Coordinating protective devices in mission critical facilities
A coordination study ensures that the most reliable electrical system has been installed. Applicable codes and standards help engineers get it right.
- Illustrate the basics of protective-device coordination studies.
- Outline elevator protection coordination as required per the NEC.
- Apply NEC Article 517 and ground-fault coordination studies required for health care facilities.
A sudden power failure will have a dramatic effect on business, especially in a critical environment. Isolating a fault condition to the smallest area possible is essential in providing the most reliable electrical system with maximum uptime for your facility. Expensive electronic distribution protection equipment is not worth the extra cost unless a proper protective-device coordination study is provided by an experienced engineer.
NFPA 70-2014: National Electrical Code (NEC) defines selective coordination as: "Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents."
In other words, a properly coordinated system will limit disconnection to the nearest upstream protective device.
Protective-device coordination study basics
The main types of overcurrent protection used in mission critical environments are circuit breakers, fuses, and relays. This article focuses on circuit breakers and fuses. Relays are not addressed due to space constraints.
Depending on the circuit breaker type, there may be several parameters that can be selected for each protective device. A thermal magnetic breaker may have no adjustment at all, or only minimal adjustment to the instantaneous region, whereas a fully adjustable electronic trip breaker may have many.
Adjustment of these parameters allows for what is referred to as "curve shaping." Curve shaping enables better coordination between upstream and downstream overcurrent-protection devices. Typical parameters include:
- Overload region (long time trip unit): This is the long time trip setting of an overcurrent protective device. This parameter, also known as continuous amperes, is a percentage of the breaker’s nominal rating.
- Long time delay: This setting allows for inrush from motors to pass without tripping the breaker. This setting affects the position of the I2t slope just below the continuous-current setting.
- Short-time pickup: This setting is typically provided with an adjustment of 5 to 10 times the inrush current. This setting allows downstream overcurrent-protection devices to clear faults without tripping upstream devices. This setting can also be adjusted to allow for transformer inrush current.
- Short time delay, instantaneous override: This setting postpones the short-time pickup. Setting this parameter can be done on a fixed setting or an I2t ramp setting. This allows for better coordination between upstream and downstream devices. An instantaneous override can be set at high-current value to override this function and to protect electrical equipment. The I2t function of the short time delay can provide better coordination when coordinating a breaker with a fuse.
- Instantaneous: This setting will trip the overcurrent-protective device with no intentional delay.
- Ground fault setting (ground fault trip unit): This is the percentage of the rating of the breaker for the ground fault setting. According to the NEC, ground fault cannot exceed 1,200 A, regardless of the size of the breaker.
- Ground fault delay: This setting allows for a time delay before ground fault pickup. This allows for better selective coordination between multiple levels of ground fault protection. In addition, the time delay cannot exceed 1 sec (60 cycles) for ground-fault currents of 3,000 A or more.
- Reduced arc flash mode: This setting allows the breaker to be manually taken out of coordination for short periods of time during maintenance to reduce the arc flash hazards on the system.
When performing electrical engineering studies for mission critical environments, the required documentation includes:
- Description, rating, make, and catalog numbers of protective devices
- Full-load current at the protective device (3-phase and line-to-ground)
- Transformer kVA, impedance, and inrush current data and connection type (delta-wye, etc.)
- Available fault current at the protective device
- Cable and conductor sizes
- Protective-device design requirements from the serving utility
- Voltage at each bus.
After the aforementioned critical information is typed into the software database, the function of protective devices can be graphically presented. The resulting graphic representation is called a time-current curve (TCC). When more than one electrical device is overlaid on a single graph, the relationship of the characteristics among the devices is presented. Any potential issue, such as overlapping of curves or long time intervals between devices, are illustrated. Fault-current conditions can be illustrated by indicating on the current scale the maximum and minimum value of short-circuit currents (3-phase and line-to-ground) that can occur at various points in the circuit (see Figure 1). It is common today to perform complicated electrical protection coordination studies with computer software. These software platforms typically contain libraries that include most of the common overcurrent protective devices and their available adjustments.
Elevator protection coordination per the NEC
Selective coordination is required when more than one elevator is supplied by a common feeder, according to NEC Article 620-62. Figure 2 shows an example of a coordination study illustrating the feeder-breaker overcurrent protection, the elevator fuse overcurrent protection, and the elevator motor startup curves.
The study must ensure that the two fuses will trip in a fault condition in any one of the separate elevator feeders and will not trip the 200-A main breaker. A fault in one of the elevator feeders that takes out the main breaker would essentially take out both elevators.
Ground fault studies for health care facilities
NEC Article 517.17 requires that if ground fault protection is provided for service disconnecting means, an additional step of ground-fault protection shall be provided in the next level of feeder disconnecting means downstream toward the load (see Figure 3). There is clear separation between the ground-fault curves.
NEC Article 230.95 indicates that all 480 V, 3-phase services rated 1,000 A and higher must be installed with a ground fault relay. The setting of the ground fault relay cannot exceed 1,200 A regardless of the size of the overcurrent-protection device. In addition, the time delay cannot exceed 1 sec (60 cycles) for ground fault currents of 3,000 A or higher. There shall be a minimum of 0.1 sec (6 cycles) of ground-fault delay between ground fault devices in health care facilities.
Ground fault settings for main breakers serving downstream motors that are set too low or too fast may trip a main overcurrent-protection device before tripping the local thermal-magnetic overcurrent-protection device during motor-starting ground faults. On the other hand, ground fault settings that are too high can cause undue damage before a ground fault is interrupted. It is important to provide the ground fault setting that will not permit nuisance tripping, but will protect the electrical equipment from excessive damage during an event.
On many occasions, projects are completed without protective-device studies. In such cases, the breaker manufacturer will ship the breakers with all settings set to the most sensitive. This will ensure the most protection, but will increase false trips and is typically not good for the reliability and uptime of the systems. As soon as the owner complains of a false trip, the facility personnel will probably set all of the dials to least sensitive. This will reduce false trips, but may not adequately protect the electrical system and could also reduce selective coordination of the system. Sometimes perfect coordination between a set of devices cannot be obtained. Certain settings may be required on a breaker that could affect the settings of many breakers. In some cases, there may be many levels of breakers that may cause overlap of the breaker curves within the tolerance of the curves. It is at these times that experience will allow engineers to make judgment calls regarding certain compromises in coordination between devices. The engineering behind providing protective coordination studies is not a perfect science. It is important to ensure that protection is not compromised, even if perfect coordination is not achieved.
A coordination study is typically required to ensure that the most reliable electrical system has been installed. In addition, there are instances where the NEC requires that a study be performed. In either case, the cost of a coordination study is pretty cheap insurance for most installations that would be adversely affected by an extensive power outage.
To understand code-related issues involved in selective coordination studies, it is important to quote the new definition of selective coordination and the new codes in NEC Articles 100, 517, 700, and 701.
NEC Article 100, 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 and settings.
NEC Article 700.27, Coordination: Emergency system overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
NEC Article 701.18, Coordination: Legally required standby system overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
NEC Article 517.26, Application of Other Articles: The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.
For clarity, it is important to include the NEC definition (Article 100) of overcurrent and the fine print notes (FPNs) defining emergency systems and legally required standby loads in Articles 700 and 701.
Overcurrent: Any current in excess of the rated current of the equipment or ampacity of the conductor. It may result from overload, short circuit, or ground fault.
It is significant to note that a "short circuit" is listed as one of the items that can cause an overcurrent condition. The typical molded-case circuit breaker combination with the upstream breaker that is somewhat larger than the downstream breaker does not have a problem coordinating in the overload area of the TCC, but a high level of current in the short-circuit area of the TCC can present significant problems to selective coordination.
NEC Article 700.1, FPN No. 3: Emergency systems are generally installed in places of assembly where artificial illumination is required for safe exit and panic control. Emergency systems may also provide power for such functions as ventilation, fire detection and alarm systems, elevators, fire pumps, public safety communication, and industrial processes.
NEC Article 701.2, FPN: Legally required standby systems are typically installed to serve loads—such as heating and refrigeration systems, communication systems, sewage disposal, lighting systems, and industrial processes—that, when stopped during any interruption of normal electrical supply, could create a hazard or hamper rescue or firefighter operations.
Elevators are noted in Article 700.1 as an emergency system load. Some jurisdictions also consider elevators to be a legally required standby load. In either case, the NEC has required elevators in certain situations to be selectively coordinated for some time. This selective coordination has been required by NEC Article 620.62.
NEC Article 620.62, Selective Coordination: Where more than one driving-machine disconnecting means is supplied by a single feeder, the overcurrent protective device in each disconnecting means shall be selectively coordinated with any other supply-side overcurrent protective devices.
NEC Article 240.6 (C) Restricted-Access Adjustable Trip Circuit Breakers: A circuit breaker that has restricted access to the adjusting means shall be permitted to have an ampere rating that is equal to the adjusted current setting (long-time pickup setting). Restricted access shall be defined as located behind one of the following:
- Removable and sealable covers over the adjusting means
- Bolted equipment-enclosure doors
- Locked doors accessible only to qualified personnel.
Arc flash mitigation via noncoordination
The NEC also includes information to help minimize the risk of an arc flash incident.
NEC-2014 Article 240.87, Arc Energy Reduction: Where the highest continuous-current trip setting for which the actual overcurrent device installed in a circuit breaker is rated, or can be adjusted, is 1,200 A or higher, Article 240.87(A) and (B) shall apply:
- (A) Documentation shall be available to those authorized to design, install, operate, or inspect the installation as to the location of the circuit breaker.
- (B) As a method to reduce clearing time, one of the following or approved equivalent means shall be provided:
- Zone-selective interlocking
- Differential relaying
- Energy-reducing maintenance switching with local status indicator
- Energy-reducing active arc flash mitigation system
- An approved equivalent means.
A method for complying with this code section is to provide breakers that have a reduced arc flash mode (RAM) setting. Most major switchgear manufacturers are providing this in their breakers to enable compliance with this new code section. While each manufacturer may have a different twist on the solution, the basic principal is the same: provide a temporary setting where the breaker will trip faster in a fault condition. While this RAM system is effective in reducing the arc flash hazard, it may take all or some of the breakers downstream from the RAM breaker out of coordination and increase the risk of losing critical loads.
It is always recommended that all systems be powered down prior to personnel working on electrical equipment. However, in mission critical facilities qualified professionals may need to access and work near energized equipment. Testing, troubleshooting, and diagnostics may require that power remain on to complete the task when removing power is infeasible.
Keith Lane is president/CEO of Lane Coburn & Associates. He is a member of the Consulting-Specifying Engineer editorial advisory board and is a 2008 40 Under 40 award winner. Scott Coburn is principal of Lane Coburn & Associates. He oversees design constructability to ensure the highest level of construction documents. He is the co-founder of the Neher-McGrath Institute and Neher-McGrath.com.