In power system studies – the critical four I said:
The purpose of the protection coordination study is to verify that the various protection devices in your system, relays, breakers, fuses, etc. are coordinated correctly and are sized appropriately for the equipment that they are protecting.
which is very true. To expand on that, a protection coordination study really has two purposes. First it ensures that the electrical equipment is properly protected from over-currents and overloads. Secondly the study determines that the selective coordination is employed such that the system reacts as intended and in a predicable manner. You should (almost) always sacrifice equipment protection for selective coordination. The cases where the opposite is true is a topic for a future article.
A protection coordination study will consist of time-current curves (TCC) of all the electrical equipment, protective devices and large motors. Above is a sample TCC showing a circuit for a 50hp motor, it shows the breaker (CB16) that protects the motor from over-currents, the over-load protection (OL_H1), and the fuse (Fuse3) that feeds the MCC. The dashed line at the far left shows the motor starting and running curve, the next shaded area is CB16 and OL_H1. ETAP models them as a system and only shows the parts of the curves that are relevant. At the far right is the fuse.
There is no overlap between the motor curve and the motor protection (CB16 and OL_H1) so therefore the motor will operate as intended, and there is no overlap between the feeder fuse and the motor protection, so if there is a fault on the motor circuit the main fuse won’t activate. If there was an overlap, then there is a chance of the fuse operating on a motor fault, removing a larger portion of the system from service, and possibly making it difficult to troubleshoot the issue to get the system back into operation.
Why do I need a protection coordination study?
You will need a current protection coordination study at your facility to ensure that your system reacts to a fault in a predictable manner and to know how long a fault will be present before the protection takes action. This time is critical information for an incident energy study, and will greatly affect the severity of an arc fault.
When should a protection coordination study be completed?
A protection coordination study will be completed during the design phase of any facility, however after that it will be reviewed on a periodic basis, but at least every 5 years. As with all power system studies, if there have been changes within the system, you should verify that the protection is still adequate.
Another time to revisit the protection coordination study is when equipment is experiencing nuisance tripping: this can be caused by abnormal currents in the equipment, or a faulty relay.
How do I get one done?
I always recommend that the protection coordination study be completed with software, unlike the short circuit study. It is extremely hard to print curves to scale for overlaying, and to try multiple options when an issue presents itself. However, it is relatively easy to add different devices to the circuit and verify protection and selective coordination when using power system modelling software like ETAP, SKM Powertools or Easypower.
To determine the best way to get a protection coordination study completed check out this post from a couple of weeks ago. In it I explain the various methods that you can use to complete a power system study, which can work for any individual study.
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