The Art of Protecting Electrical Systems: Introduction and Scope

By GEORGE W. FARRELL and BARRY FEINBERG, Ph.D., P.E., Fellow, IEEE August 10, 2005

Editor’s Note: From 1965 through 1970, Consulting-Specifying Engineer ’s predecessor, Actual Specifying Engineer, ran a series of articles on overcurrent protection. Due to the immense popularity of the 31 installments in the series, the authors, George Farrell and Frank Valvoda, P.E., reprised the series in an updated version beginning in the Feb. 1989 issue of CSE. Over the years since the last installment ran in the late ’90s, we have received many requests to re- run this series. Mr. Valvoda passed away in Dec. 2001, but his long-time friend and editorial partner, George Farrell, is reviewing and updating this valuable series of articles, with the assistance of a new collaborator, Barry Feinberg, formerly EE faculty at Purdue University and now an independent consultant. Addenda describing new information that has been added will appear at the end of each article. It seems fitting that this year, 40 years after the series first appeared, we are re-launching it in an electronic format. “The Art of Protecting Electrical Systems” has made the transition from 20thto 21stcentury—and from print to the web. We begin here with installment no. 1.

“The overcurrent protective system is the very heart of the electrical distribution system. Faulty or inadequate overcurrent protection can bring about the loss of the entire facility served by the system—or, worse, it can cause needless death or injury to personnel.”

Those words are still true. But if overcurrent protection was the “heart” then, electronic devices now being integrated into electrical systems have become the “nerves” of today’s systems. In just one generation, the introduction of new, “smarter” devices has significantly changed equipment and design practices.

Every application of electricity results in a need for overcurrent protection. Today, overcurrent protection is only one part of a protective umbrella covering such things as corrosion, lightning, preventive maintenance and similar considerations.

This series, “The Art of Protecting Electrical Systems,” will present information about devices, equipment and methods for doing so. It will be practical, based on sound principles. While directed to consulting engineers and designers, plant and field engineers will benefit much from the material.

Those with a mathematical bent may choose to use the equations we will offer throughout this series. And to speed up their work, others will use the many tables and shortcut methodsprovided. New, powerful computer programs will be discussed in detail. Both newcomers and experienced professionals will hopefully gain new appreciation and understanding of the new challenges of code and regulation revisions, rapidly changing protective systems, equipment and calculation methods.

The series stresses the importance of meeting code requirements and reducing liability exposure. We hope that the articles will speed the design of electrical distribution systems, increase their efficiency and decrease their cost over a project’s life. System safety and reliability are emphasized.

Engineering has been defined as being the art of an applied science. Nowhere is this more apparent than in electrical system design. Selecting and applying the wide variety of available equipment—some with markedly different operating characteristics—requires a proficient artisan in the highest, most professional sense.

Sources and references

Engineers concerned with the design, installation, operation and maintenance of electrical distribution systems traditionally research standards, codes, handbooks, manufacturers’ data and periodicals.

Standards set specific requirements by defining electrical terms, quantities, measurements, tests, dimensions and ratings. These standards are updated as technology and manufacturing techniques develop and change, usually at intervals of three to five years. Some relevant standards organizations are:

%%POINT%%Institute of Electrical and Electronics Engineers (IEEE)

%%POINT%%National Electrical Manufacturers’ Assn. (NEMA)

%%POINT%%National Fire Protection Assn. (NFPA)

%%POINT%%Underwriters Laboratories, Inc. (UL)

%%POINT%%Canadian Standards Assn. (CSA)

%%POINT%%American National Standards Institute (ANSI)

NFPA’s National Electrical Code (NEC) and local derivatives establish minimum requirements for fire protection and safety. These codes are usually adopted as law by responsible governing bodies. NFPA 70E, Standard for Electrical Safety In the Workplace, has become increasingly important and greatly influences design.

In addition, the federal government’s Occupational Safety and Health Act (OSHA) mandates compliance with standards and codes (including the NEC) for all new electrical installations, and sometimes retroactively if it thinks personnel safety is inadequate.

Handbooks are available that provide details on many aspects of electrical distribution. Written with specific audiences in mind, they may cover both theoretical and practical perspectives. For example, some discuss industrial power systems, and others only high-voltage equipment. Some elaborate on transformer connections or application of relays, and others concentrate on underground systems maintenance, testing, motors or lighting. In trying to cover the entire field of electrical engineering, the “standard” handbooks do not deeply examine specific subjects.

In addition, electrical equipment manufacturers publish sales information listing catalog numbers, ratings and sizes. They also make technical application data available to interested engineers in the form of recommendations, graphs, time-current coordination curves and performance tests.

IEEE publishes several “Recommended Practice” books based on standards, codes, manufacturers’ data and committee members’ experiences. The books, which are updated at regular intervals and may be purchased at reasonable prices, include the following:

%%POINT%%Electrical Power Distribution for Industrial Plants (red)

%%POINT%%Protection and Coordination of Industrial and Commercial Power Systems (buff)

%%POINT%%Electrical Power Systems in Commercial Buildings (gray)

%%POINT%%Grounding of Industrial and Commercial Power Systems (green)

%%POINT%%Emergency and Standby Power Systems (orange)

%%POINT%%Protection and Coordination of Industrial and Commercial Power Distribution Systems (buff)

Periodicals of the IEEE professional societies provide an abundance of information on distribution systems. The societies most concerned with power distribution are the Power Engineering Society, the Industry Applications Society and the Computer Applications Society. Each society publishes at least one monthly or bimonthly periodical.

Publications of the National Electrical Contractors’ Assn. (NECA) and the International Assn. of Electrical Inspectors (IAEI) provide additional insight into distribution system problems and practices.

Finally, there are the commercially oriented periodicals that offer current information on distribution systems and equipment.

“The Art of Protecting Electrical Systems” will provide detailed, current information, extracting from and supplementing available standards, codes, handbooks, manufacturers’ data and periodicals. Each topic discussed will draw from these varied sources, as well as from personal experience. It is this personal experience that makes the articles unique. We intend to present a highly personal view of system design—not a consensus, but a guide in making choices.

In addition, we will reference data supplied by manufacturers, electrical inspectors, insurance associations and others. Here’s what the series will cover:

%%POINT%% Overcurrent protection fundamentals. Types and effects of overcurrents, protection of people and property, terminology and definitions concerning electrical phenomena, fuses, circuit breakers and relays.

%%POINT%% The nature of short-circuits, including arc-flash phenomena. Equipment interrupting ratings and short-circuit ratings, calculations, computer programs, time-current and peak let-through curves, ampere-squared-seconds, quality overcurrent protection systems and the influence of fire and building codes.

%%POINT%% Principles, application and operation of protective devices. Molded-case circuit breakers; intermediate-frame circuit breakers; power circuit breakers; thermal-magnetic, magnetic-only and electric trips; low-voltage fuses and fusible devices; power service protectors; medium-voltage circuit breakers; fused switches and fuses; and protective relays. The characteristics and ratings of most of this equipment has changed drastically since we last presented this series.

%%POINT%% Distribution and utilization equipment protection. Switching equipment, switchgear, switchboards, generators, transformers, capacitors, UPS systems, industrial equipment and HVAC equipment.

%%POINT%% Protection of system conductors. Wire and cable, busway, service conductors, mains and feeders, motor feeders and branch circuits and motor control circuits.

%%POINT%% Electrical motor application and protection.

%%POINT%% Lightning Protection.

%%POINT%% Miscellaneous protection. System voltage studies and stabilization, grounding and ground faults, physical protection and guarding, and cathodic protection.

%%POINT%% Preventive maintenance.

%%POINT%% System design.

Fundamentals of protection

The need for effective overcurrent protection cannot be overemphasized. It is the cornerstone of a reliable electrical system. Basic to overcurrent protection is a thorough understanding of overcurrents, the various types and the major causes and the severe damage that may result when they are not controlled.

By definition, an overcurrent is any current that, under the conditions of use, exceeds the rating of system components such as conductors, switching devices, utilization equipment and similar items. Depending on the magnitude of the overcurrent, the result may be overheating with damage to insulation, magnetic stress with damage to components, or arcing accompanied by arc flash and erosion of component material. Unless specifically stated otherwise, all references to current in the series are to rms (root mean squared) AC current, and all references to voltage are 480 volts or less.

Heating in current-carrying components is a function of the current’s duration and magnitude (squared), component impedance, ambient temperature, air movement and heat radiated by other components. When carrying current within their ratings for the conditions of use, components reach a heat balance and an ultimate temperature within maximums for the insulation. At such temperatures, actual insulation life should equal or exceed insulation design life, which is usually 20 years.

The energy, in joules (watt-seconds), delivered to a conductor during a time (t) in seconds is:

J = I2R t

In general, this equation demonstrates the effect current variations have on the heating of insulation materials in contact with a conductor. This energy is expended as heat. For a given time, heat generated varies directly as the square of the current. Doubling the current results in four times the generated heat and increasing current by ten causes a hundred times increase in heat. (Since conductor resistance increases with an increase in temperature, the heat generated in such a circuit is even greater.)

When overcurrents are moderate, components reach a new heat balance that may be above their maximum design values. Most insulation is organic: polyvinylchloride, polyethylene, rubber, plastic, etc. With such compounds, each time sustained moderate overcurrents occur, insulation aging is accelerated. Every 10°C temperature increase above the rated operating temperature results in almost double the aging (chemical action).

If temperatures are high enough, as may occur in a short circuit, heat is generated so quickly that cooling by convection, radiation or conduction is negligible. Insulations carbonize within a few seconds and may emit poisonous, noxious or flammable gases. Many insulations simply burst into flame. This is particularly destructive when it happens in cable trays, switchboards, motor control centers and other equipment where cables are concentrated and oxygen is available to permit burning.

If cables do not have adequate flame resistance, as one cable begins burning, it heats adjacent cables, and fire may propagate quickly. There have been many such cable tray fires resulting in the complete loss of a facility. NEC requires cables used in cable trays to pass very severe fire tests before they can be approved for such use.

When this overheating occurs in conduit runs, especially if conduits are heavily filled, the insulation vapors cannot readily burn; there is not enough oxygen. The gases are forced out of their conduits into, in most cases, the switching equipment where the conductors originated. If an arc is drawn, perhaps by a breaker clearing a fault or by load switching, the gases may be ignited and cause an explosion.

A case in point: a switchgear building in a metropolitan area had such an explosion. It completely lifted the poured concrete roof slab and set it back down—but skewed about eight inches. The explosion so weakened a brick and concrete block wall that it had to be shored up, and a piece of metal ejected from the switchgear passed through a window and through both sides of a van parked nearby.

Such incidents are not rare. They exemplify one way excess heat caused by overcurrents may destroy system components.

Magnetic. When a current flows in a conductor, it sets up a surrounding magnetic field. Depending on direction of current flow, this field acts in space to either attract or repel adjacent conductors or enclosures. The strength of the field and the resulting force depend on many factors, including current magnitude, shape and spacing of conductors, and length of conductors and their configuration. This magnetic stress varies approximately as the square of the peak current. In a typical circuit the maximum magnetic stress occurs in the first half-cycle following the fault. Due to increasing impedance the current gradually decreases until it reaches a steady state.

Load currents in industrial and commercial electrical systems do not generate significant magnetic stress. However, when short circuits occur, the current may exceed 100,000 amps, and substantial stress may be present. For example, in a system consisting of parallel 4-in. wide bus bars spaced 2 in. apart (a typical 480-volt switchboard spacing), a 100,000-amp current can produce a magnetic force of 7,000 lbs./ft. of bus bar. A bus structure 10 ft. long could have a total force of 70,000 lbs. attempting to pull the bars together or force them apart.

Equipment not designed to withstand such stress is actually torn apart. This may create additional short circuits and break equipment terminals. It may destroy switchboards, switchgear and motor control centers. Magnetic stress is particularly destructive when permitted to continue for several cycles. Alternating current produces a magnetic force that reverses direction, shaking components and compounding damage.

Arcing. While some short circuits occur from crossing phases while connecting or maintaining systems, even more occur as a result of insulation breakdown often accompanied by arc flash. The impedance of the arc may restrict current flow to such a degree that overcurrent devices do not operate immediately. An arc may continue until it burns to a more solid connection. This permits additional current flow.

An arc is usually accompanied by overheating and magnetic stress. Its effect can be devastating; it often burns through enclosures or raceways, showering the area with bits of molten metal and starting fires. Gases from vaporized insulation and other materials may ignite or explode. The insulation breakdown may produce noxious or poisonous fumes.IEEE and NFPA have done studies of arc flash hazards. The result has been stringent new requirements by OSHA, NEC, and NFPA 70E.

A small arc in switchboards or motor control centers may deposit metal or carbon throughout the equipment, causing many short circuits and rendering damage that makes it too costly to repair the equipment.

In light of these potential problems, there can be no doubt that overcurrent protection should be engineered to minimize the effects of heating, magnetic stress and arcing.

Types of overcurrents

Overcurrents may be either transient or sustained. Transient overcurrents are of short duration and if protective devices are properly applied, they are harmless unless frequently repeated.

Sustained overcurrents are those that are not transient. They continue until interrupted by automatic switching or overcurrent devices. If they are not interrupted, sever damage may occur.

Overcurrents also may be classified as overloads or short circuits. Overloads are overcurrents that are confined to the normal current path. They are caused either by equipment malfunction or by connection of utilization equipment that draws more current than the circuit rating under conditions of use. Short circuits by definition are currents out of their normal path. They may be caused by insulation breakdown or by incorrectly connecting equipment or conductors.

Overloads. Transient overloads are by definition harmless. The most common form of transient overload in AC systems is motor-starting current. Motors must be permitted to start, so overcurrent protection should not open the circuit during normal staring time. Other examples of transient overload are the current increase that occurs when too much pressure is put on a drill bit, when a lathe operator makes a large cut or when large currents are switched.

Sustained overloads, however, must be disconnected before system temperature limits are exceeded. If permitted to continue, they will cause overheating of conductors and insulation.

Short circuits. A short circuit is current out of its normal path. Some short circuits are of such short duration that they clear before protective devices can open, especially when occurring on open wire distribution systems. For example, a large bird may short circuit a distribution system and then drop out of the way instantly. A short circuit that continues—regardless of its magnitude—must be removed as quickly as possible so outages will be minimized.

Short circuits can happen between phases or between phase(s) and ground. For calculation purposes,three-phase “bolted” faults usually produce the highest current and are used to determine the minimum interrupting ratings required for overcurrent protective devices and short-circuit ratings for other equipment.

New requirements for arc-flash protection may require calculations to determineminimum sustainable arcs at many points in the system.

Faults that occur between phase(s) and ground (ground faults) may be solidly connected or may be arcing. Ground fault current can vary from a fraction of an ampere to the full value of available fault current.

The next article in the series will continue discussing the importance of overcurrent protection. Legal requirements, terminology and definitions will be addressed.