1. Selective Coordination
An introduction to Selective Coordination
Selectivity between fuses
Selectivity between fuses and circuit breakers
Welcome to an introduction to Selective Coordination. By the end of this presentation you’ll have a greater understanding selectivity between fuses and selectivity between fuses and circuit breakers.

2. One-Line Diagram F1 F2 Faulted Circuit F3 Motors Fuse Selectivity
Consider this one-line diagram. F1 represents our main fuse. F2 represents our feeder fuses and F3 is the branch circuit fuses. If a fault occurs between F3 and the motor it serves, short circuit current passes through F1, F2, and F3. In addition, motors will temporarily act as generators and feed current into the fault. If our system is selectively coordinated, F3 will clear the fault before either F2 or F1 open. If our system is not selective F2 and or F1 may open as well as F3, unnecessarily shutting down our whole system or at least parts of our system that are healthy and don’t need to be shut down.
Motors

3. Definitions
Coordination (Selective) – 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.
Source – Article 100 of NEC 2008
Overcurrent Coordination Study – Is the comparison and selection of operating times of the protective devices that achieve the objectives of the protection system under abnormal system conditions.
Source – IEEE Std
Here is the definition of selective coordination as found in Article 100 of the 2008 NEC.

4. Definitions
Selectivity - A main overcurrent protective device (OCPD) and a branch OCPD are said to be selective if the branch OCPD will clear all overcurrent conditions before the main OCPD unlatches (CBs) or melts (fuses).
Jargon - The terms coordination, selectivity, and selective coordination may be used interchangeably.

5. Degrees of Selectivity
Complete selectivity – OCPDs are selective under all overcurrent conditions.
Partial selectivity – OCPDs are selective under some overcurrent conditions
Overloads
Low fault currents
Degrees of selectivity are often encountered. Some combinations of OCPD are selective under all overcurrent conditions. Other combinations may be selective under overload conditions or low fault level conditions but not under high fault current conditions. Partial selectivity is often encountered in circuit breaker protected circuits.

6. NEC References
Selective coordination is generally desirable and may be mandatory:
NEC (hazard reduction)
NEC (elevators)
NEC (healthcare essential circuits)
NEC (emergency lighting)
NEC (standby power circuits)
NEC * (critical operations)
*new in NEC 2008
Selective coordination between OCPD is generally desirable and may be mandatory. The NEC has long mandated selectivity if a lack of selectivity represents a hazard to personnel (NEC ). NEC requires selectivity in people mover equipment. Several articles were added to the 2005 NEC which mandate selective coordination in specific systems. Examples include essential circuits in healthcare facilities (NEC ) , emergency lighting and power circuits (NEC ), and for legally required standby systems (NEC ).
Article 708 was added to the 2008 NEC. This article covers critical operations power systems. Article requires selectivity.

7. Selectivity Between Fuses
In a fused system as represented here, if our fuses are selective and a fault occurs in the leads to the motor, those fuses closest to the fault open without opening or damaging upstream fuses.
Fuse closest to fault must open without opening or damaging upstream fuses
Fault
Motors

8. Fuse Selectivity To assure selectivity between fuses consider:
Fuse Time-current curve ( .01 sec or longer)
Fuse I2t (less than .01 sec)
Determining whether or not fuses are selective is a relatively simple matter.
To do so we need to consider fuse time-current curves which deal with events lasting .01 sec and longer and we must consider fuse I2t which describes fuse performance for clearing times that are less than .01 seconds.

9. Fuse Time-Current Curves
The three types of fuse time-current curves (TCC)
Minimum melting curves
Average melting curves
Total clearing curves
Fuse manufacturers publish three different types of fuse time-current curves. They are…
- Minimum melting curves
- Average melting curves
- Total clearing curves

10. Fuse Min & Max Time-current Curves
Here we are showing the minimum melting curve and the total clearing curve for a feeder fuse as shown on your right and for a branch fuse as shown on your left. If a separation is maintained between the minimum melt curve of the upstream fuse and the total clearing curve for the downstream fuses, the fuses will coordinate for currents and times covered by the graph. This graph is typical of how fuse curves are depicted in integrated systems software packages from company’s like SKM, ESA, etc

11. Fuse Average Melt TCC
If only average melting curves are available as shown here, contact the manufacturer for guidance in terms of the degree of separation needed between curves to maintain selectivity.

12. Current Limitation
I2t defines fuse performance for times below 0.01 sec
For times of less than .01 seconds we can compare fuse I2t which describes fuse performance for clearing times of less than 0.01 sec.
This slide shows what happens when a short circuit is initiated in a circuit protected by a Current-Limiting fuse. The black line represents prospective current also called available short circuit current while the red line represents the actual current waveform passed by the Current-Limiting fuse.

13. (IRMS )2 t ≈ Thermal Energy
Current Limitation Ip
(IRMS )2 t ≈ Thermal Energy
This slide illustrates the current waveform passed by the Current-Limiting fuse.
Peak Let-Thru Current (Ip) - This is the maximum value that the current will reach when a Current-Limiting fuse melts in its short circuit region. [Note the location on the chart]
Ampere Squared Seconds (I2t) - This is a good indicator of the thermal energy associated with the current that is passed by the protective device. I2t is RMS current squared X time.
Melting I2t - The I2t required to melt the fuse element(melting time).
Clearing I2t - The total clearing I2t passed by fuse (time is from initiation of the fault until final clearing).
Melting
Time
Arcing
Time
Clearing Time

14. Available Current (kA) RMS Symmetrical
Fuse IP Graph
Fuse Peak Let-thru
100
AJT200 10
Ip (kA)
2.3 X RMS Symmetrical 1
Here is a fuse Ip graph for an AJT A class J time delay fuse. The horizontal axis is labeled as available short circuit current in RMS symmetrical amperes. The vertical axis represents the maximum fuse Ip in terms of instantaneous amperes. The diagonal line is labeled 2.3 X RMS Symmetrical. This line represents the maximum instantaneous current for a fully asymmetrical waveforms assuming a 15% power factor circuit. The red line represent fuse performance. It shows the maximum fuse peak let-thru current (Ip) as a function of the available fault current.
For example, if the available fault current is 30kA RMS Symmetrical. The fuse peak let-thru will not exceed about 11kA. Without the fuse, the circuit peak current could be as much as 69kA.
For a given fuse design, the primary external factor determining fuse Ip is di/dt, i.e., the rate of current rise during the time the fuse element is melting. Thus the higher the available fault current the higher the maximum possible fuse peak let-thru current.
0.1
0.1 1 10
100
Available Current (kA) RMS Symmetrical

15. I2t Selectivity Main Fuse Melting Time Branch Fuse Arcing
Clearing Time
Clearing I2t
1000 A2s
Melt I2t
1500 A2s
If the clearing I2t of the branch fuse is less than the melting I2t of the main fuse, the fuses will be selective for events cleared in less than .01 seconds.

16. Selectivity Tables
To simplify coordination evaluations for the end user, fuse manufacturers publish selectivity tables, sometimes called ratio tables. These tables take into account fuse TCC’s and fuse I2t. Here’s an example such a table. This table shows us that if our main and branch fuses are both A6D fuses, we need to maintain a 2 to 1 ratio between upstream and downstream fuses to assure selectivity. Today’s Current-Limiting fuses are easy to coordinate.

17. Circuit Breaker Coordination
Partial Selectivity Typical Due To MCCB & ICCB Instantaneous Trips
Solution Problem
Zone Selective Interlocking Cost, Complexity
LVPCB with Short-time delay Arc Flash Energy
Add CL Fuses Selectivity???
What about coordination is a system that includes circuit breakers? Circuit breaker manufacturers publish tables which list the instantaneous selectivity capability of various circuit breaker combinations . Adequate selectivity can be difficult to achieve in an all circuit breaker system due to the instantaneous trip characteristics of molded case and insulated case circuit breakers. Electronic breakers with zone selective interlocking and LVPCB with short-time delay characteristics may be used to improve selectivity. However the use of short-time delay without instantaneous trip override can result in unacceptably high arc flash incident energies. Zone selective interlocking adds cost and complexity to the system.
A judicious mix of circuit breakers and Current-Limiting fuses may well be the most economical package providing the needed degree of selectivity while providing minimal incident energy levels. The problem with this approach is in evaluating selectivity between fuses and circuit breakers.

18. Fuse & CB Selectivity
“Selectivity Analysis in Low Voltage Power Distribution Systems with Fuses and Circuit Breakers”
Marcelo Valdes, Cindy Cline, Steve Hansen, Tom Papallo
IEEE I&CPS Technical Conference Record May, 2009
In May of 2009 Ferraz Shawmut and GE presented a joint IEEE paper discussing selectivity between fuses and circuit breakers. Much of the information in the following slides comes from that paper.

19. TCC or Published Tables
Analysis Matrix
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
Let’s now consider viable methods for evaluating selectivity between fuses and circuit breakers. We will focus our discussion on the two combinations highlighted here. Our first scenario will involve a conventional CB below a Current-Limiting fuse. Our second scenario will involve a Current-Limiting fuse below a conventional CB. We use the term conventional CB to refer to MCCB, ICCB, or LVPCB which employ magnetic or simple digital electronic trips. These are not Current-Limiting CBs.

20. TCC or Published Tables
Fuse UP - CB Down
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
Our first scenario will involve a non-CL breaker below a Current-Limiting fuse.

21. TCC Analysis Fuse Up With non-CL CB Down
Traditional TCC analysis provides a conservative analysis of where selectivity may not exist.
For this combination of OCPDs traditional TCC analysis will provide a conservative analysis. This evaluation will tell us where selectivity may not exist. Here we show a 200A CB downstream from an 800A Class L fuse. The curves tell us that these devices will be selective over the long time and short time portions of the CBs time-current curves. The combination is also selective in the instantaneous portion of the CB curve for faults up to about 8kA. If the fault current exceeds about 8kA we would expect both devices to operate. Nothing new here.

22. TCC or Published Tables
CB Up – Fuse Down
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
Our next scenario involves a Current-Limiting fuse below a conventional non-CL circuit breaker.

23. Time-Current Curve (TCC) Analysis
100 1K
10K
100K
0.01
0.10 1 10
1000
Amperes RMS
SECONDS
200A J TD
800A CB
Up – 800A CB Set at 4kA
Down – 200A Class J
Selective ?
Here we show a 200A Class J fuse down from an 800A MCCB set to trip instantaneously at 4kA. How would you interpret this combination? Would you expect these devices to be selective? The reality is that these devices are not selective for instantaneous tripping of the CB. Though we may suspect this from our TCC analysis we can’t be sure based on TCC alone.

24. CB & Fuse Selectivity TCC analysis may be inadequate - Why?
CBs can unlatch and fuses can melt in less than 0.01sec
TCC stop at 0.01 sec
How do we evaluate selectivity between fuse and CB for times < 0.01sec ?
Why is Time-current curve analysis inadequate in this case?

25. Consider TCC and IP
How do we evaluate instantaneous selectivity between fuse and circuit breaker?
Compare TCC for CB long-time and short-time operation
Compare fuse peak let-thru current to CB peak pickup current for instantaneous operation.
Perhaps the better question is how do we evaluate selectivity between the downstream fuse and an upstream breaker when instantaneous CB tripping is possible? The answer is to compare fuse peak let-thru current to the CB peak pickup current for instantaneous operation.

26. MCCB Characteristics Instantaneous Trip Setting
Magnetic or Simple Digital Electronic
RMS Calibrated
Peak Sensing
RMS × √2 = Peak Current
(√2 = 1.414)
The term instantaneous trip can cause some confusion. Instantaneous trip refers to the tripping action and NOT to the current value that initiates the trip. Though the instantaneous trip setting is calibrated in terms of RMS current, CB mechanisms are peak current sensing. Peak sensing trips are set at the square root of 2 x the RMS setting. This factor comes from the ratio between RMS and peak current of a symmetrical sine wave.

27. CB Peak Current Trip Example 1000A CB Instantaneous trip set at 10X
Peak current trip is calculated as
1000A X 10 X = 14,140A
An example is in order. If we have a 1000A CB with an instantaneous trip set at 10X this CB will trip instantaneously if it senses a peak current that exceeds 14,140A. I do not mean to suggest that the CB trip is accurate to 4 significant digits but rather displayed the digits so you could follow through with the math.

28. Available Current (kA) RMS Symmetrical
CB Trip Current
100
Peak Current Causing CB to Trip 10
Ip (kA) 1
CB Instantaneous Trip (RMS)
Here is a graphical representation of the same example.
1.414 Peak to RMS Ratio
0.1
0.1 1 10
100
Available Current (kA) RMS Symmetrical

29. Time-Current Curve (TCC) Analysis
100 1K
10K
100K
0.01
0.10 1 10
1000
Amperes RMS
SECONDS
200A J TD
800A CB
Up – 800A CB Set at 4kA
Down – 200A Class J
Selective ?
Looking again at the time current curve analysis of our original example involving a 200A fuse downstream from an 800A circuit breaker, we see that a separation is maintained between the time current curves of our fuse and our circuit breaker down to times as low as 0.01 seconds. Over these time values our devices are selectively coordinated. To evaluate selectivity for times below 0.01 seconds we must perform an Ip analysis.

30. IP Analysis C.B. Trip Current vs. Fuse Peak Let-thru 100 AJT600 AJT400
Ip (kA) 1
CB Instantaneous Trip
By performing an Ip analysis we see that with a 4kA RMS instantaneous trip setting the CB will commit to trip if it senses an peak current above about 5.6kA. With an available fault current of 10kA the fuse will pass a peak current of about 9kA, well above the peak current required to unlatch the CB. Based upon this analysis fuse maximum Ip will not be below CB trip Ip at any fault level. Thus we conclude that we cannot count on these two devices to be selective if the fault current is high enough to cause instantaneous tripping of the circuit breaker.
2.3 Peak to RMS Ratio
1.414 Peak to RMS Ratio
0.1
0.1 1 10
100
Available Current (kA) RMS Symmetrical

31. TCC Analysis Up – 800A CB Set above 10kA Down – 200A Class J
100 1K
10K
100K
0.01
0.10 1 10
1000
Amperes RMS
SECONDS
800A CB
200A J TD
Up – 800A CB Set above 10kA
Down – 200A Class J
Selective to 40kA
Our next example involves the same CB and fuses as in our previous example but in this case the CB instantaneous trip setting is set above 10kA.
The significant separation between the fuse and CB TCC at 0.01 sec suggests these devices will be selective in the CBs instantaneous trip range. What does an Ip analysis show us?

32. IP Analysis C.B. Trip Current vs. Fuse Peak Let-thru 100 AJT600 AJT200
Ip (kA) 1
CB Instantaneous Trip
The CB instantaneous trip setting is just over 10kA RMS. This equate to just over 14kA peak current. The graph shows that the 200A Class J fuses has a maximum Ip let-through of less that 14kA for fault currents up to 40kA. This analysis tells us that the fuse and circuit breaker will be selective provided the available fault current at the fuse does not exceed 40kA.
Questions?
2.3 Peak to RMS Ratio
1.414 Peak to RMS Ratio
0.1
0.1 1 10 40
100
Available Current (kA) RMS Symmetrical

33. TCC or Published Tables
Summary
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
In summary, when evaluating selectivity if we have a CL fuse up and CL fuse down we use manufacturers ration tables to evaluate selectivity.

34. TCC or Published Tables
Summary
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
If we have CL fuses up and conventional CBs down we can use TCCs to provide a conservative determination of the degree of selectivity that exists.

35. TCC or Published Tables
Summary
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
If we have conventional CBs up and down we can use TCCs or manufacturers instantaneous selectivity tables to determine the degree of selectivity that exists.

36. TCC or Published Tables
Summary
Main →
Feeder
Current-Limiting Fuse
Conventional CB (non-CL)
Ratio Tables
TCC and Ip Analysis
TCC
TCC or Published Tables
If we have conventional CBs up and CL fuses down we should use TCCs to evaluate selectivity in the long time and short time portions of the CB TCC and use Ip analysis to evaluate selectivity in the CBs instantaneous operating range.

37. Thank you
Thank you.