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 Post subject: Generic Group Motor Load at Service Entrance
PostPosted: Sun Mar 06, 2016 6:06 am 
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One of our consultants who performs arc flash calculations always puts 10% of the service entrance onto the main as a direct connected motor contribution. For example, if the service main gear is rated for 4000 amps they will slap on 400 amps worth of motor contribution onto the main (400 FLA). Despite the fact that the vast majority of our motor loads are on non-regenerative VFDs with no bypass they insist on doing this. There argument is that this 10% motor contribution symbolizes the potential motor contributions during a fault from the motor loads not on a VFD and also symbolizes future motor loads that may be added to the system.

Is this 10% motor contribution scheme a industry rule of thumb? Is it recommended from one of the industry reference sources? I suppose it is a smart & conservative way to approach an arc flash study but it also appears to be driving incident energy up unnecessarily, especially if we happen to know precisely the loads that are and are not on non-regen / non-bypass VFDs.

Any thoughts? Thank you for your time.


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Sun Mar 06, 2016 6:53 am 
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One way to look at it is that they are adding a bit of conservatism into the study if every motor has not been modeled. You stated the majority of your motors are on non-regen vfds with no bypass but what is the remaining amount. Of course adding onto the main service would only really add the backfeed to to that bus. What if the motors are not on that bus but farther down stream.

Typically the main bus is your highest incident energy as the protective device is usually the utility device on the transformer primary side. Adding the additional load to that may not make a difference. Does it matter if the IE is 60 cal/cm2 and adding the additional load makes it 65 cal/cm2?

Of course it could also put one into a different class of PPE making it more cumbersome for the worker. For example, 10 cal/cm2 versus 13 cal/cm2.

I would ask to see the results with and without the additional load to see how much of a difference it makes.

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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Sun Mar 06, 2016 10:21 pm 
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Never heard of it. Seems pointless. The place where motor loads make a difference is large motor loads on long cables with weak fault protection.


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Mon Mar 07, 2016 8:22 am 
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Ditto - PaulEngr.

Why not model the system accurately instead of adding some bogus motor contribution?


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Mon Mar 07, 2016 9:10 am 
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That isn't always a "conservative" assumption either. The added fault current might take the arcking fault from below to above the pickup point of a Short time or inst element, thereby reducing, not increasing the calculated I.E.

The only case where I could see making the argument would be a commercial/office occupancy situation, where you might have multiple customers on a single utility XF, and you wanted to account for unknown motors in another occupants space. Even that is a fairly weak argument.

The main bus, is almost certainly the wrong location to add any motor loads, I almost never see any motor loads connected there, except for fire pumps.


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Mon Mar 07, 2016 9:10 am 
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I agree, it's not a policy I'd follow. Implied in his idea is the added fault current produces a more conservative result. But what if the additional fault contribution actually caused the arc flash hazard to decrease? That additional current could push over current devices into their instantaneous trip range. That reduction in trip time could significantly reduce the calculated arc flash incident energy. With that reduction, the hazard would appear to be lower, possibly a lot lower. With that false reduction, the minimum level of PPE and arc flash boundary would also be incorrectly reduced. Not only would the worker be placed at added risk, others could be as well. Remember, the standard is there to protect not just the person working on the energized device, but other workers who might be in the area. The practice makes no sense to me.


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Mon Mar 07, 2016 12:14 pm 
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The thing to remember about modelling when it comes to arc flash is that the "shape" of the output is not what you would expect. There are three "regions" to consider:

1. When arcing time is fixed, incident energy decreases as fault current decreases. This seems intuitively obvious but it's restated here. This means that once the trip time is in excess of 2 seconds or that the trip timing is "instantaneous" (not inverse-time), everything happens as expected.

2. When we are in the inverse-time region of a circuit breaker or fuse, things do not happen as they would be intuitively expected. In this condition arc "power" is decreasing as expected but arcing time is increasing at a rate that is faster than the decrease in arc power. The net result is that decreasing fault current increases incident energy.

In the "real world" typically we would expect to see 3 regions: an instantaneous region where incident energy falls, a time-overcurrent region where incident energy increases, and finally a long trip time condition where again incident energy decreases as expected. Depending on the maximum available fault current and distances involved, most of the time one or more regions are not visible based on actual operating conditions.


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Thu Mar 10, 2016 7:17 am 
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Thank you everyone for your thoughtful responses. This is a useful forum, glad I stumbled upon it!

Sincerely,
Brian


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Thu Mar 10, 2016 9:11 am 
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DMB5mil wrote:
Thank you everyone for your thoughtful responses. This is a useful forum, glad I stumbled upon it!

Sincerely,
Brian


Thank you Brian and I hope you will come back frequently and join in the discussions. Lots of knowledge on this site.

Barry

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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Thu Mar 10, 2016 10:16 am 
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DMB5mil wrote:
One of our consultants who performs arc flash calculations always puts 10% of the service entrance onto the main as a direct connected motor contribution. For example, if the service main gear is rated for 4000 amps they will slap on 400 amps worth of motor contribution onto the main (400 FLA). Despite the fact that the vast majority of our motor loads are on non-regenerative VFDs with no bypass they insist on doing this. There argument is that this 10% motor contribution symbolizes the potential motor contributions during a fault from the motor loads not on a VFD and also symbolizes future motor loads that may be added to the system.

Is this 10% motor contribution scheme a industry rule of thumb? Is it recommended from one of the industry reference sources? I suppose it is a smart & conservative way to approach an arc flash study but it also appears to be driving incident energy up unnecessarily, especially if we happen to know precisely the loads that are and are not on non-regen / non-bypass VFDs.

Any thoughts? Thank you for your time.


Motor contribution can vary greatly and may have a very significant impact on calculated incident energy and arc flash boundary.

I would recommend using minimum available short circuit current coming from utility only while ignoring contribution from motors and generators upstream when determining the duration (values from Figure 1 column Min Isc3, KA below). I would also recommend using maximum available short circuit current when calculating arcing current and incident energy (values from Figure 1 column Max Isc3, KA below). The above approach results in the worst case incident energy scenario associated with the longest arc duration and the largest energy flux effectively compensating for uncertainty in fault current contribution and current decay from all different type and size of motors being a part of power system.

Image

Fig1. Screen from Short Circuit Analytic V1.0 software showing calculated short circuit current values.

Image

Fig 2. Screen from Arc Flash Analytic V5.0 software showing controls for Available Short Circuit Current, Part of ASSC thru Protection Device, predicted arcing current values and arc duration as a function of predicted arcing current thru protection device and the device time-current characteristics.

Are there any other assumptions your consultant is making when calculating available fault currents, such as infinite bus assumption, ignoring the equipment x/r ratios etc?


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 Post subject: Re: Generic Group Motor Load at Service Entrance
PostPosted: Thu Mar 10, 2016 8:18 pm 
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Using minimum kA for time and maximum kA for current is nuts because neither will occur at the same time. Just check incident energy at both conditions. Usually one or the other is a maximum. While it is possible to pick say a maximum fault current that is on an inverse time curve and a minimum fault current that is unprotected and thus exceeds two seconds and miss the peak incident energy in between but this would be an obvious condition and suggest lowering the long term trip setting or getting there another way.


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