Today, more than ever, one of the most important parts of any facility is the electrical distribution system. Nothing will stop all activity, paralyze production, inconvenience and disconcert people, and possibly cause a panic, more than a major power failure . Selective coordination is critical for the reliability of the electrical distribution system an must be analyzed.
Selective coordination of over current protection devices is required by the NEC for a few buildings systems for a limited number of circuits that supply power to vital loads. For circuits supplying power to all other loads, selective coordination is a very desirable design consideration, but not mandatory. It is important to deal with selective coordination in the design phase. After switchboards, distribution panels, motor control centers, lighting panels boards, etc. are installed, there typically is little that can be done to retroactively fix a system that is not selectively coordinated.
While it's very important, it is not enough to select protective devices based solely on their ability to carry the system load current and interrupt the maximum fault current at their respective points of application. It is important to not that the type of overcurrent protective devices and ratings (or setting) selected determine if a system is selectively coordinated. A properly engineered and installed system will allow only the nearest upstream overcurrent protection device to open for both and all types a of short-circuits, leaving the remainder of the system undisturbed and preserving installation is critical in today's modern electrical systems.
POWER BLACKOUTS CANNOT BE TOLERATED. Especially at data centers. (Bussmann Field Manual on selective coordination pg. 102)
Circuit Breaker Operation Basics
Circuit breakers are mechanical over current protective devices. all circuit breakers share three common operating functions:
1.) Current sensing means:
A. ) Thermal
2.) Unlatching mechanism: mechanical
3.) Current/voltage interruption means (both)
A.) Contact parting: mechanical
B.) Arc chute
The circuit breaker's physics of operation is significantly different from that of a fuse. First, the circuit breaker senses the overcurrent. If the overcurrent persists for too long, the sensing means causes of signals the unlatching of the contact mechanism. The in Latching function permits a mechanism to start through the air and arcing between the contacts commences, The further the contacts separate the longer the arc, which aids in interrupting the overcurrent. However in most cases, especially for fault current, the contacts alone are not sufficient to interrupt. The arcing is thrown to the arc chute which aids in stretching and cooling the arc so that interruption can be make. Figure 8 Shows a simplified model with the three operating functions shown for a thermal magnetic circuit breaker, which is the most commonly used circuit breaker. Also, it should be noted that there are various contact mechanism designs that can significantly affect the interruption process.
Circuit Breakers instantaneous trip operation due to a short-circuit current. The magnetic element senses higher level over current conditions. This element is often referred to as the instantaneous trip, which means the circuit breaker is opening without intentional delay. In some circuit breakers, the instantaneous trip sensing is performed by electronic means. In either cases the unlatching and interruption process is the same as illustrated in the picture. The last picture in the sequence illustrates the high rate of change of current due to a short-circuit causing the trip bar to be pulled toward the magnetic element. If the fault current is high enough the strong force causes the trip bar to exert enough force to unlatch the circuit breaker. This is a rapid event and is referred to as instantaneous tri
Circuit Breaker Curves
When using molded case circuit beakers of this type, there are there basic curve considerations that must be understood. These are:
1.) Overload region
2.) Instantaneous region with unlatching
3.) Interrupting Rating
1.) Overload Region:
Overloads typically can be tolerated by the circuit components for relatively longer times than faults and therefore, the opening g times are in the range of seconds and minutes. As can be seen, the overload region has a tolerance band, which means the breaker should open within that area for a particular overload current.
The circuit breaker will open as quickly as possible. the instantaneous trip (T) setting indicates the multiple of the full load rating at which the circuit breaker starts to operate in its instantaneous region. Circuit Breakers with instantaneous trips either have (1`) fixed instantaneous trip setting or (2) Adjustable instantaneous trip setting. The instantaneous region is represented in figure 14 and for this Example is shown to be adjustable from 5x to 10x the breaker amp rating. When the breaker senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed (unlatches). Unlatching permits the contact parting process to start.
3.) Interrupting Rating (AIC):
The interrupting rating is represented on the drawing by a vertical line at the right of the curve. The interrupting rating for circuit breakers varies based on the voltage level; se the interrupting rating table in Figure 14 which list the interrupting ratings for this specific circuit breaker. For coordination purposes, the vertical line is often drawn at the fault current level in lieu of the interrupting rating(if the interrupting rating is grater than the available short -circuit current). However, if the fault current is above the interrupting rating a misapplication and violation of NEC 110.9 is evident. In Figure 14, the circuit breaker interrupting rating at 480 volts is 30,000 amps.
Interrupting Rating Explanation
Achieving Selective Coordination with Low Voltage Circuit Breakers (PDF)
To achieve selective coordination with low voltage circuit breakers, the general rule is that the time curves will have no overlapping regions (including the unlatching time). The ability of circuit breakers to achieve coordination depends with other circuit breakers depends on its time current curve, its settings, amp ratings, and options of the circuit breaker, and the available short-circuit currents. The type of circuit break selected could be one of three types: circuit breakers with instantaneous trips; circuit breakers with short time-delay but incorporating instantaneous overrides.
Example: Two Instantaneous Trip Circuit Breakers
Figure to the left illustrates a 90 amp circuit breaker and an upstream 400 amp circuit breaker having in instantaneous trip setting of 5x (5 tunes 400A circuit breaker having instantaneous trip current for the 400 amp circuit breaker could be as low as 200A times 0.75 = 1500A (+/-25% band ). If a fault above 1500 amps occurs on the load side of the 90 amp breaker, both breaker could open. The 90 amp breaker may unlatch before the 400 am breaker. However before the 90 amp breaker can part is contacts and clear the fault current the 400 amp breaker could have unlatched and started the irreversible contact parting process.
Assume a 4000A Short-circuit exists on the load side of the 90A circuit breaker. The sequence of events would be as follows:
The 90A breaker will unlatch (point A) and free the breaker mechanism to start the contact parting process.
The 400A breaker will unlatch (Point B) and it too would begin the contact parting process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible. it is similar to pulling a trigger on a gun.
At Point C, the 90A breaker will have completely interrupted the fault current
At point D the 400A breaker also will have opened, which unnecessarily disrupt power to all other loads.
Mandatory Selective Coordination Requirements
Fir building electrical systems, the topic of selective coordination of over current protective devices can be segmented into two areas:
1.)Where it is desirable design consideration and
2.) Where is a mandatory NEC requirement
In most cases, selective coordination is a desirable design consideration and not a NEC requirement. However , it is in the best interest of the building owner or tenants to have selectively coordinated overcurrent protection to avoid unnecessary blackouts. Selective coordination should be evaluated in the contest of the reliability desired for the power system to deliver power to the loads, In today's modern commercial , institutional and manufacturing building systems, what owner would not want a selectively coordinated system?
Selective coordination is mandatory per the NEC for a few applications. In some buildings systems, there are vital loads that are
important for the life safety, national security or business reasons. Continuity of power to these loads and the reliability of the power supply to these loads is a high priority. The sections of the NEC defining selective coordination and those requiring the overcurrent protection devices in the circuit paths supplying these vital loads to be selectively coordinated are as follows:
Article 620 Elevators
Where more than on e driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shell be selectively coordinated with any other supply side overcurrent protective devices.
Selective coordination shall be selected by a licensed professional engineer or other qualified person engaged primarily in the design, installation, maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.
Article 645 Information Technology Equipment
Critical operations data systems overcurrent protective devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Article 695 Fire Pumps
Multi-building campus style complexes. If the sources in 695.3(A) are not practicable and the installation is part of a multi-building campus style complex , feeder sources shall be permitted if approved by the authority having jurisdiction an installed in accordance within (C) (1) and (C)(3) or (C) (2) and (C) (3). (C)(3) Selective coordination. The over current protective device(s) in each disconnecting means shall be selectively coordinated with any other suppl-side overcurrent protective device(s)
Selective Coordination . Conclusion
Selective coordination is an easy concept to understand, the complex calculations and understanding of how motor loads, transformers X/R ratios, and Harmonics affect the available short circuit current and power distribution system is the move complex concepts. But before selective coordination can be performed an electrical engineer must first understand these more complex ideas of a power distribution system.
(Bussmann Field Manual on selective coordination pg. 102-112)
Transmission Protective Relaying
Relay are utilized in all aspects of modern activity; the home ,communication, transportation, commerce ,and industry . Wherever electricity is utilized there is a high probability that relays are involved. Thus protective relays and their associated systems are compact units connected through a power system for the purpose of sensing problems. They are utilized in all parts of electric power systems for the detection of undesirable conditions.
The primary objective of all power systems is to maintain a very high level of continuity of service, and when undesirable conditions occur isolate the system from the abnormal voltages and currents.