to the correct thinking? Thanks in advance.

You may also want to reference the IEEE Red Book. Having said that, I don't know the status of your 1-lines, but the Red Book allows one to lump motors together under a certain size, to expedite the analysis.

If he's not building the model per the Red Book, I would be suspect of the study and its assumptions.

Mike

Not really.

NFPA70E leaves it up to the EMPLOTER to decide the likelihood of an incident as part of their Electrical Safe Work Practices (ESWP) program.
The person conducting the study needs to perform it based on the ESWP. If an outside party is performing the study the contract should be agreed to ahead of time and refer to the ESWP.

However, I know of several employees that abrogate their responsibilities and let providers conduct studies with little to no guidance.

They should be following the guidance of IEEE 551-2006 (The Violet Book, "Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems"). That standard says for short-circuit calculations it is permissible to omit motor contributions from motors of less than 50 HP. So when doing an arc flash study, I'll show all motors 50 HP and larger as a specific motor load and "bulk load" all the rest of the motors on a bus. If they aren't modelling motors 50 HP and larger, then they aren't following industry standards.

If you're having an arc flash study done, normally, you would tell them how far you want to take labelling. If you want labelling on the motor disconnects, then that needs to said up front because every label becomes a bus for them to model and the amount of time it takes to model a facility scales with the number of buses. If you don't need to label disconnects, then there isn't a reason to model every single small motor other than having a thoroughly complete model which in itself has value. But at the same time, they should be paid for the effort.

You want the worst case scenario for your arc flash label. But is that worst case going to be with 'motors running contribution' or 'motors off and no contribution'. Adding in the motor contribution will typically increase fault currents about 20%. That much more current may throw you into a faster time response on your overload device. Since IE is based on time squared, the IE you calculate may be lower, than if you had no motor current and the 't' value was longer. Motors off, may actually give you a higher IE rating.

The goal of the exercise is NOT to arrive at a precise IE value. Its actually impossible to calculate a precise IE as the formulas are all empirical, based on testing. The goal, is to make sure personnel PPE equipment is rated at or greater than the risk IE exposure.


Its the same thing with Utility Service Fault Current Values. The utility can and does make changes to their distribution grid. Yes, you have to do a refresh study every five years, but do you really want to change a few hundred labels in your plant because the utility fault values have changed. You need to take the utility value given you and create an upper and lower bandwidth around their value, say 50% more and 50% less, and run the calcs both ways. Plus, each of these are run with and without motor contribution. Whichever scenario produces the highest IE, is what you use as the label.

For me, anything 208 and up is covered. As far as VFD’s go, I include them even if they don’t have a bypass. And as far as motor size goes, part of what we give the customer is a complete one line. So I generally cut off motors below 20 HP, not 50.