The importance of circuit protection in electrical distribution system design
- Understand the different types of overcurrent protective device strategies to apply in building electrical systems.
- Recognize the difference between ground-fault protection for equipment (GFPE) versus ground-fault circuit interrupters (GFCI; protection for personnel).
- Understand how to protect against the various types of faults.
The electrical engineer carries a large responsibility to the public when designing power distribution systems for buildings. The design must protect against faults and overloads while also providing adequate personnel protection and minimizing disruption. Unfortunately, there is not a clear, concise "recipe" to follow for such designs. Rather, it requires constant study of the ever-changing codes and standards that can be interpreted in various ways, and then properly applying them into an actionable design. Even codes themselves reinforce that, while providing a practical guide for safeguarding people and property from electrical hazards, they are "not intended as a design specification or an instruction manual for untrained persons" (NFPA 70: National Electrical Code [NEC], Article 90.1). Therefore, it is exceedingly important for the electrical engineer to understand and properly apply circuit-protection strategies in their designs to ensure safe operating systems. When it comes to circuit protection, the NEC is the primary code book with which electrical engineers must familiarize themselves. The NEC contains fundamental safety principles that encompass protection against electric shock, thermal effects, overcurrent, fault currents, and overvoltage. It’s critical to also understand circuit-protection strategies as they relate to the NEC.
Nearly every article in the NEC includes some form of circuit protection, stressing the importance of the issue. The basic goals of circuit protection are to 1) localize and isolate the condition or fault and 2) prevent and minimize any unnecessary power loss. There are several types of abnormal conditions that may occur throughout a building’s life, in which an electrical system must be designed to correct or overcome. These include overloads, short circuits, under/overvoltages, transient surges, and other power issues, such as single-phasing of 3-phase systems and reverse power-phase rotation.
An overload is caused by an excessive demand from utilization equipment that is higher than its rated capacity. System overloads can be tolerated for a short period of time before corrective action must be taken. Short-circuit faults, on the other hand, are caused by failed electrical components. Since the damage can be immediate, the faulted part of the system must be isolated as quickly as possible. Several types of faults exist including arcing line-to-line faults, line-to-ground faults, and 3-phase bolted faults. Many faults start out as intermittent, arcing faults with variable impedance and relatively low magnitude currents, characterized by the uncontrolled release of energy. A 3-phase bolted fault, on the other hand, is one that creates immense amounts of current on the system and will sustain this current until the circuit is opened or isolated by some means. Although a designer must account for the worst-case scenario, a 3-phase bolted fault is quite rare. The most common type of fault is a line-to-ground fault, typically caused by inadvertent contact between an energized conductor and ground or equipment frame, which causes an unintentional flow of current through a path other than the utilization equipment. This could arise from issues such as a breakdown in equipment insulation, conductor insulation, or a loose termination. When this happens, the return path, which would normally be through the grounding system, now travels through any equipment frame, metal surface, or person in contact with the system as they essentially become part of an electrical circuit back to the source.
Overcurrent protective devices
The service entrance equipment offers the first step in protecting against thermal overloads and faults where circuit-protection devices are introduced into the system. Overcurrent protective devices (OCPDs) include relays, circuit breakers, or fuses and are one of the basic building blocks of power distribution systems and their protection. At the most basic level, these devices are inserted into the distribution system to "break," isolate, or disconnect the circuit if there is an overload or short-circuit condition. These devices have been used since the late 19th century and continue to be applied today. However, circuit protection continues to evolve with ever-changing technology. Today, there are technologies that use intricate communication and control strategies and can report which type of overload or fault opened a breaker, provide insights on power quality, measure harmonics, alarm certain events like ground fault, and more.
The most basic levels of circuit protection include fuses and thermal magnetic-type circuit breakers. Fuses contain a fusible element, which responds to the heat generated by the passage of current through it with a typical operating curve. A typical thermal magnetic circuit breaker includes a long-time trip operation region as well as an instantaneous zone. Some are adjustable in the instantaneous region, but these, along with fuses, are not inherently smart devices and have no built-in intelligence. They offer basic wire and equipment protection. They are designed to "break the circuit" when a fault occurs beyond their operating range. A distribution system should be designed when the OCPD isolates a fault close to the event without affecting unnecessary equipment upstream. This is referred to as selective coordination. With a standard fuse, or thermal magnetic device, you have basic circuit protection, but due to limited flexibility, they only offer basic protection from significant arc flash dangers. A thoughtful design assures the downstream feeder breaker has enough time to "clear" before the fault condition pushes the upstream breaker into its trip curve. This is referred to as selective coordination. In Figure 3, the diagram on the left shows a system lacking selective coordination. The upstream and downstream devices highlighted opened, since the OCPD closest to the fault did not trip first, thus all the red devices and their associated downstream loads would see an unnecessary power loss. Again, in Figure 3, the diagram on the right indicates how a correctly coordinated system would isolate the fault condition as close as possible and leave the rest of the system up and running as usual.
A means to produce a more reliable and coordinated system is to add intelligence to a circuit breaker in the form of integral trip units and protective relays. Another type of circuit breaker is an electronic trip-adjustable circuit breaker. This breaker has a long time-operating region, a long-time delay, a short time pickup, a short time delay, and finally, an instantaneous pickup. These parameters are adjustable over a given range. This adjustability makes the electronic-trip circuit breaker very flexible when coordinating with other devices. However, these devices are still not "smart" devices. The settings are initially set, but they are not communicating with other devices to provide optimum protection. Electronic breakers allow a design to be better coordinated, but they still tend to drive the circuit breaker sizes higher the further upstream you go to minimize the overlap in the trip regions. A design engineer must use experience and judgment to optimize the inherent trade-off for reliability and safety. The engineer must be careful; if he or she designs an electrical system based solely on safety by minimizing arc flash, it will be difficult to coordinate all devices. The system could be plagued with nuisance tripping, and costly unplanned downtime would be imminent. Likewise, designing a system singularly focused on uptime would place people at risk as well as the plant equipment. Thankfully, circuit breaker technology advancements are available that enable a better balance of safety and uptime so a compromise isn’t forced.
A technology that provides further reduction in the let-through energy for a fault in the region between two electronic-trip circuit breakers can be accomplished through ZSI (zone-selective interlocking). ZSI consists of wiring two circuit breaker trip units together so a fault is cleared by the breaker closest to the fault in the minimum amount of time possible. They operate such that if the downstream circuit breaker senses a fault, it sends a restraining signal to the upstream circuit breaker. The upstream circuit breaker will then continue to time out as specified on its characteristic curve, tripping only if the downstream device does not clear the fault. The primary goal is to switch off the fault current within the shortest time possible while impacting the least amount of connected equipment. ZSI is not new a technology, but tends to be more expensive. Manufacturers have different ways of accomplishing the same principle, so it is important to understand the nuances. However, the 2014 NEC added a requirement to provide arc-energy reduction (Article 240.87) and lists ZSI as an acceptable method-making ZSI a more common practice.
Additional circuit-protection strategies include using protective relays in the OCPD. Protective relays and devices can be applied to a system to help protect the circuits from conditions, such as reverse-power flow, single phasing, or transients and surges. Directional power or reverse-power relays monitor the direction of current and have the ability to respond by disconnecting the circuit. Differential relays measure the difference between two values of current and respond accordingly if it senses an error. A surge-protective device is an appliance inserted into the electrical system; it is designed to protect from voltage spikes by limiting the voltage supplied to an electrical circuit. Surge-protective devices help protect equipment against the damaging effects of transients caused by lightning, utility anomalies, or even internal load switching. There are hundreds of different types of protective relays, and the more complex a system is (such as having multiple sources of power and different voltage levels), the more complex protection systems become. They must be analyzed by the electrical engineer.
While properly selecting OCPDs and relays will provide protection against thermal overloads faults, these strategies alone cannot protect against arcing-type ground faults. For these types of faults, another level of defense must be added to the system. Because of the relatively higher resistance of an arcing fault and its intermittent nature, resulting fault currents are much smaller than those for bolted faults and are therefore harder to detect. There are two types of ground-fault protection: ground-fault protection of equipment (GFPE) and ground-fault circuit interrupters (GFCI), which is for protection of personnel. GFPE by definition is "a system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device." (NEC Article 100). GFPE senses faults down to 30 mA and does not provide protection for personnel. For personnel protection, GFCI is required, which senses faults down to 5 mA (this is discussed later). GFPE is required by the NEC for solidly grounded wye electric services ranging between 150 and 1,000 V to ground and 1,000 amps or greater (NEC 230-95; exceptions apply). And for vital electrical systems, such as hospitals, two levels of GFPE are required (NEC 517-17). However, codes are only minimum standards; it is good engineering practice to apply GFPE-type ground-fault detection even further downstream in the distribution system where ground faults are of concern and the desire is to isolate the fault closer to the source.
The importance of grounding
Any distribution system design should either include an ungrounded system or a solidly grounded system. An ungrounded system is not necessarily as safe as a grounded system, and there are only five different electrical power circuits noted in NEC Article 250.22 where the hazards of an ungrounded system may outweigh safety benefits of grounding. To avoid confusion, we will focus on solidly grounded systems. Proper system grounding is a major player in personnel and equipment protection. Grounding is the intentional connection of a current-carrying conductor to ground. The two main reasons for grounding per the NEC are to 1) limit the voltages caused by lightning or by accidental contact of the supply conductors with conductors of higher voltage and 2) stabilize the voltage under normal operating conditions. Properly grounded equipment provides a ground reference for exposed noncurrent-carrying parts of the electrical system and provides a path for the ground-fault current to get back to the source. The purpose is to prevent the flow of objectionable current. Grounding is a commonly misunderstood topic and the NEC devotes an entire article (Article 250) to grounding requirements. Figure 6 represents a summary of the requirements in NEC, Article 250. Figure 6 shows the important concept of a full-grounding electrode system. Rather than completely relying on one grounding electrode to perform its function, the NEC requires the formation of a system of electrodes in which all electrodes that are present in a building or structure are bonded together. This includes metal structural members, metal water pipe, and even rebar in concrete footings.
Additional circuit-protection strategies
Once the system grounding is properly designed, additional protection strategies may be applied to the feeder and branch circuits. Another form of circuit protection is GFCI. GFCI operates similarly to GFPE; however, it is typically an end-use device that de-energizes a receptacle within an established period of time when a ground fault is sensed. In contrast to GFPE, which is applied at the OCPD to primarily provide equipment protection, GFCI is typically applied at the end-use device to primarily provide personnel protection, as previously mentioned. This form of protection may also be applied at the branch circuit OCPD but provides the same personnel protection. Requirements for GFCI reside in NEC Article 210.8. GFCI is required for commercial facilities in bathrooms, kitchens, rooftops, outdoors, within 6 ft of a sink, wet locations, locker rooms, garages, and service bays. Other articles of the NEC also list GFCI requirements for specialty locations, such as vending machines, dwelling units, mobile homes, etc.
An arc-fault circuit interrupter (AFCI) is another form of circuit protection. An AFCI is "intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected" (NEC Article 100). Requirements for AFCI devices can be found in NEC Article 210.12. They are required in dwelling units and dormitory units, but not in a lot of commercial construction.
One last form of circuit protection worth mentioning is that of physical protection. Several articles of the code require physical or mechanical protection for feeders and even branch circuits for things like services or emergency power circuits in hospitals. Strategies for this form of protection can be found in NEC 230.50 or NEC 517.30 and include routing underground, installation in a more supportive conduit, or other approved means.
Danna Jensen is a senior vice president and lead electrical engineer at WSP USA and has been responsible for the design of all types of facilities ranging anywhere from office buildings to large greenfield hospitals. Jensen is a member of the Consulting-Specifying Engineer editorial advisory board.