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 Post subject: Motor contribution to arc flashPosted: Mon Mar 02, 2020 11:06 am

Joined: Tue Oct 30, 2018 7:57 am
Posts: 8
Hi you all!

I would like to ask you if there is any document/metodology to calculate the motor energy contribution to the arc flash incident energy, taking into account that the motor stopping time is fast, and may be the main protection of the motor won't trip.

I know that there is a motor current contribution to the bolted fault current of each node, but, should it be consider or there is another metodology?

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 Post subject: Re: Motor contribution to arc flashPosted: Mon Mar 09, 2020 11:51 pm

Joined: Mon Mar 09, 2020 11:36 pm
Posts: 2
Location: Denmark
There is a tradition in Norway to include the motor contribution in the arc flash results.
Short-circuit current is calcalated at ½-cycle, 3-cycles, 5-cycles and 30-cycles to find the decaying current.
Arc-flash is calculated in 4 intervals where:
- ½-cycles value is used in first interval 0-0.06s
- 3-cycles value is used in second interval 0.06-0.1s
- 5-cycles value is used in third interval 0.1-0.250s
- 30-cycles value is used in forth interval 0.25-2s
The energy from the 4 intervals are added to have the total energy of the event
Best regards
Preben Jakobsen

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 Post subject: Re: Motor contribution to arc flashPosted: Mon Mar 16, 2020 9:13 am
 Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 510
Location: New England
This is part of the problem with Arc Flash, is that we focus on obtaining the correct 'value' for IE, when in reality its impossible to truly predict it accurately.
The objective is to protect personnel and assure they have on sufficient PPE. If you included Motor Contribution, you'll get higher fault current values, which render higher arc fault current, which means your further out on the Trip Curve, and the arc duration is less. IF you don't include MC, then AFC is less, your closer on the TC and tripping times are longer, the arc duration is longer, and IE probably goes up. Since the IE formula is proportional to Time Squared, IE can go up a lot with longer trip times even with lower AFC.

Same can be said about the utility supply. You have no control over their modifications or the fault current they claim they are supplying to your service. Its their estimate that moment in time. If they change transformers with different impedance do you really think they are going to call you to notify you?

What I do is look for best and worst case. I first calculate fault current with no MC, and 20% less utility fault current than the utility estimates, run for IE. Then I calculate fault current with MC, and plus 20% of the utility estimate. Calculate IE. Whichever IE is larger is the value I use. This gives a large range of upset tolerance and should cover the worse case for some time to come.

In many cases the difference aren't huge, and the larger value is acceptable. IF you can't live with it, then you trim to you liking.

In general through, a lot of the breaker trip times we are getting are in the 2 cycle range, and MC can last 2 cycles, so best to include.

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