Arc Flash and Small Motors <50HP

[ish1284]

Hi All,

IEEE1584 2002 implies that motor contribution for motors <50HP can be excluded from arch flash calculations. IEEE1584 implies something similar but takes out "<50HP" and rather says "small motors".

The debate I'm having is on why small motor contributions can be excluded.

I thought somewhere I saw that small motors can be excluded because their inertia just isn't enough to sustain an arc for more than 0.5 cycles. However, I can't find the reference where I thought I read/heard this. If this is the true reason why, it would lead me to believe that a lumped group of small motors that totals >50HP wouldn't need to be included in the arc flash calculations as well.

This leads to the question, if there is a group of motors smaller than 50HP that adds up to more than 50HP, should the lumped contribution be counted? If it's related to motor inertia, I'm inclined to say no because it is dependent on the individual motor size rather than what the lumped motor total HP is.

Note that I'm referring to arc flash calculations only. I understand per the ANSI standards, any size LV motor should be included in short circuit calculations. For the sake of this topic, let's stay in the low voltage realm (i.e. 480V).

 

[Jraef]

As I understand it, the issue of excluding motors under 50HP originally applied to ANSI short circuit calculations on the MV circuit breakers back in the days when SC calculations were done by hand, not with computers. It was based on the fact that the magnetic fields in small motors collapse much more quickly than they do on larger motors, so their contribution of fault current only lasts a couple of cycles, so for determining breaker withstand ratings, they were, even in aggregate, insignificant. Later in life I worked on a project designing DC injection brakes and tested AC induction motor field collapse rates (to know how long to delay applying DC), we found this to be true, although we actually found it to be up to 100HP.

IEEE1584-2018 does not say that motors <50HP can be excluded for arc flash studies (that I can see), but the practice seems to have carried over any way in that software programs like SKM have an "option" to exclude them. The general consensus however in some arc flash forums I have participated in is that small motors WILL contribute to arc flash energy, but what most people do is to just lump them all together, i.e. 20 x 15HP = 300HP.

 

[ish1284]

As I understand it, the issue of excluding motors under 50HP originally applied to ANSI short circuit calculations on the MV circuit breakers back in the days when SC calculations were done by hand, not with computers. It was based on the fact that the magnetic fields in small motors collapse much more quickly than they do on larger motors, so their contribution of fault current only lasts a couple of cycles, so for determining breaker withstand ratings, they were, even in aggregate, insignificant. Later in life I worked on a project designing DC injection brakes and tested AC induction motor field collapse rates (to know how long to delay applying DC), we found this to be true, although we actually found it to be up to 100HP.

IEEE1584-2018 does not say that motors <50HP can be excluded for arc flash studies (that I can see), but the practice seems to have carried over any way in that software programs like SKM have an "option" to exclude them. The general consensus however in some arc flash forums I have participated in is that small motors WILL contribute to arc flash energy, but what most people do is to just lump them all together, i.e. 20 x 15HP = 300HP.

Hi Jraef, thanks for the input. I was referring to IEEE1584-2018, section 6.2 ("Collect the system and installation data"), 7th paragraph which states: "Obtain the available fault current and X/R ratio that represents the source. For transformers, large motors, and switchgear, collect the relevant nameplate data...". To me, this seemed to imply that they are not putting emphasis on the modeling of small motors. However, I just didn't know the history and theory behind this.

 

[jim dungar]

Thirty 10HP motors does not have the same short circuit current effect as do one 300HP motor. Lumping small motors together into a single large motor may produce inappropriate arc flash incident energy values. For example, if the motor group is connected to a switchboard main bus the supposed additional fault current may result in protective devices opening faster than they really would. Most of the major software manufacturers allow you to group small motors but then have that group ignored during the arc flash calculation.

The company I worked for would run an arc flash scenario with all motors on line and another with all motors off line. The off line version almost always had the highest incident energy. The all motors off line is a typical operating condition during periods of maintenance.

 

[ish1284]

Thirty 10HP motors do not have the same short circuit current effect as do one 300HP motor. Lumping small motors together into a single.large motor may produce inappropriate arc flash incident energy values. For example, if the motor group is connected to a switchboard main bus the supposed additional fault current may result in protective devices opening faster than they really would. Most of the major software manufacturers allow you to group small motors but then have that group ignored during the arc flash calculation.

The company I worked for would run an arc flash scenario with all motors on line and another with all motors off line. The off line version almost always had the highest incident energy. The all motors off line is a typical operating condition during periods of maintenance.

A "motors on" run can result in an increase if you're already in the instantaneous region of the feeder breakers before the motor contribution as it is in my case. However, in my case I found it to be a pretty insignificant increase because of being in the inst region.

However, the "motors on" run doesn't decipher between one 300HP motor and thirty 10HP motors when you don't have the software set to ignore small groups of motors. Therefore, it's an engineering judgment on whether to include or exclude small motors and/or to have motors on, motors off scenarios when the motor loads in the model consist of almost 100% motors smaller than 50hp. It's not as easy of a decision as it looks when you have a very large model already with a large number of scenarios.