When designing a safe and reliable power distribution system, it is imperative to consider life safety and equipment protection.
Marius Popescu, PE; Ali Ashur, PE; and Michael Stevens
Even the most robustly designed and well-maintained electrical systems can experience faults. Faults can be the result of natural events including lightning; environmental factors, such as aging and deterioration of electrical equipment; or human error, like the shorting of bus bars with a metallic tool. If left unchecked, faults in the electrical system can lead to equipment damage, arc blasts, and building fires. This is due to the mechanical forces and thermal energy that result from the huge fault currents that flow in the system. Protection, therefore, is vital for the electrical power system and its components.
The most basic function of a protection system is to recognize abnormal or faulty circuit operation and to then remove the faulted circuit from the electrical system to minimize damage to equipment and safeguard personnel and property. Protection can be defined based on the electrical system component being protected-generator protection, transformer protection, transmission-line protection, bus protection, feeder protection, and motor protection (which is the most common type).
An alternative way to define protection is based on the principle used in the protection scheme. This includes overcurrent protection, differential protection, distance protection, over/under voltage protection, and over/under frequency protection.
The following discussion will be limited to coordinated and selectively coordinated overcurrent protection-the most common form of circuit protection. Coordinated protective devices provide an optimal balance between fault localization and circuit protection based on the responsible engineer's judgment. Selectively coordinated protection is required for a few select power systems, such as emergency systems, critical operations power systems, and fire pumps. The objective of selective coordination is to ensure coordination in the full range; there is no engineering judgment as to what level of coordination is acceptable.
Key electrical codes, standards, and definitions
There are multiple electrical codes and standards that apply when designing and constructing projects, and they all require diligent consideration to ensure life safety, reliable power, and equipment protection. NFPA 70: National Electrical Code (NEC), is the industrywide standard in the United States regarding safely installing electrical wiring and equipment. It is typically adopted by state and local agencies to standardize the enforcement of safe electrical practices in their jurisdictions. An overview of key terms from 2017 Edition of NEC Article 100 includes the following:
NEC's selective coordination requirements specifically define selective coordination in Article 100 and mandate the proper selection and coordination in Article 110.10. Selective coordination shall be selected by a licensed professional engineer or another qualified person primarily in the design, installation, or maintenance of electrical systems. Selective coordination is not typically required between overcurrent devices connected in series if no loads are connected in parallel with downstream devices. Additional key selective coordination requirements defined in NEC include:
The 2018 edition of NFPA 99: Health Care Facilities Code also includes requirements regarding selective coordination in Article 22.214.171.124.2. It states coordination is required for essential systems when the fault duration exceeds 0.1 second. The coordination will be revised every 3 years (if changes are made to the power system).
Additionally, there are professional association standards related to circuit protection that require consideration, such as the Institute of Electrical and Electronics Engineers (IEEE) and the National Electrical Manufacturers Association (NEMA). Key standards and practices include:
The formerly IEEE Color Book set that includes 13 standards is now part of the IEEE 3000 Standards Collection for Industrial & Commercial Power Systems. This collection is organized into approximately 70 IEEE "dot" standards that cover specific technical topics.
Overcurrent protective devices
An OCPD is installed to protect against instances when the current exceeds the rating of the conductors or the equipment. These instances can result from either a short circuit, a ground fault, or general overload. There are several devices that are designed to protect against overcurrent, the most common include:
The insulated-case circuit breaker (ICCB) is derived from a MCCB and is used to designate a circuit breaker with a supportive and enclosing housing of insulating material and a stored-energy mechanism. ICCB uses characteristics of design from both the power and molded-case classes. Examples of trip units used in LVPCBs include:
Electromechanical trip devices. In the past, LVPCBs were equipped with an electromechanical trip device of the moving armature type, using a heavy copper coil carrying the full-load current to provide the magnetizing force. In this trip unit, overload protection is provided by a dashpot restraining the movement of the armature. Short-circuit protection is provided when the magnetic force suddenly overcomes a separate restraint spring.
Solid-state trip devices. Solid-state trip devices operate from a low-current signal generated by current sensors or current transformers in each phase. Signals from the sensors are fed into the solid-state trip unit, which evaluates the magnitude of the incoming signal with respect to its calibration setpoints and acts to trip the circuit breaker if preset values are exceeded.
The total clearing time of a circuit breaker is the sum of the breaker's sensing time, unlatching time, mechanical operating time, and arcing time. A fuse's total clearing time is the total opening time from the occurrence of an overcurrent until the fuse stops current flow. This is the sum of link-melting and arcing time. The time current curve (TCC) for an adjustable circuit breaker includes the following zones:
Selective coordination and protection study
System protection is achieved if the overcurrent devices settings are above the load operating levels and below electrical equipment damage curves. The selectivity is met when an OCPD on a circuit is interrupted and only the closest upstream device opens, such that only the section of the electrical system with a problem is removed from service. The circuit protection and selective coordination among protective devices associated with the power distribution system is not an easy process and requires knowledge and experience with local and national codes and standards.
The circuits protection and coordination should meet the following characteristics:
To ensure proper circuit protection and coordination, the electrical engineer needs to perform the following studies: load analysis, short-circuit calculations, protective device selection, and coordination. There are several power-analysis programs on the market that can be used to model the power distribution system and evaluate its behavior on faults at different locations.
The four main steps summarized in Table 1 are used in evaluating the circuit's protection:
TCCs are provided by the protective device manufacturer to show the amount of time required for a protective device to trip at a given overcurrent level. These curves are essential for the proper coordination of protective devices (breakers, fuses). Generic TCC curves for different protective devices are shown below in Figure 5. Fuses operate in a time-current band between maximum clearing times and minimum melt (or damage) times. The difference is the arcing time within the fuse. The minimum melt time is important when the fuse backs up other devices.
There are several protective device coordination methods that can be used, including current-type selectivity, time-type selectivity, zone selectivity, energy selectivity, and backup protection. Description of these protective device coordination methods are as follows:
Methods to improve selective coordination include, but are not limited to, increasing the withstand capabilities of the upstream line-side OCPDs, changing the type of circuit breaker (for example, from molded-case type to ICCB or LVPCB), selecting a current-limiting-type protective device, using fuses that do not overlap in the instantaneous range, or using manufacturer-tested, series-rated protective devices.
Even the most robust, well-designed and maintained electrical systems can experience faults. If left unchecked, faults in the electrical system can lead to equipment damage, arc blasts, and building fires. Protection, therefore, is vital for the electrical power system and its components. Selectively coordinated protection is required for a few select power systems, such as emergency systems, critical operations power systems, and fire pumps. Coordinated protective devices minimize disruption to plant operations by restricting an outage to only the faulted circuits and therefore provide an optimal balance between fault localization and circuit protection based on the responsible engineer's judgment.
Case study: Improving the electrical utility system at NSA Bahrain
Naval Support Activity (NSA) Bahrain is a U.S. naval base located in the Kingdom of Bahrain and serves as a central point of U.S. military operations in the region. NSA Bahrain is also the home of the U.S. Naval Forces Central Command and U.S. Fifth Fleet. In recent years, NSA Bahrain has undergone significant changes, expansions, and upgrades with the addition of new operations and facilities to support military operations. An electrical equipment photograph from the project is shown in Figure 6.
Using Power Tools for Windows (PTW) software, also known as SKM, CDM Smith performed an electrical system analysis that included a protective device coordination study from the 66-kV utility supply to the 415-V level. The existing protective devices' settings showed a lack of selective coordination between the medium-voltage (MV) relay and the low-voltage (LV) bus-feeder circuit breakers. The MV relay curve, time dial, and instantaneous pickup settings were adjusted to better coordinate the overcurrent protective devices. [WEB ONLY]Before and after time-current curve samples from the MV/LV side of the power distribution system are shown in Figures 7a and 7b.
To perform the study, CDM Smith conducted onsite data-collection activities using mobile computing devices. The final report included:
The final report documented the findings and made more than a dozen recommendations to the U.S. Navy to improve the electrical utility system coordination at NSA Bahrain.
Marius Popescu is a senior electrical engineer at CDM Smith. Popescu has more than 20 years of experience in the power industry (generation and distribution). Ali Ashur is a senior electrical engineer at CDM Smith. Ashur has 10 years of electrical engineering experience, specializing in both design and studies of electrical power distribution systems and involving complex projects for municipal, federal, and industrial clients. Michael Stevens is a technical writer at CDM Smith. Stevens has been supporting technical submittals and deliverables across multiple engineering disciplines for more than 20 years.