Understanding time current curves: Part 3

The final installment of a three-part series about time current curves (TCCs) reviews the coordination of sample curves and the importance of coordination.

02/04/2014


Continued from Part 2

Now that the basics of TCCs have been explained, a review of coordination is in order. Our sample curves to coordinate will consist of an MCC with main 800-A fuses, a 1,200-A feeder circuit breaker and the switchgear 3,200-A main circuit breaker. In the uncoordinated system there is overlap of the circuit breaker trip curves, and in some instances the main circuit breaker will trip before the feeder circuit breaker. The main fuse in the MCC is also uncoordinated. While the fuse is not required, it is included in this example because it is typical of an industrial installation. The purpose of the fuse is to provide current limiting to increase the short circuit withstand rating of the MCC bus. For example purposes, it is assumed the MCC fuse is required and the cable feeder sizes cannot be changed. These assumptions make the example case here realistic as there are often constraints, such as this found in real world coordination problems in industrial facilities. “Coordinated” means that selectivity between the feeder and the main circuit breaker is maintained. Per the National Electric Code, “coordinated” is defined as “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.”



In the coordinated example the main and feeder circuit breakers are selectively coordinated and the main circuit breaker provides adequate protection for the power transformer. Coordination between the MCC main fuse and the feeder circuit breaker was also improved. Coordination improvements to these circuit breakers included the following changes:

 - Main Circuit Breaker Long Time Pickup (LTPU) from 2880 to 3200
 - Main Circuit Breaker Short Time Pickup (STPU) from 1.5 to 5
 - Main Circuit Breaker Short Time Delay (ST DLY) from Min. to Int.
 - Main Circuit Breaker Instantaneous Pickup from 3 to disabled
 - FB3 Circuit Breaker Long Time Pickup (LTPU) from 1 to .95
 - FB3 Circuit Breaker STPU from 4 to 5
 - FB3 Circuit Breaker Instantaneous Pickup from 15 to 9

There is overlap of the MCC main fuse and the feeder circuit breaker time current curves for long term low level overloads. This overlap could be eliminated if a larger long time pickup setting was used in the feeder circuit breaker. Increasing this setting would then require the upsizing of the feeder cable to maintain conformance with the National Electric Code. Coordination involves tradeoffs and selections that require engineering experience and judgment to find the most optimal settings. In many real world cases it is impossible to coordinate all possible cases. As such, engineering judgment is required to coordinate the most likely scenarios and create the most reliable system. Additionally, Arc Flash hazard category reductions generally result in diminished selective coordination. Conversely, improved coordination may result in increased arc flash hazard categories in some cases. In the above example Arc flash hazard category ratings for both the uncoordinated and the coordinated cases were unchanged even with improved coordination. It is possible to achieve these optimized results through the use of engineered selections. It is for these reasons that the selection of overcurrent device ratings and settings be left to power system engineers experienced in industrial power systems.

This post was written by David Paul. David is a Principle Engineer at MAVERICK Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.



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