What is the Difference Between Interrupting Rating and Interrupting Capacity?

 

Main Distribution Panel

Many electrical engineers thought that interrupting rating and interrupting capacity has a similar meaning. For this reason, we need to know its key differences and its effect when applied to electrical design or installations. 

Interrupting Rating

Interrupting rating is the maximum short-circuit current that an overcurrent protective device can safely interrupt under standard test conditions. One should notice the term “under standard test conditions” which means, it is important to determine how the overcurrent protective device is tested in order to assure it is properly applied. 

Interrupting Capacity

According to the IEEE Standard Dictionary of Electrical and Electronic Terms, interrupting capacity is the current at rated voltage that the device can interrupt. 

Standard Test Conditions - Fuses

The branch circuit fuses are tested without any additional conductor in the test circuit. For example, if a fuse has an interrupting rating of 300 kA, the test circuit is calibrated to have at least 300 kA at the rated fuse voltage. During the test circuit calibration, a bus bar is used in place of the fuse to check the proper short-circuit current. Then the bus bar is removed and the fuse is inserted; the test is then conducted. If the fuse passes the test, the fuse is marked with this interrupting rating (300 kA). 

In the procedures just outlined for fuses, there are no extra conductors inserted into the test circuit after the short-circuit current is calibrated. A major point is that the fuse interrupts an available short-circuit current at least equal to or greater than its marked interrupting rating. 

In other words, because of the way fuses are short-circuit tested (without additional conductor impedance), their interrupting capacity is equal to or greater than their marked interrupting rating.

Standard” Test Conditions - Circuit Breakers

Compare to fuses, it is not the case with circuit breakers. This is because of the way circuit breakers are short circuit tested (with additional conductor impedance), their interrupting capacity can be less than their interrupting rating. 

When the test circuit is calibrated for the circuit breaker interrupting rating tests, the circuit breaker is not in the circuit. After the test circuit has been verified to the proper level of short-circuit current, the circuit breaker is placed into the circuit. Accordingly, in addition to the circuit breaker, important lengths of conductors are permitted to be added to the circuit after the calibration. This additional conductor impedance can result in a significantly lower short-circuit current. 

So a circuit breaker marked with an interrupting rating of 22,000A may in fact have an interrupting capacity of only 9,900A.

Reference: 

• Cooper Bussman.

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What are the Characteristics of Fuse as Circuit Protection?

Fuse

 

A fuse is the simplest device for interrupting a circuit experiencing an overload or a short circuit. A typical fuse, like the one shown below, consists of an element electrically connected to end blades or ferrules. The element provides a current path through the fuse. The element is enclosed in a tube and surrounded by a filler material. 

The National Electrical Code® defines overcurrent as any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault (Article 100-definitions). Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, overloads and short circuits do occur. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source.

Nontime-delay Fuses

Nontime-delay fuses provide excellent short circuit protection. Short-term overloads, such as motor starting current, may cause nuisance openings of nontime-delay fuses. They are best used in circuits not subject to large transient surge currents. Nontime-delay fuses usually hold 500% of their rating for approximately one-fourth second, after which the current carrying element melts. This means that these fuses should not be used in motor circuits which often have inrush (starting) currents greater than 500%.

Time Delay Fuses

Time-delay fuses provide overload and short circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds. This is normally sufficient time to allow a motor to start without nuisance opening of the fuse unless an overload persists.

Read: How to Select Overcurrent Protection Devices?

Fuse Ratings

Fuses have a specific ampere rating, which is the continuous current carrying capability of a fuse. The ampere rating of a fuse, in general, should not exceed the current carrying capacity of the circuit. For example, if a conductor is rated for 10 amperes, the largest fuse that would be selected is 10 amperes. There are some specific circumstances when the ampere rating is permitted to be greater than the current carrying capacity of the circuit. For example, motor and welder circuits can exceed conductor ampacity to allow for inrush currents and duty cycles within limits established by the NEC.

The voltage rating of a fuse must be at least equal to the circuit voltage. The voltage rating of a fuse can be higher than the circuit voltage, but never lower. A 600 volt fuse, for example, can be used in a 480 volt circuit. A 250 volt fuse could not be used in a 480 volt circuit.

Fuses are also rated according to the level of fault current that they can interrupt. This is referred to as ampere interrupting capacity (AIC). When applying a fuse, one must be selected which can sustain the largest potential short circuit current which can occur in the selected application. The fuse could rupture, causing extensive damage, if the fault current exceeds the fuse interrupting rating.

UL Fuse Classification

Fuses are grouped into current limiting and non-current limiting classes based on their operating and construction characteristics. Fuses that incorporate features or dimensions for the rejection of another fuse of the same ampere rating but with a lower interruption rating are considered current-limiting fuses. Underwriters Laboratories (UL) establishes and standardizes basic performance and physical specifications to develop its safety test procedures. These standards have resulted in distinct classes of low voltage fuses rated at 600 volts or less. The following chart lists various UL fuse classes.

UL Fuse Classification

Reference: 

• Siements