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This guide provides a comprehensive overview of inspection and test procedures for protective relays in electrical power systems. Photo: TestGuy.
Protective relays are used extensively across the power system to remove any element from service that suffers a short circuit, starts to operate abnormally or poses a risk to the operation of the system. The relaying equipment is aided in this task by instrument transformers that sense power conditions and circuit breakers that are capable of disconnecting the faulty element when called upon by the relaying equipment.
Due to their critical role in the power system, protective relays should be acceptance tested prior to being placed in service and periodically thereafter to ensure reliable performance. In a normal industrial application, periodic testing should be done at least every 2 years in accordance with NFPA 70B 2016.
Surge arresters are voltage limiting devices used to protect electrical insulation from voltage spikes in a power system.
Similar to how a fuse functions to protect an electrical system from damage due to overcurrent conditions, the job of a surge arrester is to protect the system from damage due to overvoltage conditions.
This guide provides a general overview of inspection and test procedures for simple residual and zero-sequence ground fault protection systems. Photo: TestGuy.
A ground fault is a type of electrical fault or short circuit condition that results from any unintentional connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.
This article provides an overview of some commonly used maintenance and diagnostic techniques that are commercially available for performing tests in the field on medium- and high-voltage power cables. Photo: TestGuy.
Field testing of medium- and high-voltage cables may performed for various reasons, such as acceptance after installation, charting the gradual deterioration of insulation over time, verification of splices and joints, and for special repairs. This assessment applies to both the cable itself, as well as the associated accessories (splices and terminations) — referred to as the "cable system."
This guide provides a basic overview of zone selective interlocking principles and test procedures. Photo: TestGuy
Zone selective interlocking is a communication scheme used with circuit breakers and protective relays to improve the level of protection in a power distribution system. Also called zone restraint or ZSI, the main purpose of this scheme is to reduce the stress on electrical distribution equipment during short-circuit or ground-fault conditions.
This guide provides an introduction to transformer winding resistance test methods and procedures. Photo: TestGuy
Winding resistance measurements are an important diagnostic tool for assessing possible damage to transformers resulting from poor design, assembly, handling, unfavorable environments, overloading or poor maintenance.
Personal protective equipment (PPE), including arc flash rated clothing, provide additional safeguards against electrical hazards. Photo: PEO ACWA (Flickr CC)
Electricity has long been recognized as a serious workplace hazard, exposing employees to electric shock, electrocution, burns, fires, and explosions. Hundreds of deaths and thousands of injuries occur each year on the job due to electrical shock and arc flash, yet almost all of these tragic events are preventable.
In addition to being properly qualified for the task at hand, personal protective equipment (PPE), including arc flash rated clothing, can provide additional safeguards against the hazards posed by working around electrical energy.
This guide provides a general overview of the various PPE that may be necessary for safe electrical work, including insulated gloves or sleeves, face shields, footwear, eye or hearing protection, and other electrical protective equipment.
Mimic bus symbols accurately reflect the distribution system arrangement that they are producing. Photo: Sage Controls, Inc.
The primary function of the electric power distribution system in a building or facility is to receive power at one or more supply points and to deliver it to lighting, elevators, chillers, motors and all other electrical loads.
The best distribution system is one that will, cost-effectively and safely, supply adequate electric service to both present and future probable loads. The selection of a system arrangement has a profound impact upon the reliability and maintainability of the electrical system.
Each transformer construction and application is different, requiring its own unique approach and test plan.
Determining the existing condition of power transformers is an essential step in analyzing the risk of failure. The following is a summary of testing and diagnostic techniques, and tools used to assess the condition of transformers.
It is essential to inspect and test current transformers and their connected instruments at regular intervals. Photo Credit: ABB
Current transformers play an important role in the monitoring and protection of electrical power systems. CT's are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments.
The field-testing and calibration of solid-state trip units can be performed by either primary current injection method or secondary current injection method. Photo: YouTube.
The Electronic Trip Unit is a microprocessor controlled, multifunction overcurrent protective device for application with low voltage power circuit breakers. The adjustment flexibility provided by the Electronic Trip Units allows users to easily accommodate load changes and other protective requirements while still assuring optimum coordination.
In addition to the adjustable protection functions, the trip unit is designed to use field interchangeable rating plugs. These rating plugs allow the ampere rating of the circuit breaker to be changed in order to meet specific applications.
Excellent maintenance is essential for reliability and safety of power circuit breakers. Maintenance programs must be tuned to the specific application, well planned and carried out consistent with both industry experience and manufacturer's recommendations.
