Short Circuit Motor Contribution

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Short Circuit Motor Contribution Jim Phillips, P.E. Short Circuit Motor Contribution When a short circuit occurs, all connected running motors act like generators for a few cycles and produce short circuit current that is fed back into the power system. The motor  contribution can total thousands of amps when all motors are taken into account. During a short circuit, the voltage collapses at the point of the fault (short circuit). As the voltage collapses, the motor’s field does not instantaneously collapse so there is a rotating magnetic magnetic field and motor windings windings – exactly what it takes takes to produce electricity. Although the motor’s short circuit current only lasts a few electrical cycles, while the field collapses. It is during the first few electrical cycles that overcurrent devices must respond and they must either interrupt or  withstand this additional current. Motor contribution is equal to locked rotor current. i.e. if a motor has a full load current of 40 Amps and a starting current (locked rotor current) of 200 Amps, then the motor  can contribute 200 Amps to the short circuit. When there are many motors on a power system, as there usually are, you can see the total motor contribution can be significant. To calculate motor contribution manually, it is easiest to total all of the motor contribution and add it to the calculated short circuit current. Although this is somewhat simplified and therefore not 100 percent accurate, it is the most straight forward method. Other considerations would be that motors are distributed throughout the power system and not lumped at one bus. In that case, you would want to consider what motor contribution is at each local bus. In last months example, we calculated a maximum three phase short circuit current at the 480 Volt secondary bus of  26,845 Amps. Let’s say there are 20 motors motors with a short circuit motor contribution of 200 Amps each. That would be a total of 20 X 200 Amps or an additional 4000 Amps of short circuit current. If you add this to 26,845 Amps, the total short circuit current would be 30,845 Amps. If you were evaluating equipment rated 30,000 Amps, motor contribution would make a difference between an adequate equipment application and an inadequate application.

electric power system must have an adequate short circuit interrupting capability. If an overcurrent device such as a fuse or circuit breaker was subjected to a short circuit current beyond its rating, the device could possibly explode leading to equipment damage and jeopardizing the safety of  personnel. The circuit breaker below has an Ampere Interrupting Rating (AIC) of 10 kA or 10,000 Amps. This means the device’s design was tested in accordance with Underwriter Laboratories test procedures and is capable of  being applied at locations where the short circuit current is no greater than 10,000 Amps at the device line side terminals.

It is fairly easy to determine a device’s rating from the labeling. However, it is often difficult to determine how much short circuit current would be available at a device if there was a fault. Hopefully the past several months of example calculations, formulas and procedures, will help give you a better understanding of the calculation process. Of course, the best method for calculating short circuits is to use as much detailed data as possible and factor in X/R ratios for  more precise numbers. Due to the complexity of the calculations involved with this level of detail, it is usually easier to use one of the many available computer programs. The manual methods will get you close, and in most cases will be conservative, which is good for estimating short circuits.  _________________________  ________________________________________ _______________________  ________ 

NEC® Article 110.9 Short Circuit Interrupting Ratings National National Electrical Code® Article Article 110.9 requires that all equipment that could interrupt short circuit current in an

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Copyrigh Copyrightt © 2005 T2G Technical Training Group You can contact Jim Phillips, Phillips, P.E. at: [email protected]