understanding timecurrentcurves
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3/17/2015 Understanding Time Current Curves Part 1
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Understanding time current curves: Part 1Posted by David Paul on January 28, 2014 @ 2:36 pm
The first installment of a threepart series about time currentcurves (TCCs) provides a quick overview of item identificationand how to read TCC plots.
A time current curve (TCC) plots the interrupting time of an overcurrent device based on a givencurrent level. These curves are provided by the manufacturers of electrical overcurrent interruptingdevices, such as fuses and circuit breakers. These curves are part of the product acceptance testingrequired by Underwriters Laboratories (UL) and other rating agencies. The shape of the curves isdictated by both the physical construction of the device as well as the settings selected in the case ofadjustable circuit breakers. The time current curves of a device are important for engineers tounderstand because they graphically show the response of the device to various levels overcurrent.The curves allow the power systems engineer to graphically represent the selective coordination ofovercurrent devices in an electrical system. Modern power system design software packages, such asEasyPower, SKM Power Tools, and Etap, contain graphical libraries of curves to allow the powersystem engineer the ability to plot, analyze, and print the curves with minimal effort compared to theprevious methods used when coordinating a power system.
TCC item identification
The TCC diagram shown in Figure 1 (below) plots the interrupting response time of a currentinterrupting device versus time. Current is shown on the horizontal axis using a logarithmic scale andis plotted as amps X 10X. Time is shown on the vertical axis using a logarithmic scale and is plottedin seconds X10X .The light blue curve is a switchgear feeder circuit breaker curve. The violet curve isthe switchgear main circuit breaker curve. The red curve is the transformer primary fuse curve. Theorange curve is the transformer damage curve. The green curves are the cable damage curves. Each
3/17/2015 Understanding Time Current Curves Part 1
http://www.mavtechglobal.com/blog/2014/01/28/understandingtimecurrentcurvespart1/ 2/7
one of these items will be explained. The system represented by this curve is well coordinated andadequately protected from damage. It also has minimal arc flash hazard category ratings due to lowinstantaneous circuit breaker trip values.
The one line diagram (see Figure 2) and TCC plot show a typical hypothetical industrial powersystem. There is a utility delivery point with power supplied at a medium voltage level (in this case4,160 V), which feeds the primary side of a 2.5 MVA power transformer through a medium voltagefused switch containing an E Class fuse. The 480 V secondary side of the transformer feeds a piece oflow voltage power switchgear utilizing draw out low voltage power circuit breakers for the main andfeeder circuit breakers. The TCC plot also displays the transformer and cable damage curves. Thesecurves are based on accepted industry consensus standards published by the American NationalStandards Institute (ANSI, for the transformer) and the Insulated Cable Engineers Association(ICEA,for the cables). Interpreting the damage curves is fairly straight forward. Operating conditions(overcurrent protection) must be kept to the left of the damage curve to guarantee no permanent
3/17/2015 Understanding Time Current Curves Part 1
http://www.mavtechglobal.com/blog/2014/01/28/understandingtimecurrentcurvespart1/ 3/7
damage is done to the transformer or cable in question. Operating conditions that allow operation tothe right of the damage curve subject the device in question to currents that can cause permanentirreversible damage, a shortened lifespan, and possible catastrophic failure. Therefore, the overcurrentand circuit breaker coordination schemes must take this into account during the initial design phase.
There are two transformer damage curves (shown in orange on Figure 1) – one is dashed, the other issolid. The solid damage curve is unbalanced and takes into account a derating factor for transformerwinding type and fault type. The dashed damage curve is the 100% rating curve with no deratingconsideration. The transformer inrush current is also plotted a single point on the TCC diagram.Again, as part of the initial design, the transformer inrush current must be to the left of the transformerprimary fuse curve, otherwise the fuse will open when the transformer is energized. These differencesin the unbalanced and 100% damage curves can be mitigated with additional protective relaying toallow the 100% curve to be used for power system design without the risk of transformer damage.
There are three cable damage curves (shown in green on Figure 1). There is one curve for each cablerepresented on the one line diagram. As part of the initial design, the overcurrent interrupting devicemust limit the fault current to left of the damage curve to prevent permanent damage. The damagecurves for the cables are dependent on size, insulation type and raceway configuration.
This post was written by David Paul. David is a Principle Engineer atMAVERICK Technologies, aleading automation solutions provider offering industrial automation, strategic manufacturing, andenterprise integration services for the process industries. MAVERICK delivers expertise andconsulting in a wide variety of areas including industrial automation controls, distributed controlsystems, manufacturing execution systems, operational strategy, business process optimization andmore.
3/17/2015 Understanding Time Current Curves Part 1
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Discussion – 4 Comments
There are 4 responses to "Understanding time current curves: Part 1".
David Hollister says:February 2, 2014 at 7:07 am
Thank you for this article. Very well done and easy to comprehend. As a newly graduate, I amthankful to have found this since noone at my plant could explain the TCC in such clarity.
ReplyGabe Mendez says:February 14, 2014 at 9:11 am
This is a good explanation of TCC’s and will help in understand why a coordination study isneeded! Not only for equipment safety and reliability but as you touched on, reducing energyavailable for Technicians!!
Good Job
ReplyDan Holtzclaw says:July 30, 2014 at 2:41 pm
Where can I find a guideline for the minimum amount of time between curves for a givencurrent level?
ReplyUnderstanding time current curves: Part 2 | MAVERICK Ideas says:January 30, 2014 at 9:17 am
[…] from Part 1… The light blue curve represents the circuit breaker settings for the feedercircuit breaker. […]
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3/17/2015 Understanding Time Current Curves Part 2
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Understanding time current curves: Part 2Posted by David Paul on January 30, 2014 @ 9:17 am
The second installment of a threepart series about time currentcurves (TCCs) covers short and long time settings, includingtheir purpose and examples of such overcurrents.
Continued from Part 1…
The light blue curve represents the circuit breaker settings for the feeder circuit breaker. The lowerportion of the curve (below 0.05 sec or three cycles on the time axis) is the instantaneous trip function.The purpose of the instantaneous trip is to trip the circuit breaker quickly with no intentional delay (nomore than a few cycles) on high magnitude fault currents. This quick trip protects electricaldistribution equipment from damage and keeps arc flash hazard categories low. Clearly these typefaults must be interrupted quickly and do not allow the system to wait and see if the fault will selfclear. The minimum instantaneous setting determines the minimum trip setting for the circuit breaker.In the case shown in Part 1, the instantaneous setting is 2,400 A and the maximum value displayed isavailable fault current at the circuit breaker. Small changes in the instantaneous setting can result insignificant changes in the Arc Flash Hazard category, so this is a setting which must be carefullyselected according to sound engineering principles.
The next section of the curve moving up the time axis is determined by the short time settings. Shorttime settings cover the time range from 0.05 to 0.5 sec (three to 30 cycles). The purpose of short timesettings is to allow a timebased delay to elapse before tripping the circuit breaker for moderatecurrent faults. This allows moderate faults time to clear themselves without tripping the circuitbreaker. Examples of these types of overcurrents would be the inrush on a large motor starting or atransformer inrush when it is first energized. The short time pickup setting shifts the curve on thecurrent axis. Increasing the short time pickup settings shifts the curve to the right and conversely
3/17/2015 Understanding Time Current Curves Part 2
http://www.mavtechglobal.com/blog/2014/01/30/understandingtimecurrentcurvespart2/ 2/7
lower settings shift the curve to the left. The short time delay setting moves the “knee” of the curvevertically on the time axis. Increasing the short time delay setting moves the “knee” vertically up thetime axis. Similarly decreasing the settings moved the “knee” lower on the vertical axis. Thedescribed curve shifts are depicted on the TCC plots below.
The next section of the curve moving up the time axis is the long time section. Long time settingscover the time range from 0.5 to 1000 sec. The purpose of long time settings is to allow a timebaseddelay to elapse before tripping the circuit breaker for low level current faults. This allows low levelfaults time to clear themselves and allows electrical equipment to operate in a temporarily overloadedcondition provided it will not produce permanent damage to the equipment. Examples of these typesof overcurrents would be the overloading of a power transformer or large motor for a few minutes.The long time pickup setting sets ultimate trip value of the circuit breaker. Generally circuit breakersare set at their maximum long time setting and cannot exceed the rating of the circuit breaker. Theycan be set at reduced values which shifts the curve to the left on the current axis. There is also a“knee” in the long time portion of the curve. The long time “knee” can be shift up the time axis byincreasing the long time delay setting. Conversely, the “knee” can be shifted lower by decreasing thelong term delay setting. The described curve shifts are depicted on the TCC plots below.
3/17/2015 Understanding Time Current Curves Part 2
http://www.mavtechglobal.com/blog/2014/01/30/understandingtimecurrentcurvespart2/ 3/7
Another factor to notice in the curves is that they have a width to them. This is due to tolerance of thetrip elements in the circuit breaker. The curves below will show the differences in this deadband forthe same circuit breaker using an electronic versus non electronic trip mechanism.
Notice the difference in the tolerance in the trip curve of the feeder circuit breaker. The circuit breakerunit itself is the same for both curves so the tripping delays are constant. Settings for both trip unitswere set as similar as possible to each other so performance could be easily compared. Notice there ismore tolerance +/ in the nonelectronic trip unit due to analog component tolerances and electromechanical devices inside the trip unit. The transition points on the electronic trip curve betweeninstantaneous, short time and long time are much more distinct and accurate because of the digitalmicroprocessor based trip. In the nonelectronic trip the transition points have time based decaycurves associated with them due to the physics of the electromechanical trip elements. It is simplyimpossible to improve the performance of the non electronic trip because of the physical limits of thecomponents.
3/17/2015 Understanding Time Current Curves Part 2
http://www.mavtechglobal.com/blog/2014/01/30/understandingtimecurrentcurvespart2/ 4/7
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As can be seen in the above TCC curves electronic trip units provide the engineer greater ability toaccurately and selectively coordinate the electrical power system. Because of the obsolesce and lackof repair parts for older nonelectronic trip units the replacement of these units with electronic tripsunits can improve system coordination and reduce arc flash values in a properly designed electricalpower system.
This post was written by David Paul. David is a Principle Engineer at MAVERICK Technologies, aleading automation solutions provider offering industrial automation, strategic manufacturing, andenterprise integration services for the process industries. MAVERICK delivers expertise andconsulting in a wide variety of areas including industrial automation controls, distributed controlsystems, manufacturing execution systems, operational strategy, business process optimization andmore.
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Discussion – 4 Comments
There are 4 responses to "Understanding time current curves: Part 2".
T Feld says:February 14, 2014 at 4:50 pm
Nice explanation. Would be nice to be able to enlarge the curves in order to read the marginnotes and axes. Or better yet, provide them as pdf downloads.
ReplyLuc says:March 3, 2014 at 8:01 am
Copy paste the images in Excel and enlarge, that’s how I did it.
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3/17/2015 Understanding Time Current Curves Part 2
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ReplyMario says:October 31, 2014 at 6:52 am
Where is the link for part 3?
Replyliken says:December 9, 2014 at 2:56 am
http://www.mavtechglobal.com/blog/2014/02/13/understandingtimecurrentcurvespart3/
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3/17/2015 Understanding Time Current Curves Part 3
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Understanding time current curves: Part 3Posted by David Paul on February 13, 2014 @ 2:53 pm
The final installment of a threepart series about time currentcurves (TCCs) reviews the coordination of sample curves andthe importance of coordination.
Continued from Part 2…
Now that the basics of TCCs have been explained, a review of coordination is in order. Our samplecurves to coordinate will consist of an MCC with main 800A fuses, a 1,200A feeder circuit breakerand the switchgear 3,200A main circuit breaker. In the uncoordinated system there is overlap of thecircuit breaker trip curves, and in some instances the main circuit breaker will trip before the feedercircuit breaker. The main fuse in the MCC is also uncoordinated. While the fuse is not required, it isincluded in this example because it is typical of an industrial installation. The purpose of the fuse is toprovide current limiting to increase the short circuit withstand rating of the MCC bus. For examplepurposes, it is assumed the MCC fuse is required and the cable feeder sizes cannot be changed. Theseassumptions make the example case here realistic as there are often constraints, such as this found inreal world coordination problems in industrial facilities. “Coordinated” means that selectivity betweenthe feeder and the main circuit breaker is maintained. Per the National Electric Code, “coordinated” isdefined as “localization of an overcurrent condition to restrict outages to the circuit or equipmentaffected, accomplished by the choice of overcurrent protective devices and their ratings or settings.”
3/17/2015 Understanding Time Current Curves Part 3
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In the coordinated example the main and feeder circuit breakers are selectively coordinated and themain circuit breaker provides adequate protection for the power transformer. Coordination betweenthe MCC main fuse and the feeder circuit breaker was also improved. Coordination improvements tothese 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 longterm low level overloads. This overlap could be eliminated if a larger long time pickup setting wasused in the feeder circuit breaker. Increasing this setting would then require the upsizing of the feedercable to maintain conformance with the National Electric Code. Coordination involves tradeoffs andselections that require engineering experience and judgment to find the most optimal settings. In manyreal world cases it is impossible to coordinate all possible cases. As such, engineering judgment isrequired to coordinate the most likely scenarios and create the most reliable system. Additionally, ArcFlash 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 aboveexample Arc flash hazard category ratings for both the uncoordinated and the coordinated cases wereunchanged even with improved coordination. It is possible to achieve these optimized results throughthe use of engineered selections. It is for these reasons that the selection of overcurrent device ratingsand 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, aleading automation solutions provider offering industrial automation, strategic manufacturing, andenterprise integration services for the process industries. MAVERICK delivers expertise andconsulting in a wide variety of areas including industrial automation controls, distributed controlsystems, manufacturing execution systems, operational strategy, business process optimization andmore.
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3/17/2015 Understanding Time Current Curves Part 3
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Discussion – 2 Comments
There are 2 responses to "Understanding time current curves: Part 3".
Bob Quammen says:June 24, 2014 at 1:43 pm
David Paul has written a clear and easy to follow guide to understanding some of the uses of thetime current curves. Part three of the discussion makes the complex coordination problemdoable. Unfortunately for me, part two was not available online and I will search for it. Welldone David Paul, thank you very much.
Replydave white says:October 7, 2014 at 11:09 pm
Your guides are clear and to the point. They answered many questions I had, particularly how toprovide protection with respect to the transformer inrush and the cable damage curves. Thankyou for producing this series of articles on understanding TCCs!
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THE BIG IDEA
Every day, MAVERICK employees connect with our customers on different projects across abroad spectrum of categories. We work with executives out in the field to define strategies toachieve their vision. We help plant and operations managers implement change and encourageengineers to integrate and sustain solutions on the plant floor. Now we're sharing ourexperiences with you through this blog. It's written by engineers, for engineers, and we hope itwill help you take your operation to the next level.
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All of us at MAVERICK are dedicated to delivering value for our clients. We don't pushproducts, we drive your potential. Greater safety. Higher efficiency. More profitability. We'llhelp you achieve it all. Learn more about MAVERICK.
Browse by Category
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