Relay control and protection guides

Protection Relays

The relay is a well known and widely used component. Applications range from classic panel built control systems to modern interfaces between control microprocessors and their power circuits or any application where reliable galvanic separation is required between different circuits. Altough considered to be a relatively simple component, the electromechanical relay and its technology is complex and often misunderstood.

History of relay

The earliest electrical relays were developed in the 1830s, as people began to recognize that such switches could be extremely useful. Historically, electrical relays were often made with electromagnets, which continue to be used today, although for some applications solid state relays are preferred. They key difference between electromagnetic and solid state options is that electromagnetic relays have moving parts, and solid state relays do not.

Electromagnets also conserve more energy than their solid state counterparts do.

Usage of relay

One of the reasons an electrical relay is such a popular tool for electricians and engineers is that it can control electrical output which is higher than the electrical input it receives. In the example discussed above, if the ignition connected directly to the battery, heavy duty insulated wiring would be needed to connect the steering column to the battery, and the ignition switch would also need to be much more robust.

A variety of circuits can be connected to an electrical relays. Relays can be used as amplifiers for electrical energy, as in the car example, and they can also connect to things like alarm switches, activating when a circuit is broken to trigger an alarm.

Many electrical failsafe systems utilize electrical relays which turn on or off in response to things like a current overload, irregular current, and other issues which may arise. These electrical relays trip to shut the system down until the problem can be addressed.

Browse guides and papers

Note that not all documents in this section are free to download. Navigate through sub-pages to discover all documents.

Lecture notes on switchgear and protection devices for students

The neutral point of the generator is usually earthed, so as to facilitate the protection of the stator winding and associated system. Impedance is inserted in the earthing lead to… Read more

May 15, 2017

Guide through applications for SIPROTEC protection relays

Transformers are valuable equipment which make a major contribution to the supply security of a power system. Optimum design of the transformer protection ensures that any faults that may occur… Read more

Dec 04, 2016

Basic principles in modern substation automation protection systems you MUST know

This guide outlines some of the principles used in modern substation automation protection systems, as well as some of the underlying theory. Simple forms of overcurrent protection have the advantage… Read more

Nov 23, 2016

Operation, maintenance, and field test procedures for protective relays

Protective circuit functional testing, including lockout relay testing, must take place immediately upon installation, every 2 years thereafter, and upon any change in wiring. If applicable, documentation is required detailing… Read more

Oct 18, 2016

Guide to protection of power systems with distributed generation

Distributed generation, dispersed, decentralized or embedded generation (DG, EG) are keywords for an upcoming probable paradigm shift in electric power generation. There is no standing international definition for these terms, but… Read more

Sep 25, 2016

Schematic Representation Of Power System Relaying

A schematic is a diagram that represents the elements of a power system using abstract, graphic symbols rather than realistic pictures. The reason for not emphasizing reality is to communicate… Read more

May 28, 2016

The basics of power system protective relaying

Protective Relaying The IEEE defines protective relays as: “Relays whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and… Read more

Dec 19, 2015

Electrical Plan Review Booklet

Content of Electrical Plan Review Booklet: Overcurrent Protection and Devices, Short-Circuit Calculations, Component Protection, Selective Coordination and Other Considerations By reviewing this brochure, the Electrical Inspector, Electrical Contractor, Plan Examiner,… Read more

May 16, 2015

Transmission Line Protection Principles

The protection system selected should provide redundancy to limit the impact of device failure, and backup protection to ensure dependability. The high level factors influencing line protection include: The criticality… Read more

May 31, 2014

Protective Relaying – Principles and Applications

Protective relays using electrical quantities are connected to the power system through current transformer (CT) or voltage transformer (VT). These input devices or instrument transformers provide insulation from the high-power… Read more

Mar 15, 2014

The Philosophy Of Protective Relaying

The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts… Read more

Nov 06, 2011

Electrical Relay Diagram and PID Symbols

Disconnect; Circuit Interrupter; Circuit Breaker; Limits (normally open, normally closed, neutral position, maintained position, proximity switch); Liquid Level; Vacuum & Pressure; Temperature; Flow (Air, Water); Foot; Toggle; Cable Operated (Emerg.);… Read more

Nov 05, 2011

Fundamentals of line protection with distance relays

Distance relaying should be considered when overcurrent relaying is too slow or is not selective. Distance relays are generally used for phase-fault primary and back-up protection on subtransmission lines, and… Read more

Dec 24, 2010

Distance Relay Fundamentals

Distance functions have been in use for many years and have progressed from the original electromechanical types through analog types and now up to digital types of functions. The purpose… Read more

Dec 24, 2010

Relay Selection Guide

Power distribution systems serving industrial or commercial facilities represent a variety of costs to the owner. Obviously, the capital cost of the equipment is a concern, as is the cost… Read more

Oct 13, 2010