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 Post subject: Motor Contribution
PostPosted: Sat Oct 20, 2007 11:26 am 
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This is a solution I shared on the power eng forum a few days ago that may be of use to someone trying to calculate borderline Arc Flash exposure.

(I realize that we are fooling ourselves by trying to make this 'too accurate' but from a professional engineering liability perspective, the best I can do is follow the set forth calculations as though they were perfect).

Motor contribution to fault current (and thus arc flash exposure) will typically decay within a few cycles. Existin software to calculate Arc Flash exposure assumes motor contribution will continue uniformly until the governing device clears. That may result in higher than actual calculated Arc Flash Exposure. In a lineup with many motors, and barely exceeding a workable calculated exposure, the following calculations might make a difference:

1) calculate the Arc Flash exposure with clearing time manually set at approximately 6 cycles (or what is deemed appropriate for the motors).

2) repeat the above calculation but with the motors disconnected.

3) Calculate arc flash exposure without the motors and with clearing time determined by actual protective device. Add the difference (in Cal/cm2) between the above two calculations which should represent true motor calculation.

It is understood this is approximate, i.e: the motor contribution would decay within the 6 cycle period and device clearing times could be affected by motor contribution, however; these differences should err on the conservative side yet will result in a lesser calculated value than if the motors are allowed to contribute MAX current for full fault duration as the software programs will default.

(I have heard many times the statement that we should not seek lesser calculated values, however; my experience in continuous process plants is that an outage also constitutes a safety liabiilty).

Gary B.


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PostPosted: Sun Oct 21, 2007 9:56 am 
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Great info Gary! ANSI C37 also gives a few multipliers for reducing medium voltage motor contribution as well. Something that I don't think has been addressed is motor contribution is dependent on the voltage collapsing during a short circuit. Total collapse means a bolted fault. Since these are arcing faults with some fault impedance, the voltage does not collapse completely so I doubt if the maximum contribution would be experienced. Until all this is resolved - hopefully with the next wave of IEEE tests, ideas like your to work around the problem are great. For everyone out there, try many what if scenarios with motor contribution and see if any of them give you a drastic change in incident energy or PPE requirements.

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PostPosted: Sun Oct 21, 2007 10:24 pm 
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I've spoken to the software provider (ETAP) on this and they hope to someday include diminishing motor contribution. In the meantime, motor contribution usually is not governing the risk category.

Gary B


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PostPosted: Mon Oct 22, 2007 11:52 am 
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Gary, to clarify… (I may be the original poster on the engineering forum you are referring to.) I also have yet to find a “certifiable” solution to the initial problem, so any additional information here would be helpful. I’m going to re-post the original question here for anyone else who may be able to help.

Here at our facility we have several large synchronous motors. The question of motor contribution to our arc flash hazard has recently come up. On one bus in particular, because there was no protective device on the motor that would clear the fault quickly, (we are using a two second maximum for our calculations, unless other conditions warrant otherwise) the motor contribution to the arc flash hazard was very significant. The calculations revealed 17 cal/cm2 with all 6 synchronous machines running, but only 2 cal/cm2 with no motors running.

The software we are using (SKM) assumes motor contribution for the full time of the calculation.

While trying to research our options, I came across an article by J. C. Das, "Design Aspects of Industrial Distribution Systems to Limit Arc Flash Hazard" in the Nov/Dec 2005 issue of IEEE Transactions on Industry Applications. Mr. Das states that "The synchronous motor short-circuit contribution can be neglected in six to eight cycles." Although he doesn't reference it directly, I found support for this in the 1997 edition of IEEE 399, the IEEE Brown Book, Section 7.4.1, as well as IEEE 141, the IEEE Red Book, Section 4.5.4.3.

The IEEE references apply to breaker duty calculations, so the question remains: Can this be applied to arc flash studies? Otherwise, what is a reasonable time to assume for the motor contribution to an arc flash event at the upstream bus?

I am assuming that the motor will see a “dynamic brake” effect due to the resistance of the arc. My own calculations (such as they are) indicate that it will take as much as 10 cycles before the motor contribution is negligible, but my current software (SKM) assumes a linear cutoff, and so I am approximating this effect by using a reduction to 100% of rated current after six cycles.


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PostPosted: Mon Oct 22, 2007 2:46 pm 
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I would say 'YES', the same limit on motor contribution that you have found referenced for equipment duty, would be appropriate for Arc Flash calculations. If the energy isn't there it won't contribute.

I do not know what you mean by 'certifiable', but in performing calculations I have been required to make several determinations that are not prescribed in NFPA 70E or the calculations.


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PostPosted: Tue Jan 22, 2008 9:41 pm 
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'While the synchronous motors may last 6 to 10 cycles, all the literature I've read from GE and Square D on 'how to calculate fault currents' say that induction motors lose their magnetic field extremely fast and will only contribute for 1 to 2 cycles. 6 cycles for induction motors seem way too long to me.


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PostPosted: Wed Jan 23, 2008 8:32 am 
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haze10 wrote:
'While the synchronous motors may last 6 to 10 cycles, all the literature I've read from GE and Square D on 'how to calculate fault currents' say that induction motors lose their magnetic field extremely fast and will only contribute for 1 to 2 cycles. 6 cycles for induction motors seem way too long to me.


Isn't the loss of magnetic field proportional to the voltage? So if it is NOT a bolted fault, but an arcing one and there is a large indertia that drives the rotor, the now induction generator would sustain the contribution in proportion of the decaying voltage. Am I right? :cool:


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PostPosted: Wed Jan 23, 2008 10:36 am 
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LaszloZW wrote:
Isn't the loss of magnetic field proportional to the voltage? So if it is NOT a bolted fault, but an arcing one and there is a large indertia that drives the rotor, the now induction generator would sustain the contribution in proportion of the decaying voltage. Am I right? :cool:


Laszlo - This is one problem I ran into when trying to plot curves. The current is proportional to the decaying voltage. The voltage is dependent on the arcing resistance, which determines the arcing current.........

So the 10 cycle decay is a very rough approximation. I think the 6 cycle reduction to 100% of rated current estimation would be more accurate, without being too limiting on your arc energy.

You would think that the software people could integrate a motor decay curve, given Xd'', Td, etc, and include this in their arc flash calculations.

Do I have to think of everything for people?? :D


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PostPosted: Wed Jan 23, 2008 11:43 am 
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WDeanN wrote:
Do I have to think of everything for people?? :D


Yep, it's a thankless job, but somebody's gotta do it....... :p


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PostPosted: Fri Feb 01, 2008 2:29 pm 
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LaszloZW wrote:
Isn't the loss of magnetic field proportional to the voltage? So if it is NOT a bolted fault, but an arcing one and there is a large indertia that drives the rotor, the now induction generator would sustain the contribution in proportion of the decaying voltage. Am I right? :cool:


I am not a motor expert so the jargon is probably all wrong, however; the inertia has little to do with fault sustainence. Fault current will drop off much faster (i.e.: 1 or 2 cycles as Haze suggested, depending on the motor) as a function of the magnetic field collapsing, NOT the motor spinning. I use 6 cycles understanding that is conservative for most applications.

I learned this on account of a reversing circuit for large cooling fan motors, that would spin for over a minute. They were safe to apply reverse power to, after a few seconds (actually less than a second) even though the fans didn't noticeably slow down. The reverse voltage acted like a brake and as long as the thermal capacity wasn't exceeded all was okay. (on the other hand if you reverse a motor within a few cycles, you might be closing in on 180 out of sync and draw a huge current resulting in a break (not to be confused with brake :p ))

Even if the arc voltage maintains a longer arc duration, I suspect it will be in the order of a few more cycles.


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PostPosted: Mon Apr 21, 2008 8:40 am 
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How to decide motor contribution

Another question regarding the motor contribution. If I have quite a few big motors(more than 50hp) in the system. How to decide how many motors I need to include in arc flash analysis? Do I need to run my scenarios from one motor to all and find out the worst condition? Thanks!


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PostPosted: Tue Apr 29, 2008 6:50 pm 
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I'm sure I'll get roasted for this but.....

For the accuracy we need for Arc Flash I would just model your motors into groups, ie, 0 to 100, 125 to 300, Synchronous, etc. If all your motors are under 100HP, then just approximate the total and sum them.

The other thing is to model the two worst cases, full motor contribution gives you a higher fault current and no motor gives the lower fault current but potentially longer clearing time. Rule of thumb is to assume 25% of transformer kVA as motor contribution. Run the calcs both ways and use which either gives the higher IE.

Motor contribution current does not allows flow through the Overload device, it flows to the fault location.


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PostPosted: Wed Apr 30, 2008 6:38 am 
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Thanks Haze10, always learn a lot from you!


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PostPosted: Mon May 05, 2008 10:24 am 

Joined: Wed Oct 17, 2007 5:20 pm
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Hello guys, I might be missing something in the discussion, as far as the SKM softwares, there is an allowance in the arc-flash options where you can select the number of cycles after which the motor contribution is eliminated, so the question of motor contribution until the fault clearance is not true. Typically the induction motors a 5 cycle decay time is conservative approach. So SKM does eliminate the contribution after the preset number cycles. The arc-flash values are calculated based on the accumulated energy over the time of clearing time. Let me know if I am wrong.


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