Coordinated Protection for Critical Environments

These days, it's common to perform complicated electrical protection coordination studies with the help of advanced computer software. These software platforms will typically contain libraries that include required settings for most of the common overcurrent protective devices.

And as noted above, with the review of protective coordination study basics, the reliability of an electrical system and its equipment can only be assured if proper coordination is implemented among all the various protective devices.

There are instances where the National Electrical Code (NEC) requires a protective coordination study. There are also times when K-rated transformers employed to deal with electronics and non-linear loads can reduce reliability if not properly coordinated.

On a typical transformer, the current and associated magnetic field is 90 degrees out of phase with the voltage. When you close a breaker and turn on a transformer, the instantaneous magnetic field can be twice as high as normal. In the "ideal" transformer, the current required to supply this magnetic field would also be twice as high. However, in a real transformer the core is saturated and the actual current required to create the field can be 12 times as high as normal. Factors such as the size of the core of the transformers and the time the voltage is applied play roles in determining the amount of inrush current.

The actual inrush current mentioned above would be different based on the actual transformer manufacturer. It is critical for the consulting engineer to contact the specific manufacturer of the transformer supplied in the field. If actual transformer inrush data is not known, common industry standard is to assume the inrush is 12 times for 0.1 seconds and 25 times for 0.01 seconds. Figure 1 (p. 30) illustrates the transformer inrush at 12 times for 0.1 seconds.

Engineers were running into trouble some years back when the K13-rated transformer was becoming more prolific in regular office environments. A K13-rated transformer is oftentimes just a larger transformer with a smaller rating to compensate for harmonics. The same 110-amp breaker typically on the primary side of a regular 75-kVA transformer may trip when protecting a 75-kVA, K13 transformer. For sizing of the primary side, overcurrent protective device for K13 or higher transformers, I recommend multiplying the input full load amps of a transformer by 125% and going to the next common size up. In addition, a breaker with the instantaneous setting is often required to allow for the transformer current inrush. As a final step, I recommend a coordination study to ensure the system will work before it is too late, after construction is complete and the engineer is stuck with an angry owner.

Selective coordination is required when more than one elevator is supplied by a common feeder, per NEC 620-62. Figure 2 (above) is an example of a coordination study illustrating feeder breaker overcurrent protection, elevator fuse overcurrent protection and elevator motor start-up curves.

NEC 517-17 requires that if ground-fault protection is provided for the service disconnecting means, then an additional step of ground-fault protection shall be provided in the next level of feeder disconnecting means downstream toward the load. Figure 3 (above) is a good example of what a properly coordinated ground fault study should look like.

NEC 230-95 indicates that all 480-volt 3-phase services 1,000 amps and above must be installed with a ground-fault relay. The setting of the ground-fault relay cannot exceed 1,200 amps, regardless of the size of the overcurrent protection device. In addition, the time delay cannot exceed one second (60 cycles) for ground-fault currents of 3,000 amps or more. There shall be a minimum of six cycles (0.1 second) ground-fault delay between ground-fault devices in health-care facilities.

Ground-fault settings for main breakers serving downstream motors that are set too low or too fast may trip a main overcurrent protection device before tripping the local thermal magnetic overcurrent protection device during motor starting ground faults. On the other hand, ground-fault settings that are too high can cause undue damage before a ground fault is interrupted. It is important to provide the ground-fault setting that will not permit nuisance tripping, but will protect the electrical equipment from excessive damage during an event.

In my experience, sometimes perfect coordination between a set of devices cannot be obtained. Certain settings may be required on a breaker that could affect the settings of many breakers. In some cases, there may be many levels of breakers that may cause overlap of the breaker curves within the tolerance of the curves. It is at these times that experience will allow the engineer to make judgment calls as to certain compromises in coordination between devices. The engineering behind protective coordination studies is not an exact science, by any means.

On many occasions, I have seen completed projects with no protective device study. In such cases the breaker manufacturer will ship the breakers with all settings set to the most sensitive. This will ensure the most protection, but will increase false trips and is typically not good for the reliability and uptime of the systems.

As soon as the owner complains of a false trip, the facility personnel will probably set all of the dials to least sensitive. This reduces the likelihood of false trips, but might not adequately protect the electrical system and reduces selective coordination of the system.

In short, a coordination study is typically required to ensure that the most reliable electrical system has been installed. In addition, there are instances where the NEC requires that a study be performed. In either case, the cost of a coordination study is cheap insurance for most any facility that would be adversely affected by an extensive power outage.

The important point to remember is that protection equipment can be an expensive investment, and it's well worth the cost of investing in a coordination study by an experienced electrical engineer.