Methods of Calculating Incident Energy

[VinceS]

When obtaining short circuit current for a bucket in a 600V class MCC. Given the following: 3ph 480VAC 30A adjustable trip CB, size 2 NEMA rated, contactor w/OL, and a 15HP motor.

I understand, per the 70E using the IEEE Calculation method, the max current is obtained at the Instantaneous Trip of the short circuit protection point ( the CB ). The debate at work is to somehow use the FLA of the motor. I might even understand if the value used was the locked rotor current. Yet this also has its issues as normal CB selection allows for the selection of CB's which can avoid tripping during startup.
I believe using motor related data will give a calculated value for incident energy much lower than actual. I understand there are many methods for calculating incident energy, I'm not an EE and don't know all of them.

1. Is there a method which uses FLA of the motor as the data for the calculation?

2. If this method is accurate, does it give a liberal lower incident energy?

 

[iwire]

I am no expert on this by any means but it is my understanding that the motor will add to the actual fault current and must be considered.

 

[WDeanN]

A 15HP motor will have a negligible effect on short ckt current at the MCC. Larger motors may contribute to the total current, but the largest part of it should still be from the upstream service.

 

[jim dungar]

1. Is there a method which uses FLA of the motor as the data for the calculation?

Effectively during the first moments of a short circuit the motor will act like a fault current generator. One method for approximating the short circuit current from the motor is similar to that used with a transformer - take the motor FLA and divide it by the motor impedance (actually called transient reactance and abbreviated is Xd). Except for very large (above 200HP) you will probably need to use an approximate value. Some people keep it even simpler (meaning more of a guess) by just using the inrush current of the motor (i.e 10x FLA).

Fault current contributions from motors is why "series-rated" combinations of overcurrent devices are restricted in the amount of motor loading allowed.

 

[VinceS]

Thank you all very much...

Thank you all very much...

As I thought... It's the upstream current potential, not the load which determines the potential energy able to be discharged during an arc fault occurrence.

 

[jim dungar]

As I thought... It's the upstream current potential, not the load which determines the potential energy able to be discharged during an arc fault occurrence.

Downstream motors do contribute to the current at an arc fault, although it may not be much from a single motor.

 

[zog]

Keep in mind that your highest current may not result in the highest Ei. Look at the Ei for 38% of your calulated Ibf.

 

[richxtlc]

The available arc fault current is dependent on many factors:
The available bolted fault current
The cable size, length, impedance.
Is the cable in conduit, steel, aluminum, pvc or db?
The settings of the ocpd
The speed of the ocpd
Where on the ocpd curve the fault current hits. (if it is in the instantaneous, its fast, if not it's a lot slower)
If the ocpd is a circuit breaker, is it maintained or not.
Is the switchgear very dusty?

All the above affect the level of arc fault current and there are probably a few that I missed.

 

[weressl]

When obtaining short circuit current for a bucket in a 600V class MCC. Given the following: 3ph 480VAC 30A adjustable trip CB, size 2 NEMA rated, contactor w/OL, and a 15HP motor.

I understand, per the 70E using the IEEE Calculation method, the max current is obtained at the Instantaneous Trip of the short circuit protection point ( the CB ). The debate at work is to somehow use the FLA of the motor. I might even understand if the value used was the locked rotor current. Yet this also has its issues as normal CB selection allows for the selection of CB's which can avoid tripping during startup.
I believe using motor related data will give a calculated value for incident energy much lower than actual. I understand there are many methods for calculating incident energy, I'm not an EE and don't know all of them.

1. Is there a method which uses FLA of the motor as the data for the calculation?

2. If this method is accurate, does it give a liberal lower incident energy?

Your and your colleagues assumptions are incorrect.

The available energy at the point of short circuit should be calculated and its major component is what is called the Utility Contribution.

Small motors like yours in case have minimal effect on the total.

If it is an industrial facility with a distribution system, the short circuit study should be performed by a professional, along with the coordination of the protective devices and appropriate arc-flash labels may be produced based on the calculated data.