The importance of circuit protection in electrical distribution system design

The electrical engineer is responsible for designing power distribution systems for buildings. Understanding the full circuit-protection requirements will enable the engineer to design the safest and most reliable electrical distribution systems for buildings.


Learning objectives

  • 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.

Figure 3: Comparison of a system with and without selective-coordination strategies applied. The diagram on the left shows a system lacking selective coordination. Both the upstream and downstream devices highlighted are open, 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. 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. All graphics courtesy: WSP USA 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.

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