1. Sistemas Eléctricos Industriales
Coordinación Selectiva de los elementos de protección
Referencia:
Definiciones
Coordinación con fusibles
Coordinación con interruptores
Ejemplo de coordinación en una planta industrial
Presentado por: Jesús A. Baez M.
ITESM, Departamento de Ingeniería Eléctrica
Monterrey, NL Octubre 2007

2. Selective Coordination
Definition:
The act of isolating a faulted circuit from the remainder of the electrical system, thereby eliminating unnecessary power outages. The faulted circuit is isolated by the selective operation of only the overcurrent protective device closest to the overcurrent condition.
As you can see, selective coordination assures that ONLY the device closest to the fault opens. This assures that power is not lost to unfaulted circuits (blackouts).
This can be extremely important in critical power applications like continuous manufacturing process or health care facilities where blackouts can not be tolerated.
The next slide shows how this can happen and what effects it can have on the distribution system

3. Selective Coordination: Avoids Blackouts
Without Selective Coordination With Selective Coordination
Selective Coordination is the ability of a system to isolate a fault.
Without selective coordination, many or all of the upstream devices can also open causing unnecessary power losses to other non-faulted loads.
With selective coordination, the device closest to the fault is the only device which opens, unnecessary power loss is avoided.
Again the concept is easy to understand. Most people just assume that selective coordination is achieved if the upstream overcurrent protective devices have larger ampere ratings. You will discover that this is not the case.
OPENS
OPENS
NOT AFFECTED
NOT
AFFECTED
UNNECESSARY
POWER LOSS
Fault
Fault

4. Selective Coordination: NEC®
240.2 Definitions
Coordination.
The proper localization of a fault condition to restrict outages to the equipment affected, accomplished by the choice of selective fault-protective devices.
This is the definition in the NEC®

5. Selective Coordination: Normal Supply
Emergency
Source
Normal
Source N E
ATS
Typical one-line circuit diagram with emergency circuit.

6. Unnecessary Feeder Outage
Selective Coordination: Normal Supply
Emergency
Source
Normal
Source
Unnecessary Feeder Outage N E
ATS
Normal power
If overcurrent protective devices in the emergency system are not selectively coordinated, a fault at X1 on the branch circuit may unnecessarily open the sub-feeder; or even worse the feeder or possibly even the main. In this case, emergency circuits are unnecessarily blacked out. With selective coordination as a requirement for emergency, legally required standby, and essential electrical systems, when a fault occurs at X1 only the nearest upstream fuse or circuit breaker supplying just that circuit would open. Other emergency loads would remain powered.
This shows a fault on an emergency branch circuit when the power is from the normal power source. In this case the overcurrent protective devices are not selective coordinated and the feeder OCPD also opens. This unnecessarily blacksout the other branch circuits fed by that feeder. This is no longer permitted.
Opens
Not Affected
Unnecessary
Power Loss
Fault X1

7. Unnecessary Main Outage
Selective Coordination: Normal Supply
Without
Emergency
Source
Normal
Source
Unnecessary Main Outage
Blackouts Possible! N E
ATS
Normal power
This shows the fault on the branch unnecessarily opening the feeders and main on the normal power source. In this case, emergency circuits are unnecessarily blacked out. This system does not comply with the selective coordination requirements for emergency, legally required standby, and essential electrical systems.
Opens
Not Affected
Unnecessary
Power Loss
Fault X1

8. Blackouts Possible! Selective Coordination: Normal Supply Without With
Emergency
Source
Emergency
Source
Normal
Source
Normal
Source
Blackouts Possible! N E
ATS N E
ATS
Normal power
This diagram on the right will illustrate the benefits of selective coordination when the emergency branch circuit is connected to the normal power source.
Opens
Not Affected
Unnecessary
Power Loss
Fault X1

9. Blackouts Prevented! Blackouts Possible!
Selective Coordination: Normal Supply
Without
With
Emergency
Source
Emergency
Source
Normal
Source
Normal
Source
Blackouts Prevented!
Blackouts Possible! N E
ATS N E
ATS
Normal power
This illustrates the benefits of selective coordination when the emergency branch circuit is connected to the normal power source. A fault on the emergency branch circuit is opened only by the branch circuit overcurrent protective device and no other upstream devices open. Therefore, all the others loads remain powered by the normal power source. With selective coordination as a requirement for emergency, legally required standby, and essential electrical systems, when a fault occurs at X1 only the nearest upstream fuse or circuit breaker supplying just that circuit would open. Other emergency loads would remain powered.
Isolated to
Branch Only
Opens
Not Affected
Unnecessary
Power Loss
Fault X1
Fault X1

10. Selective Coordination
Circuit Breaker Coordination
Different devices have different characteristics. If you understand how the devices operate you will be able to interpret the curves better. Let’s first look how the operation of circuit breakers relate to their curves and how you assess coordination.

11. Coordination - Thermal-Mag Circuit Breakers
Thermal Magnetic Molded Case Circuit Breaker Time-Current Curve
Overload Region
Instantaneous Region
Interrupting Time
Unlatching Time
(The SPD has a section that explains this in depth. Use that section to understand the time current curves.)
This slide shows a typical time-current curve of a standard thermal magnetic circuit breaker.
As you can see the curve basically has two areas:
Overload region
Instantaneous region
As you can see the instantaneous region has a wide area between unlatching and interrupting time. This area extends to the interrupting rating of the device. After unlatching, the overcurrent is not halted until the breaker contacts are mechanically separated and the arc is extinguished. Consequently, the final overcurrent termination can vary over a wide range of time, as indicated by the wide band between the unlatching time curve and the maximum interrupting time curve.
The relatively long delay between unlatching and the actual interrupting of the overcurrent in the instantaneous region is the primary reason that molded case circuit breakers are very difficult to coordinate.
Interrupting Rating

12. Coordination - Thermal-Mag Circuit Breakers (See SPD)
(This curve and explanation is covered in the SPD.)
Here is an example that shows two circuit breakers – a 400 amp feeding a 90 ampere. It illustrates the short circuit unlatching and interrupting characteristics for each circuit breaker. As you can see, the two curves overlap in the short-circuit region.
If a short-circuit occurs, in this example above 1500A on the load side of the 90A breaker, both breakers will open. This is due to the construction of the circuit breaker. As we discussed on the previous slide, the molded case circuit breaker relies upon mechanical means to interrupt the current in the short-circuit region. Thus, the downstream breaker can not clear the fault before the upstream breaker unlatches (above 1500A). As you can see this would be very easy to achieve on a 90A circuit. Thus, in this case, selective coordination is not achieved.
The next slide has a close up view of the overlap area.
No se tiene selectividad para corrientes de corto circuito en este rango

13. Selective Coordination
(This curve and explanation is covered in the SPD.)
This is a close-up view of the prior graph. It is the instantaneous trip region for both the 90 A and 400 A circuit breakers.
The following curve illustrates a 400 ampere circuit breaker ahead of a 90 ampere breaker. Any fault above 1500 amperes on the load side of the 90 ampere breaker will open both breakers. The 90 ampere breaker will generally unlatch before the 400 ampere breaker. However, before the 90 ampere breaker can separate its contacts and clear the fault current, the 400 ampere breaker has unlatched and also will eventually open. Assume a 4000 ampere short-circuit exists on the load side of the 90 ampere circuit breaker. The sequence of events would be as follows:
1. The 90 ampere breaker will unlatch (Point A) and free the
breaker mechanism to start the actual opening process.
2. The 400 ampere breaker will unlatch (Point B) and it, too,
would begin the opening process. Once a breaker unlatches, it will
open. At the unlatching point, the process is irreversible.
3. At Point C, the 90 ampere breaker will have completely
interrupted the fault current.
4. At Point D, the 400 ampere breaker also will have completely
opened the circuit.
Consequently, this is a non-selective system, causing a complete blackout to the load protected by the 400 ampere breaker. As printed by one circuit breaker manufacturer, “One should not overlook the fact that when a high fault current occurs on a circuit having several circuit breakers in series, the instantaneous trip on all breakers may operate. Therefore, in cases where several breakers are in series, the larger upstream breaker may start to unlatch before the smaller downstream breaker has cleared the fault. This means that for faults in this range, a main breaker may open when it would be desirable for only the feeder breaker to open.”
Time B A
4000A Fault
Current

14. Lacking Coordination OPENS Fault NOT AFFECTED UNNECESSARY POWER LOSS
In the case just worked through the upstream circuit breakers would have opened on a branch fault of certain magnitudes.
OPENS
NOT AFFECTED
UNNECESSARY
POWER LOSS
Fault

15. Selective Coordination - Insulated Case Circuit Breakers (See SPD)
2000A Insulated Case Circuit Breaker
STD Is an Option - Allows breaker to delay opening
Instantaneous Override built-in: may be as low as 12X the breaker rating
Will often result in lack of coordination
Here we have the curve for an insulated case circuit breaker with short-time delay. This allows a little more delay in the circuit breaker but the instantaneous override, which can be as low as 12X, can still prevent selective coordination depending upon the magnitude of the fault as shown in the next slide.

16. Selective Coordination - Insulated Case Circuit Breakers (See SPD)
2000A Insulated Case with STD and Instantaneous Override and 100A Molded Case Thermal Magnetic Circuit breaker - NO Coordination in Short-Circuit Region (above 21,000A)
As shown in the graph, at a fault above 21,000A both devices open and selective coordination is not achieved. Thus the short-time delay feature can yield coordination at relatively low fault currents, in this case below 21,000A. But for faults above 21,000 amperes, in this case, coordination can not be achieved.
The only way to achieve selective coordination with circuit breakers is to use a low voltage air frame circuit breaker with short-time delay as shown in the next slide.
2000A
100A

17. Selective Coordination - LV Air Power Circuit Breakers (See SPD)
Short Time Delay - Allows the fault current to flow for up to 30 cycles.
Used to coordinate with downstream
Subjects equipment to high mechanical and thermal stresses, often violating
Arc Flash/ Blast Risks Much Higher
High Cost, Larger Equipment
As shown in the curve, the instantaneous override can be disabled in this style of circuit breaker so that the short-time delay can ride out the fault and give the downstream device time to clear before this breaker unlatches.
This style breaker can do this because the heavy-duty construction of the device. This feature can provide coordination. But what happens when a fault occurs on the cables or busway protected by this device? This feature allows equipment to be subjected to high mechanical and thermal stress.
In addition, the heavy-duty construction causes this to be about several times larger and more expensive than using standard switchboard construction with molded case circuit breakers.
So how do we achieve selective coordination economically and without drastically increasing the size of switchgear?

18. Selective Coordination: Fuses
Now let’s investigate fuse coordination.

19. Time Current Curves Selective Coordination:Fuses (See SPD)
(There is a section in the SPD that covers this – review that section.)
This curve shows the time current curves of a 400 A fuse feeding a 100 amp fuse.
This shows how fuses can coordinate in the overload region. As we will see in the next slide, fuses can coordinate in the short circuit region too. This is due to the ability of a current-limiting fuse to be extremely fast acting in the short-circuit region, compared to the standard molded case circuit breaker.
Now lets look for coordination in the short circuit region using the ampere squared seconds (I squared t) values from the fuses as shown in the next slide.

20. Selective Coordination:Fuses (See SPD)
Not Melt
Selectivity Ratio
Guide Based on
Thermal Principle
Based on I2t
This slide shows that in order to achieve selective coordination in the short circuit region, the I squared t clearing value of the downstream fuse must be lower than the I squared t melting of the upstream device.
In order to achieve selective coordination, we do not have to draw the fuse curves or check the I squared t values, we simply just need to utilize the ratio table, as shown on the next slide.
Thus, if we use current-limiting fuses and follow the ratio table, we have assured a properly designed system assuring that the system is fully rated and can handle a minimum of 300K fault as well as isolate the fault and assure selective coordination.
So what is the ratio that we need to keep?
The ratio table shows us as shown on the next slide.
Clear
I2t melting > I2t Clearing
1200 A A

21. Selective Coordination (See SPD )
Load Side Fuse
Line Side Fuse
At Bussmann, we have made that easy because we have reviewed all of our testing data and placed fuse ratios into a table. This is our selective coordination ratio table. All you have to do is verify the ratio between the upstream and downstream device in order to obtain coordination. With the yellow labeled Low-Peak® fuses, the ratio is 2:1, all the way through the line. There is no need to plot all of the curves. Just verify the ratio. If you have a 100 amp Low-Peak® fuse, all you need to do is make sure that the upstream Low-Peak® fuse is at least 200 amps, and you are coordinated. ( The selectivity ratios may not be valid when the two ampere ratings being analyzed are in the same ampere rating case size such as a 30 A and 15 A fuses.)
LOW-PEAK® : LOW-PEAK®
2:1 Line:Load Ratio
Selectivity Ratio Table Assures Coordination!
No Plotting required!

22. Selective Coordination - Fuses
480V, 3 phase
Example:
Main: KRP-C 1200 SP
Feeder: LPS-RK 200 SP
Branch: LPS-RK-30 SP
1200 A
GFP
MSB
Let’s check for selective coordination for the systems shown.
Using the selectivity table for fuses, determine if this is a selectively coordinated system.
200 A
MCC
30 A

23. Selective Coordination - Fuses
Use Selectivity Table
Main KRP-C 1200 SP
Feeder LPS-RK 200 SP
Branch LPS-RK 30 SP
What happens: Branch Circuit Isca = 5000 A or 50,000A or 300,000A ?

24. Selective Coordination- Fuses
Lineside KRP-C 1200SP to Loadside LPS-RK 200SP
1200/200 = 6: Table only need 2:1
Selective Coordination
Lineside LPS-RK 200SP to Loadside LPS-RK 30SP
200/30= 6.67: Table only need 2:1
Using the selectivity table, these are the results.

25. Selective Coordination
If we look at the results from a system view, only the fuse closest to the short-circuit will open. The other upstream fuses remain in operation. This is a selectively coordinated system.
NOT
AFFECTED
OPENS
Fault

26. Are These Selectively Coordinated?
Fault
10000 Amps
Low Peak
KRP-C 1000 SP
LPJ 200 SP
LPJ 20 SP
Are These Selectively
Coordinated?
For the fuse system, does it assure selective coordination?
Use the selectivity ratio table. For Low Peak Fuses the ratio is 2:1.

27. Are These Selectively Coordinated?
Fault
10000 Amps
Low Peak
KRP-C 1000 SP
LPJ 200 SP
LPJ 20 SP
Are These Selectively
Coordinated?
OPENS
NOT
AFFECTED
Fault
10000 Amps
Low Peak
KRP-C 1000 SP
LPJ 200 SP
LPJ 20 SP
Selectively Coordinated
The answer is that there is a ampere rating ratio maintained of at least 2:1 or greater so this system is selectively coordinated. It is that simple.

28. Are These Selectively Coordinated? 1000 A. CB IT @ 10 X 200 A. CB
Fault
10000 Amps
1000 A. CB
10 X
200 A. CB
20 A. CB
10X
Now let’s use the simple method to assess coordination of circuit breakers with instantaneous trips.
Take some time to do your simple calculations. Circuit breaker amp rating X its instantaneous trip setting.

29. Are These Selectively Not Coordinated Coordinated? 1000 A. CB
Fault
10000 Amps
1000 A. CB
10 X
200 A. CB
20 A. CB
10X
Not Coordinated
OPENS
NOT
AFFECTED
Fault
10000 Amps
1000 A. CB
10 X
200 A. CB
20 A. CB
10X
So for a fault of 10,000 amps on the 20 amp circuit, the 200 amp and the 1000 amp circuit breaker will also open.

30. The Issue: Lack of coordination takes out other loads 1000 A. CB
10 X
200 A. CB
10 X
Let’s review this in a system. A fault of 10,000 amp
(mouse click)
Opens the 20 amp.
But the 200 amp also unlatches and opens too.
In addition the 1000 A circuit breaker also unlatches and opens. The result is that other loads are unnecessarily blacked out.
Unnecessary
Blackout
20 A. CB
10X
OPENS
Fault
10000 Amps

31. Ejemplo de coordinación mostrando protección de los cables