Overview of IEEE Standard 242
Overview of IEEE 242, the IEEE Buff Book
IEEE 242 is one of the "color books" offered by IEEE. IEEE 242 is contained in a "buff" or "yellow" colored book.
IEEE standard 242 has been adopted by the American National Standards Institute (ANSI) and can be called ANSI/IEEE 242.
The IEEE 242 standard is periodically reviewed and maintained by the Buff Book Working Group, which is overseen by the Power Systems and Protection Committee. IEEE 242 can be considered a power system protection book, outlining IEEE recommended practices to protect electrical power systems from damage when a fault occurs.
Electrical system protection and coordination as described by IEEE standard 242 is intended to remove a component from the power system after it short circuits or starts to malfunction. When an electrical fault occurs, the electrical system protection built through the power system should act to isolate the malfunctioning unit. If the closest fault protection fails to work, additional backup protection should be triggered.
Why Protective Measures Are Important
Power systems can have different levels of reliability and protection. Reliability measures the odds of failure. Protection measures how well it continues to function after a failure. A high reliability system may have poor protection, causing total failure after a voltage surge.
A system with moderate reliability may have a well designed system of protection with extra fuses, fault tolerances, transformers and backup generators so that an occasional malfunction has no impact on the remaining system's performance. Setting high standards for reliability reduces the risk of serious damage to equipment when electrical faults occur. Inappropriate circuit breaker replacements can cause sparks or shorts, endangering people and equipment around it.
IEEE 242 Design Requirements
IEEE 242 outlines the curves of short circuit capacity that are to be used for transformers. The short circuit capacity is based on the voltage rating and impedance. Current transformers deliver a secondary current proportional to the primary current.
IEEE 242 provides a method to evaluate the accuracy of current transformers both in distortion and proportion to the primary current. Fuses may or may not limit current, but they are used in both low and high voltage circuits. Current limiting fuses only work within a specific range of currents. IEEE 242 provides calculations to determine which current limiting fuses should be used given the circuit's design.
IEEE standard 242 allows fault currents to be analyzed directly or on a per-unit basis. Per-unit analysis is better suited to systems that operate on different voltage levels. Direct analysis of fault currents uses a one-line diagram and is better suited for power systems that only operate on one or two voltage levels.
Power System Protection and Coordination Requirements
Protective relays protect a power system in case of a short circuit or overload. Protective relays trigger circuit breakers when abnormal conditions such as a sudden voltage spike. Protective relays are generally used on applications running on 480 volts or more.
Low voltage applications may use fuses to provide protection. The type of electrical protections required on a system depends on the type of equipment used. Greater protection is required by IEEE 242 as the voltage goes up. IEEE 242 discusses how to implement Ground Fault Protect coordination, also called GFP coordination.
IEEE 242 permits over-current to be managed with pre-programmed solid state relays. It also allows electro-mechanical over-current relays to be used. IEEE 242 permits system designs to use alternate circuits to keep systems operational when a primary circuit fails. The alternate circuit is activated when automatic throw-overs are triggered by abnormal conditions such as low voltage, high voltage or when the current gets too high or too low.
IEEE 242 references other IEEE standards for components in the power system. The IEEE C37 series of standards covers circuit breakers and switch gears. The IEEE C57 series covers protective relays and transformers. IEEE C62 standards cover surge capacitors and surge suppressors.
IEEE 62.2 outlines the approved method for diagnostic testing of electric power apparatus in machinery. IEEE 32-1972 describes the terminology and test procedures used for grounding devices such as grounding transformers and ground fault neutralizers.