The Art of Protecting Electrical Systems, Part 6


This sixth part in our series discusses the need for short-circuit calculations and the current-limiting effect of some overcurrent protective devices.

Whenever a short circuit occurs, every component in the system carrying the fault current must safely withstand the heating and magnetic stresses caused by the current. In addition, the protective device interrupting the fault current must do so safely and reliably.

The National Electrical Code (NEC) requires two principal short-circuit ratings for electrical system components up to 600 volts: interrupting and withstand. Devices that interrupt current, especially overcurrent protective devices, may have one or both ratings. Noninterrupting components have only withstand ratings.

Interrupting ratings are discussed in NEC Section 110-9, which states:

“Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage and the current which is available at the line terminals of the equipment.”

“Equipment intended to break current at other than fault levels shall have an interrupting rating at system voltage sufficient for the current that must be interrupted.”

NEC Article 100 defines interrupting rating as:

“The highest current at rated voltage that a device is intended to interrupt under standard test conditions.”

“(FPN): Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.”

Withstand ratings defined

NEC paragraph 110-10 establishes the basic requirement for withstand ratings:

“Circuit Impedance and Other Characteristics. The overcurrent protective devices, the total impedance, the component short-circuit withstand ratings, and other characteristics of the circuit to be protected shall be so selected and coordinated as to permit the circuit protective devices used to clear a fault without the occurrence of extensive damage to the electrical components of the circuit.”

NEC Article 100 (Definitions) does not define withstand ratings. However, Underwriters Laboratories, Inc. (UL) and other recognized testing organizations have published withstand ratings (sometimes termed short-circuit ratings) in their standards for noninterrupting equipment. These include switches, busway (bus duct), switchgear and switchboards, motor control centers and similar equipment. Such equipment listed by one of the organizations will have its withstand or short-circuit rating stated on the label.

A withstand rating is the maximum RMS symmetrical short-circuit current at which the equipment has been tested under specified conditions. At the end of the test the equipment must be in “substantially” the same condition as prior to the test.

Withstand ratings, in general, are based on tests of three-cycle duration. The test circuits have short-circuit power factors paralleling those used for testing protective devices, that is from 50% to 15%. If the protective device for a piece of equipment will require more than three cycles to clear a fault, the installation may be in violation of the NEC.

An exception to three-cycle testing occurs when the equipment is protected during the test by a circuit breaker with instantaneous trips or by fuses. In such cases, the test time is the time required for the overcurrent device to open the circuit.

If withstand ratings are established under these limitations, the equipment instructions and labels are required to state the specific make, type and rating of the circuit breaker or the fuse class and rating which shall be used to protect the equipment. It is in violation of NEC Section 110-3(b) to use other devices.

System interrupting ratings

In systems up to 600 volts, device-interrupting ratings usually are stated in symmetrical amperes as provided in the UL standards. Therefore, devices commonly are selected with interrupting ratings equal to or greater than a system’s available symmetrical short-circuit current. This is not always a safe practice, nor does it meet the intent of the NEC. During a fault, the “highest current” present is the asymmetrical current, and it is this current that protective devices must interrupt.

A totally symmetrical fault can occur only in a single-phase circuit, and then only if the fault occurs at a point in the voltage wave where the power-factor angle prior to the fault minus the power-factor angle of the fault is equal to

During a multiphase fault, there will always be some asymmetry. Even if the fault occurs when the fault current in one phase is totally symmetrical, the other phases are 120 degrees removed from the symmetrical phase and will have some degree of asymmetry. Even though the protective device rating is stated in symmetrical amperes, this asymmetrical current is the highest current the device must interrupt under standard conditions.

If the system short-circuit power factors are lower than those used when testing the protective device being considered, the resulting asymmetrical currents may be higher than the devices have been tested to interrupt. When this occurs, the protective devices may be damaged or destroyed, sometimes violently. This does not meet NEC intentions.

Protective device design

Designers must ensure that the asymmetrical interrupting ability of protective devices exceeds the system’s available asymmetrical current at the point where the device will be applied. Fortunately, electrical system power factors usually exceed those of the test circuits, and device-interrupting ratings usually may be selected on the basis of available symmetrical current.

Short-circuit power factors lower than those used in the test circuits most often are caused by equipment such as current-limiting reactors, current-limiting busway, high-impedance transformers, large induction motors and on-site generation at utilization voltage. X/R ratios may reach 20 to 1 in circuits containing such equipment (a short-circuit power factor of 5%). The highest short-circuit X/R ratio used in any of the 600-volt or less test circuits is 6.6 (short-circuit power factor of 15%).

Many authorities recommend that when available symmetrical short-circuit current exceeds 75% of a device’s symmetrical interrupting rating, the available asymmetrical current should be determined.

ANSI Standard C37.13 for Low Voltage AC Power Circuit Breakers requires such studies should be considered when the symmetrical short-circuit current exceeds 80% of the breaker’s rating. However, power circuit breaker test circuits are calibrated to the line terminals of the breaker rather than to a bus. For molded-case and intermediate-frame circuit breakers, the more conservative 75% is recommended. Short-circuit calculations must determine system short-circuit power factors or X/R ratios at each point where fault current is calculated in addition to the available symmetrical fault current.

It is also important to consider whether the standard UL test circuits reflect the system conditions. The test circuit in UL Standard No. 489 covers molded-case and intermediate (insulated-case) circuit breakers.

Rated current is available at the bus, and the circuit breakers are connected to the bus with four-foot conductors. Breaker load terminals are shorted with 10-in. conductors, leaving a circuit breaker’s internal impedance in the test circuit. The actual current interrupted by the circuit breaker is, therefore, substantially less than its rating.

Typical breaker testing

Consider UL testing of a three-pole, 100-ampere, 480-volt circuit breaker with an interrupting rating of 14,000 amperes. The actual values for a specific manufacturer’s breakers will vary slightly, as each breaker has its own impedance and arc characteristics. The principle, however, is the same for all such testing. In this example the current interrupted by the 14,000-ampere interrupting breaker is 12,832 amps (92% of the rating).

The engineer and inspection authorities must decide if the circuit breakers in a specific system are adequately covered by standard testing. Breakers are most often connected directly to the bus in panelboards, switchgear and switchboards. There are no conductors to reduce the available current.

While the rules for safety suggest that equipment should not be worked on while energized, it is common to test and adjust motor control or other equipment while operating. In such cases, it is possible that the circuit breaker may be shorted accidentally at its terminals. If these conditions are likely to prevail, and if the available fault current is close to the interrupting rating of the equipment, it may be necessary to select the next higher interrupting rating device.

In most cases, this causes no problem in application because most short-circuit calculations overstate the amount of fault current to some extent, and most devices are not applied at their full interrupting rating.

UL test circuits for fused switches are similar, but fuses with a 200,000 A.I.R. are used in testing most switches, and switch short-circuit ratings are commonly specified at 100,000 or 200,000 amperes in engineered systems.

Selective protective devices

Protective device selection requires extreme care under one or more of the following conditions:

  • Short-circuit calculations are based on complete resistance and reactance values, including all component impedances such as short bus runs, large switches, circuit breakers, etc. This provides accurate rather than overstated short-circuit currents.

  • Short-circuit calculations made to the line terminals of protective devices or bus systems that have directly connected devices. This is typical for panelboards and some motor-control centers. In such cases, there are no conductors to decrease the fault current. In addition to adequate protective-device ratings, it is important that the panels also have adequate short-circuit ratings with the specified devices installed.

  • Circuit breakers are applied at more than 75% of their interrupting rating.

Future installments in this series will focus on fuse and circuit breaker application with a discussion of these conditions in detail.

Most fuses specified today—as well as certain circuit breakers—are labeled current limiting. A current-limiting device will, within its current-limiting range, do all of the following: interrupt all currents; limit the peak current to a value less than would occur if the device were replaced with the same impedance solid conductor; and at 60 Hertz, open the circuit in less than 180 electrical degrees (1/2 cycle) after fault inception.

Some definitions of current limitation state that clearing time must be equal to or less than the first major or symmetrical loop duration. Because the first major loop may be thought of as an asymmetrical wave, its duration may be greater than 180 electrical degrees.

Current-limiting devices are not current limiting for all values of overcurrent: their opening time is inversely proportional to the overcurrent. As overcurrents increase, the opening time of the device decreases exponentially.

For an individual protective device there is a specific current that cause the device to open the circuit in less than one-half cycle and reduces the peak current. This current is called the threshold current. This point on the time-current curve is indicated in Figure 6.6. The device will be current limiting for all currents that exceed the threshold current up to the interrupting rating of the device. This range of currents is termed the current-limiting range.

Our next installment in the “Art of Protecting Electrical Systems” series will continue the discussion of equipment short-circuit ratings and nonfault interrupting equipment.

Related Stories:

The Art of Protecting Electrical Systems, Part 1: Introduction and Scope

The Art of Protecting Electrical Systems, Part 2: System Analysis

The Art of Protecting Electrical Systems, Part 3: System Analysis

The Art of Protecting Electrical Systems, Part 4: System Analysis

The Art of Protecting Electrical Systems, Part 5: System Analysis