During the past revision cycle for the 2005 NEC, a new definition was created in Article 100 for selective coordination. The need for this definition can be taken from its creation and shows the expansion of selective coordination requirements throughout theNEC. One of the basic requirements for the creation of a definition in Article 100 is the use of the term in two or more sections of the Code. The 2005 NEC now has multiple sections containing requirements for selective coordination of overcurrent protective devices and most are contained in sections pertaining to protection requirements for critical circuits involving life safety. Compliance with these selective coordination requirements is achieved through the selection of overcurrent protective devices with appropriate operating characteristics. The choice of overcurrent protective devices with the proper operating characteristics is not difficult. However, if proper review is not given to the overcurrent protective device used, it is easy to have systems that are not compliant and, thus, systems that do not provide the proper safety of human life demanded in critical circuits.
One of the areas containing requirements for selective coordination of overcurrent protective devices is elevator circuits in 620.62 of the 2005 NEC. This article will cover the requirements for OCPDs used in elevator circuits, provide background of the critical nature of these requirements and discuss other considerations for the proper selection of OCPDs.
Selective Coordination and the NEC
The 2005 NEC contains definitions and requirements for selective coordination of overcurrent protective devices in the following sections:
- Article 100 Definition
- 517.17 Health Care
- 517.26 Health Care Essential Electrical System
- 620.62 Multiple Elevators on single feed
- 700.27 Emergency Systems
- 701.18 Legally Required Standby Systems
Selective Coordination of Elevator Circuits
As can be seen from the list above most applications requiring selectively coordinated overcurrent protective devices are critical electrical circuits or systems. Selective coordination of overcurrent protective devices fits well with requirements that focus on these critical circuits. Selective coordination provides overcurrent protection that keeps these critical electrical systems operating as intended when needed and maximizes the portion of the system that is operational during an emergency. Putting this into the context of elevator circuits it is important to provide power for elevators as much as possible due to their relation to means of egress and use by emergency personnel. This is particularly important for multiple elevators supplied by a single feeder as an overcurrent on a single elevator circuit could easily cascade up to the feeder and take down the whole bank of elevators. Therefore NEC 620.62 contains a requirement for selective coordination of overcurrent protective devices where more than one elevator is supplied by a single feeder that reads as follows:
620.62 Selective Coordination.Where more than one driving machine disconnecting means is supplied by a single feeder, the overcurrent protective devices in each disconnecting means shall be selectively coordinated with any other supply side overcurrent protective devices.
What Is Selective Coordination?
Selective coordination is defined in the 2005 NEC Article 100 as: "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.”
In other words, isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open (figures 1 and 2).
Figures 1 & 2. System lacking, and one system providing, selective coordination
Figure 1 shows a system that does not contain overcurrent protective devices that are selectively coordinated. The flow of fault current is shown by the blue arrow. The reason is that the overcurrent protective device nearest to the fault is too slow, so the upstream overcurrent protective device(s) also open. The result is an unnecessary power outage to all the loads with the hashed red and white boxes. This figure 1 system would not be compliant where selective coordination is required.
Figure 2 illustrates a system with overcurrent protective devices that are selectively coordinated for all the possible overcurrents that can occur in the system. In this example, the fault occurs on the load side of a branch circuit. The blue arrows represent the fault-current flow from the source to the fault. The branch-circuit overcurrent protective device clears this fault—depicted by the solid red box. However, in this example, no other upstream overcurrent protective devices open. All the feeders and main overcurrent protective devices represented by the boxes with solid white remain in operation. All loads, other than the faulted branch circuit, remain energized and there are no unnecessary load power outages. This is the result of the overcurrent protective devices being properly chosen so that they are selectively coordinated for the entire range of overcurrents that may occur on this system.
For a system to be selectively coordinated, each overcurrent protective device in each branch circuit, feeder and main must be analyzed for selective coordination with the other overcurrent protective devices in the system. A fault that unnecessarily opens one or more upstream overcurrent protective devices is not selectively coordinated. So if the branch-circuit overcurrent protective device and the next level feeder overcurrent protective device can open on a fault, the system is not selectively coordinated.
Also, if the fault occurs on a feeder circuit, to be selectively coordinated, only the overcurrent protective device protecting that feeder circuit shall open; no other upstream overcurrent protective devices shall open.
Comparing this to the requirements in 620.62, note that there are three levels of feeders in the systems depicted in figures 1 and 2. It is important to note that 620.62 requires selective coordination of the branch overcurrent protective device with all supply side overcurrent protective devices, from the branch levels through all three feeders up to and including the service overcurrent protective device. If emergency power is provided through a transfer switch the overcurrent protective devices in the emergency system including any supplied with a generator need to be selectively coordinated as well.
Important Considerations for Section 620.62
A common error in elevator circuit applications is when multiple circuits are run to the elevator equipment room in an effort to bypass the selective coordination requirements contained in 620.62. It is important to note that even in this situation, there will be a single feeder supplying multiple elevator loads and consideration of the service or main overcurrent protection in relation to the separate feeders needs to be reviewed
Figure 3. Example of single feed supplying an elevator room
In figure 4, observe that each of the feeders supplies its own elevator. This does not bypass the requirements of 620.62, which requires selective coordination when there is more than one driving machine being fed from a single feeder. According to Article 100, a feeder is considered to be all circuit conductors between the service equipment and the branch-circuit overcurrent protective device. This would mean that the load-side conductors from M1 would be a single feeder to multiple driving machines. This would require F1, F2, F3, and F4 to be selectively coordinated with M1 in order to comply with 620.62. These situations would require selective coordination through to the main overcurrent protective device in the building. Failure to use overcurrent protective devices that are not selectively coordinated may compromise safety. A fault on a branch may cause the main overcurrent protective device to operate; so all the elevators may be rendered inoperative, negatively affecting building egress or use by emergency personnel.
Figure 4. Example of multiple feeders supplying an elevator room
Selectively Coordinating OCPDs
Selectively coordinating overcurrent protective devices is achieved by reviewing the operating characteristics of the upstream overcurrent protective devices in relation to the downstream overcurrent protective devices. Important considerations include:
- Speed of operation under fault or short-circuit conditions for phase-to-phase, three-phase, and ground-fault conditions
- Assuring that the clearing time of the downstream overcurrent protective device is less than the melting or unlatching time of the upstream overcurrent protective device(s).
- Assuring that no overlap exists in the performance curves or time-current curves for overloads and low-level fault conditions.
- Assuring documentation is provided by the overcurrent protective device manufacturer relating to selective coordination under high-fault or short-circuit conditions as these operating times are often beyond what is covered by a traditional time-current curve. (Important note, this one condition is not in and of itself sufficient to verify selective coordination. The other conditions must be met as well.)
For fuses, selective coordination can be achieved by following the selective coordination ratios provided by the fuse manufacturers. These ratios provide the level of separation needed between upstream and downstream fuses to assure an overcurrent is cleared by the downstream fuse before the upstream fuse starts to melt. (See figure 5 for an example of applying fuse coordination ratios).
Figure 5. The use of fuse coordination ratios in determining selective coordination
To ensure circuit breakers selectively coordinate with each other consideration needs to be given to the type of circuit breakers used (fixed trip molded case, adjustable trip molded case, insulated case, or power circuit breakers) and the settings or options needed to provide selectivity. One method of providing selective coordination with circuit breakers is by the use of zone interlocking. In this type of setup the circuit breakers "talk” to each other in an effort to localize overcurrents to the circuit breaker nearest the fault. Another method of achieving selective coordination with circuit breakers is by using circuit breakers with short time delay upstream and applying appropriate settings to achieve selectivity between all the circuit breakers in the system. Insulated case and power circuit breakers provide this option. Yet another option in systems with low available fault current levels is to provide circuit breakers with adjustable trip settings upstream, which can be set above the available short-circuit current levels and therefore removing the instantaneous trip operation from the upstream circuit breakers. It is important to note that depending upon the system design and instantaneous trip settings of the circuit breakers in the system it does not always require the use of the more expensive or complex type of circuit breakers. The simple rule to follow when looking to achieve selective coordination with circuit breaker-to-circuit breaker systems is to keep the instantaneous trip portion of the upstream circuit breakers beyond the available fault current levels. (See figures 6, 7, and 8 for examples of achieving selective coordination with circuit breakers).
Figure 6. Example of achieving selective coordination with circuit breakers
Figure 7. Example of achieving selective coordination with circuit breakers
Figure 8. Example of achieving selective coordination with circuit breakers
Other Considerations for Elevator Circuits
Multiple Standards to comply with
In addition to the selective coordination requirements in 620.62 there are additional requirements surrounding elevator circuits in Article 620. The requirements for the local elevator motor disconnect are located in Section 620-51, which can be summarized into four main requirements.
1. The disconnecting means must be a listed device.
2. The disconnecting means must have the capabilities of being locked in the "”open”" position.
3. The disconnecting means must be a fusible disconnect or a circuit breaker.
4. The disconnecting means is allowed to have shunt trip capabilities.
In addition to this list, there is a fine print note, which refers to the elevator safety code ASME/ANSI A17.1. If we look at 102.3 (c)(3), we can explore the conditions mandated for the elevator motor disconnecting means. The rules follow the same guidelines as the NEC, however, in ASME/ANSI A17.1: "The shunt trip capability of the disconnect is required, when the elevator shaft and/or machine room is sprinkled.”
The shunt trip in the disconnecting means is typically initiated by the closure of a set of dry contacts in the fire alarm system. This contact can be wired to close via heat detector activation or by a flow switch that operates due to the flow of water in the pipes in the sprinkler system.
The importance of this consideration lies in the fact that the disconnecting means for the overcurrent protective devices requiring selective coordination need to provide shunt trip capabilities and comply with the additional codes and standards. There are various product offerings and solutions available to comply with this multitude of requirements (figure 9).
Figure 9. Example of shunt trip disconnecting means for elevator circuit application
Further discussion on this topic can be found on the Bussmann website athttp://www.bussmann.com/products/powermod/.
During Plan Review
þAssure proper documentation is provided showing selective coordination of overcurrent protective devices used in elevator circuits where multiple elevators are supplied through a single feeder. Ensure this selective coordination is achieved for all levels of overcurrents even at fault levels that cause opening times below 0.01 seconds. Note any settings that are required or ratios that are used in achieving this compliance.
During Final Inspection
þAssure that the overcurrent protective devices called out and reviewed during plan review where installed. Assure that any and all appropriate settings are correct and documentation is provided to ensure these settings will be known in future.
þUse fuse coordination ratios to verify selective coordination in the field where fuses are used. Use the simple method of circuit breaker amp rating x instantaneous trip setting for molded case circuit breakers in the field. Ensure that the feeder instantaneous trip setting is above the available short-circuit current level at the elevator branch disconnect. For example, if a 400-amp circuit breaker is feeding a 60-amp circuit breaker, make sure the available short-circuit current does not exceed 4000 amps or 400×10.
Read more by Todd Lottmann