Coordination studies of electrical systems have always been an important tool for engineers. They are designed to protect equipment and prevent outages to large portions of the electrical system during a fault condition, which is another way of saying a short circuit. When a short circuit occurs within a portion of the electrical system, the short circuit current can get into the tens of thousands of amps which can destroy equipment immediately. A properly coordinated electrical system protects against high short circuit currents by ensuring that the breaker closest to the short is the one that trips to clear the fault.
If a fault or short circuit was experienced inside Panel H, what do we want to happen? Looking at the system layout, we want breaker G1 to open and clear the fault for two reasons:
Coordination studies are not just designed to save equipment and limit the power outage; it is also vital to electrical safety and typically part of an arc flash risk assessment. The longer it takes to clear a short circuit, the more time an arc flash can expand and the higher the incident energy. We want the faults to clear as quickly as possible and this usually means we want the breaker nearest the fault to open. Looking back at Figure 1, in a coordinated system in which Breaker G1 clears the fault that occurs at Panel H, there may be 1 calorie/cm2 of incident energy. In an uncoordinated scenario in which Breaker A6 clears the fault, the incident energy at Panel H might be 10 calories/cm2. If the main breaker is the breaker that trips, the incident energy could be dangerously high, such as 40 calories/cm2. While these numbers are for the purpose of illustration only, you get the picture. The longer it takes to extinguish the power generating an arc flash event, the higher the energy level at ground zero.
By conducting a coordination study as part of an arc flash risk assessment, we can identify any areas that are not coordinated and form a plan to address them.
However, a balance exists between coordination and safety. In some cases, the breaker nearest the fault might be an older style with longer instantaneous trip characteristics. In these cases, the next upstream breaker may be able to trip faster. That leaves us with competition between coordination and safety. For coordination, we want the breaker nearest the fault to trip; but for safety, we want the fault cleared as quickly as possible. This may result in a breaker upstream opening rather than the one closest to the arc flash.
Again, a coordination study identifies problems like these. By identifying these cases, we can run scenarios that allow us to test the instantaneous reaction time of breakers. We can often recommend upgrades that ensure the system is coordinated and poses the lowest risk possible to the worker.
In future articles, I will go into much more detail about how all this works. In the meantime, thank you for your time. Mitchell & Lindsey offers Arc Flash Risk Assessments and Electrical Safety Training. If we can be of service to you in these areas or if you have any questions about this article, please reach out to me at the email or phone number below.
Bobby Lindsey – CESCP
Mitchell & Lindsey – President
M: (502) 836-4217