A coordination study is used to determine how devices will behave when they are subjected to overcurrent or short current situations. This is important because unless protective devices are properly set, they may not be able to perform to protect your system. The devices are needed to trip and clear a fault quickly to help protect people and equipment. It is also important to make sure that the proper device trips and not an upstream breaker that could take down a large part of your facility.
To prepare for a CS, you will need to have the manufacturer’s data curves for all protective devices as well as a time-current characteristic curve (time vs current graph). Digital tools like E-Tap, SKM, and EZ Power make performing a CS much easier as they have libraries with common devices and their TCC Curves. Having a One-Line is always useful when going to perform a CS, but it is not required.
When performing a study, you should include your transfer switches, to help show how the system is sourced under normal vs emergency situations.
After having a CS done you can expect a report with the protective device settings as well as the time that it will take each device to trip or clear a fault. Some companies that will sit down and go through the report with you to help you better understand the report and highlight and priorities that need to be taken care of immediately so that they can schedule an outage to correct the issues if need be.
Typically, a CS should be completed whenever new equipment is installed into your facility or whenever load demands change. When new equipment is installed you want to make sure that you set all the protective devices properly to ensure that they act the way they are intended to.
The plant engineer or maintenance supervisor is typically the one that will take on the responsibility of having the CS done and then performing the actions from the report, whether they do it themselves or have a professional company make the necessary changes to the settings.
Some common mistakes that can be avoided include having the proper selective coordination done to ensure that a main breaker upstream does not trip when a feeder breaker downstream should be the one to trip and clear the fault. A common reason for this is having the instantaneous set too high on the feeder breaker, as high as the main is set typically. A good book to pick up is by GE and is called the Art and Science of Protective Relaying, it pertains to breaker settings as well as Relays.
Coordination studies are often overlooked but this is a great opportunity to enhance your electrical distribution system.