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 Post subject: Posted: Tue Jul 10, 2012 11:03 am
 Sparks Level

Joined: Sat Jun 07, 2008 5:11 pm
Posts: 143
Location: Connecticut
CSC: Please post the formula your using for motor fault current calculation... I would be interested in see it.

You misread the point I was trying to make. I think you are confusing FLA with locked rotor current (LRC). Locked rotor current does not contribute to system fault current. Adding system bolted fault current will change the arc flash rating for sure. But I was looking at purely the motor contribution.

ANSI C37.13 recommends using 3.6 x motor amps FLA. Google Bussman Technical Library - Short Circuit Calculations - they recommend using 4x motor amps. This was also verified using Etap ver 11 software. The ANSI method is geared for LV breakers and is asccepted for motor fault contributions.

So... 3.6 x 1.8 motor FLA x 1000 1 hp motors = 6.5kA for combined bolted fault current for approx 5 cycles at the motor T-lead connections.

From a PD coordination standpoint it may make a difference in trip settings. Now do the arc flash calculation (Ia) which will significantly less than bolted fault current. If you look at =<50 hp from a arc flash stand point does it really make a difference?

I have found based on doing close to 140 studies that lumping low power motors really doesn't give a realistic arc flash determination.

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 Post subject: Posted: Sat Jul 14, 2012 3:09 pm
 Sparks Level

Joined: Sat Jun 07, 2008 5:11 pm
Posts: 143
Location: Connecticut
CSC: Please post the formula your using for motor fault current calculation... I would be interested in see it.

You misread the point I was trying to make. I think you are confusing FLA with locked rotor current (LRC). Locked rotor current does not contribute to system fault current. Adding system bolted fault current will change the arc flash rating for sure. But I was looking at purely the motor contribution.

ANSI C37.13 recommends using 3.6 x motor amps FLA. Google Bussman Technical Library - Short Circuit Calculations - they recommend using 4x motor amps. This was also verified using Etap ver 11 software. The ANSI method is geared for LV breakers and is asccepted for motor fault contributions.

So... 3.6 x 1.8 motor FLA x 1000 1 hp motors = 6.5kA for combined bolted fault current for approx 5 cycles at the motor T-lead connections.

From a PD coordination standpoint it may make a difference in trip settings. Now do the arc flash calculation (Ia) which will significantly less than bolted fault current. If you look at =<50 hp from a arc flash stand point does it really make a difference?

I have found based on doing close to 140 studies that lumping low power motors really doesn't give a realistic arc flash determination.

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 Post subject: Posted: Sat Jul 14, 2012 3:10 pm
 Sparks Level

Joined: Sat Jun 07, 2008 5:11 pm
Posts: 143
Location: Connecticut
CSC: Please post the formula your using for motor fault current calculation... I would be interested in see it.

You misread the point I was trying to make. I think you are confusing FLA with locked rotor current (LRC). Locked rotor current does not contribute to system fault current. Adding system bolted fault current will change the arc flash rating for sure. But I was looking at purely the motor contribution.

ANSI C37.13 recommends using 3.6 x motor amps FLA. Google Bussman Technical Library - Short Circuit Calculations - they recommend using 4x motor amps. This was also verified using Etap ver 11 software. The ANSI method is geared for LV breakers and is asccepted for motor fault contributions.

So... 3.6 x 1.8 motor FLA x 1000 1 hp motors = 6.5kA for combined bolted fault current for approx 5 cycles at the motor T-lead connections.

From a PD coordination standpoint it may make a difference in trip settings. Now do the arc flash calculation (Ia) which will significantly less than bolted fault current. If you look at =<50 hp from a arc flash stand point does it really make a difference?

I have found based on doing close to 140 studies that lumping low power motors really doesn't give a realistic arc flash determination.

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 Post subject: Posted: Thu Jul 19, 2012 9:20 am

Joined: Wed Feb 01, 2012 10:44 am
Posts: 23
Location: Atlanta, GA
For LV lumped sum motor fault contribution:

(Motor lumped HP FLA) / 0.25 = Isym (Sym fault current for 3-5 cyc, note that this is the same as 4 x FLA)

Isym x AF = Itot (total fault current at first 1/2 cyc)

Where AF is the asymetrical factor based on X/R ratio.

I am not confusing FLA and LRA. Note that typically LRA = 6 x FLA, and motor %X = 0.17, which is the reciprocal of 6. So saying LRA = Isym is a valid statement. Since LRA = Isym, then LRA x AF would be comparable to Itot. LRA is the current going INTO the motor on energization, Isym is the current coming OUT of the motor for a fault. It is the magnetic energy stored in the motor windings, and it does flow to the fault location.

The %Z = 0.25 value for lumped HP is from IEEE Std. 241 (may be referenced in other IEEE color book stds).

The motor fault current is added to the (transformer) source fault current to calculate the total fault current, and from there the total arc fault current.

If you have 10 HP of motors, ignoring them may not be an issue. But if you ignore 1,000 HP of small motors, it may give less of a PPE requirement. My point is that making a blanket statement of ignoring all motors less than 50 HP, may produce misleading results. Is that the intention of an arc flash study - to ignore the reality of the power system?

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 Post subject: Posted: Thu Oct 11, 2012 8:32 am

Joined: Wed Oct 10, 2012 12:26 pm
Posts: 4
RogO wrote:
Motor Contribution

It was generaly accepted the A/C induction motors under 75 HP do not contribute enough to available currents to make a difference in the results. I would only include in the analysis motors of 75 Hp and greater and are likely to be running at a time of an Arc Flash. For instance a hydraulic system that has two motor/pumps one the main and the other the backup, only include one of them.
Synchronous motor have a greater effect on Arc-Flash the A/C induction motors.

May I ask where did you get this information from? I was looking and looking to take make this decision, I haven't seen any official documentation that say so, could you please advise?

I have main switchboards feeding other switchboards of 15 or 20 small motors (1 HP, 5 HP, two of them only are 15 HP) I don't have any other information of these motors and I’m kind of stuck. I would like to find something official that let me disregard these motors if acceptable.

On the other hand, in the drawing they are shown and called as 480V switchboards, they have small motors and other small loads. What is the impact of modeling the switchboard as one panel only or one MCC only or one panel and one MCC for the motors, instead of a switchboard?

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