Suggestions to Gene 1203 June 24, 2001 marked by ////\\\What I did for my initial calculations is that I isolated first the branch circuit. First, I considered the main switchgear as my system where I was able to calculate the short-circuit current from the utility and other motors at 480V. After gathering all X/R ratios, I now have the "system Z" including the impedance of the feeder conductors supplying the said dry-type transformer. The second step I did was to gather the impedances after and including dry-type transformer and applying the proper formulas for each motor (type) short-circuit contributions. After reducing to a single impedance, I now have the available short-circuit current for the CB at the primary of the DT transformer and subsequently for the MCC. Is this satisfactory?
////This appears to be satisfactory, if you do not have any motor contributions in this power distribution area. If you do, then they have to be counted in addition to the Utility/System source and its contribution to faults.\\\
I came to a point where I noticed that while I was considering the 480V motors, I did not consider the 208V motors.
////The 208 motors are downstream of 480V/208V transformer, most likely, since you do not mention it. They are not counted as fault current contributors on 480V power distribution side since the 480V/208V transformer impedance mitigates their contributions to fault noticeably. However, if you wish you may add them and see the difference in the fault magnitude by yourself.\\\
As for rhatcher's tie-breaker question, it is normally open. It will close only during: an outage of any of the two power grids; a failure on any of the two transformers. Basically, the two main transformers are typically sized to carry all loads without paralleling.
////Make sure that there is an interlock, ideally electrical and mechanical.\\\As for jbartos's suggestions:
"1. Usually, the motor contributions are not considered upstream of main and upstream on the transformer primary since they are mitigated be the transformer impedance."
Please confirm whether the motors on the downstream will not affect the CB protecting the dry-type transformer however large the motor loads are (the dry-type transformer is 1000kVA).
////The motor fault current contributions upstream through the transformer are relatively small; therefore, they are not considered. If you will you may consider them and find out the difference for yourself.\\\Actually, there are three CB's to be considered: 1)branch CB in the main switchgear; 2) the CB at the dry-type transformer primary; and 3)The main CB of the MCC at the secondary of this DT transformer.
////Usually, the selective coordination will require to delay the transformer primary switchgear trip on short circuit faults. The overload can be somewhat delayed too if it is not delay automatically by the protective device curves.\\\"2. See my markups ///\\(1)the branch circuit breaker of the switchgear, where the feed comes from,
///This circuit breaker shall be protecting its branch circuit load. If load happens to be transformer it may be slightly delayed on the short circuit downstream of transformer.\\\"
I gives me comfort to hear this. However, the transformer's loads are mostly motors (about 80-90%) and the word "may"-can we use "shall" here instead?
////This depends on sizes and may become relative. If sizes are tight it shall be delayed, else it will trip. However, it should not be delay a lot since the transformer and cable thermal or damage curves have to be watched and on right hand side of switchgear curve, i.e. still protected by the time delayed switchgear. This is almost by common-sense. However, the software packages are calculating this automatically. You may learn; however, you will probably be wasting plenty of time.\\\"(2)the circuit breaker for the primary of the dry-type transformer and
///This circuit breakers are usually delayed, e.g. 0.3 sec.\\"
Could you please explain further why? My mindset is that, for any fault within this branch circuit, the available short-circuit currents shall come from the utility and motors.
////If you do not delay this circuit breaker/switchgear, you will have a hard time to coordinate it with the downstream circuit breaker, unless they are far apart in sizes. If you intend to design the power distribution without any huge redundancies (fats), then you need to consider such delays that the protective devices curves are tightly selectively coordinated.\\\"3)the circuit breakers of the MCC.
///The MCC circuit breakers (CBs) will see motor contributions and ICs or IRs of those CBS shall be sized for those. However, modeling of this system by some Electrical Analysis software shall be applied."
(Do you mean IRs = interrupting rating and ICs = interrupting
capacities?)
////Usually, IR is on the protective device nameplate. It is usually associated with low voltage circuit breakers and fuses and their highest rms alternating current that are required to interrupt. IC is more associated with medium voltage switchgear. It is defined in IEEE dictionary as a highest current at rated voltage the device can interrupt (any device). Essentially, both have the same intent, to characterize rating of protective device.\\\
Please confirm if you are saying that for this MCC, the
short-circuit current to be considered shall be from the motors only -
neglecting other sources?.
////No. You have to consider all sources: Power supply source contribution (over transformer in your case), motor contribution to the fault, capacitor contributions, standby generator contributions, etc.\\\ Can I do my own evaluation also without the use of any software?
////Yes, you can; however, the software is usually much faster and more accurate. There are various constants imbedded that you may be not aware of, e.g. for circuit breakers and for motors, grouped motors, etc.\\\
