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Fuses and Power Circuit Breakers

The Fuse Basics

The Fuse Voltage Rating

The fuse voltage is a function of its capability to interrupt the flow of overload current in the event of a fault. Generally, fuse voltage rating should be higher than the circuit where they are used. but never lower, since the fuse voltage rating is a function of its capability to interrupt the overload current in the event of a fault. If the voltage rating of the fuse is much lower, say 50%, than the system voltage, the arc quenching effect of the fuse might be impaired when the fuse element melts. A basic example:A 750mA 5kV microwave oven high voltage fuse tube has a diameter no bigger than fuses we find in electronic circuits, but with a much longer length. The considerably longer length enables the fuse to quench the arc when the fuse opens.

For low voltage system, fuses with 600Volts fuse rating will perform well in a 230V to 480V system. Always check the time current characteristic curve of the fuse manufacturer since you might assume that a time delay fuse (Class K5) of a lower ampere rating offers better protection than a higher rated non-time delay fuse (Class K1 or class J). This is a false assumption. In general, the rating of the fuse is an asymptotic value e.g: the equation y=1/x curve:The increasing value of x towards infinity will make the value of y getting nearer and nearer to zero and gets closer and closer to the x-axis but never touching it). This equates to a condition where the minimum fuse melting threshold is not met and the fuse does not open; inasmuch as the overcurrent is below or just slightly below the fuse's melting threshold.

Current Limiting Fuses (CLFs)

The Current Limiting Fuse's peak let-through is the CLF's instantaneous value. TheI2t average let-through value is the thermal energy produced during the fuse element's opening. The equation I2t is the square of the current multiplied by the time. It is proportional to the energy and magnetizing forces cause by a fault, generating heat and opening the fuse element. The I2t information of a fuse is provided by manufacturer's data sheet for coordinating the upstream and downstream of an electrical network.

To minimize the damage to electrical equipment during a fault, use a CLF with lower let-through current than the available short circuit current of the system.

CFLs in Series

Current Limiting Fuses limits the very high fault current to a lesser value (let-through current) in areas where there is a substantial increase in available short circuit current due to the addition of new reactive loads. This condition renders the circuit breaker's interrupting rating become marginally low for the system. Instead of replacing the circuit breaker, which entails a major investment cost, the Current Limiting Fuse is used and installed before and in series with the circuit breaker.


400kV SF6 Circuit Breaker

400kV SF6 circuit breaker Courtesy of Wikipedia Project. Author: Sushilksk"

Available short circuit current increases through the years due to addition of motor or other reactive loads in the facility that was not foreseen earlier. Another contributing factor is the increased available short circuit at the utility side due to new industrial facilities connected to them.

Another factor is the high available short circuit in the area during the design stage of a substation or switching facility. The designer opts to use CLF instead of resorting to impedance grounding or using expensive breakers with high interrupting ratings.

CFLs are usually applied in the incoming feeder to raise the circuit breaker's interrupting rating; or used in conjunction with Load Break Switches or Automatic Transfer Switches since a properly sized current limiting fuse limits the short-circuit current to within the withstand rating of the switch contacts.

Using CFLs do not mean no damage to some electrical auxiliaries, i.e. contacts, bimetals or other minor parts, unless they have been designed as a combo unit, rated and tested accordingly by the manufacturer.

Non-time Delay Fuses

Fuses have different operating time. A standard fuse may require to reach 2X its rated value to open the protected circuit in about 1 sec., while the fastblow fuse at twice its rated value, will open the circuit in about 0.1 sec or less. Fastblow blow fuses are the commonly used fuses. They are used in circuitry where a delay in the opening of a protected circuit during a fault can cause considerable damage to costly electrical or electronic equipment.


110kV, 50hz Oil Circuit Breaker MKP-110 for 110kV/3.3kV DC Traction 

110kV, 50hz oil circuit breaker MKP-110 for 110kV/3.3kV DC Traction Substation, Toliatti City Railway, Russia. Courtesy of Wikimedia OS Author: Vivan755 - March 2011

Slowblow fuses, or time delay fuses or anti-surge fuses, on the other hand, are commonly used in combination with motors. Motors have high starting current; and motors that have loads when started have starting current that can approach the "locked rotor" current in its time-current characteristic curve.

Non-time delay fuses are referred by NEC as single element fuses. This fuse type will not open under normal conditions as long as the inrush current of the motor (eg., starting current) will not exceed the fuse's rating. They are sized 300% of the Ifl of the motor per NEC Table 450-152 with exception #2 of Art. 430-52 allowing a 400% Ifl.

Low Voltage Dual Element Fuses

Low voltage dual element fuses have two series connected elements enclosed in a single tube to provide instantaneous short circuit protection with the other providing time delay function under normal load. The disadvantage of this fuse is the downtime that is loss to replace it. Its advantage is it provides the motivation by operations personnel to determine the exact cause of the fault. The type of the fuse's enclosure including ambient temperature should also be taken into account.

Expulsion Type fuses

Fuse overload conditions have maximum operating times defined in accordance with industry standards in terms of its time delay characteristic at a certain percentage of overload. Lab testing of fuses establishes parameters and data that are used to construct the fuse's time-current curves based on RMS current at 0.1 second or more. It also establishes the fuse element's average minimum melting time to clear an overload condition.

Pole Mounted Fuse

Pole Mounted Fuse Cutout
Courtesy of Greensburger - Wiki OS

A fuse cutout at a service drop on a single-phase overhead power distribution line supported by insulators at the top of the pole.

Fuse Opening Time:

Fuses have different operating time. A standard fuse may require to reach 2X its rated value to open the protected circuit in about 1 sec. while fastblow fuse, at twice its rated value will open the circuit in about 0.1 sec or less. Fastblow blow fuses are the commonly used fuses in circuits where a delay in the opening of a protected circuit during a fault can cause considerable damage to costly electrical equipment.

Slowblow fuses, or time delay fuses or anti-surge fuses, on the other hand, are commonly used in combination with motors. Motors have high starting current especially those that have loads when started where starting current can approach the locked rotor current of a motor during starting.

Temperature Derating

The fuse's operating parameters are affected by ambient temperature. As an example, a fuse rated 10amps may allow currents 10% or 20% or more current at lower temperature. At higher temperature, say 95°C, the fuse may open at 80% of its rated value depending on the fuse family as provided for in manufacturer's data sheet.

Low Voltage Fuse Classes

A 115kV Fuse

115kV Instrument Transformer Fuse
Courtesy of Wtshymanski - Wiki OS

A 115 kV fuse protecting an instrument transformer at Slave Falls Generating Station, Manitoba. The fuse element is the white tube.

Class CC

Class CC fuses are rated to interrupt a minimum of 200,000 amps and are rated for 600 volts . These small branch circuit fuses have overall dimensions of 13/32” x 1-1/2”. They are available from 1/10 amp through 30 amps.

Class G

Class G fuses are rated for 480 volts and are rated to interrupt a minimum of 100,000 amps. The fuse diameter is 13/32” while the length varies from 1-5/16” to 2- 1/2”. These are available in ratings from 1 amp through 60 amps.

Class H

Class H fuses are rated to interrupt a minimum of 10,000 amps. The voltage ranges from 250 and 600 volts. These fuses can be either renewable or non-renewable. These are available in amp ratings of 1 amp through 600 amps.

Class J

Class J fuses are labeled “Current-Limiting” and are not interchangeable with other classes. Voltage rating is 600 volts and rated to interrupt a minimum of 200,000 amps

Summary of Protective Devices and Their Ratings

Protective Devices and Their Ratings

Summary of Protective Devices and Their Ratings Reference: Electric Power System Protection and Coordination By: Michael Anthony - University of Michigan.

View clear image HERE

Class K

Class K fuses are current limiting fuses. They have interrupting ratings of 50,000, 100,000, or 200,000 amps. The fuses are listed as K-1, K-5, or K-9 fuses. Each subclass has designated I2t and Ip maximums. They have the same dimensions as Class H fuses. They are "current-limiting" fuses but they are not marked as so.

Class L

Labeled "Current-Limiting" Class L fuses are rated 600 volts AC. These fuses are rated for 600A to 6,000A with interrupting capacity of 200,000 amperes AC. They are bolt mounted.

Class L fuses may have a time-delay feature.

Class R

Class R fuses have 250Volts to 600Volts rating and are marked "Current-Limiting." These fuses have a minimum of 200,000Amps interrupting rating. They have similar dimension as the Class H fuse but fuses with lower interrupting capacity cannot be mounted on the Class R mounting clips.

Interrupting Rating Vs. Interrupting Capacity

Interrupting Rating Vs. Interrupting Capacity

Reference: Cooper Bussmann - pdf file 2005.

View clear image HERE

Class T

Class T fuses have a time-lag characteristic, fast acting and with an interrupting rating of 200,000Amps RMS. These are small fuses suited for limited spaces. They are rated 300Volts to 600Volts RMS (Root-Mean-Square).

High Voltage Fuses

High Voltage Current Limiting Fuses

High voltage fuses are classified under international standards as either a:

Fuse Link

These fuses interrupt current ranges from their minimum breaking current to their rated breaking capacity as specified by the manufacturer.

General Purpose Fuse Link

These fuses will interrupt all currents from rated breaking capacity down to marginally small but continuous overcurrent that accumulates enough heat within an hour to melt the fuse element.

Full Range Fuse

Interrupts any current below rated breaking capacity by total melting of the fuse element.

Recommended U.L Current Limiting Fuse Classes

Recommended U.L. Current Limiting Fuses

Ref. - Cooper Bussmann.

View clear image HERE

Current Limiting Backup Fuses

These fuses have several series arcs and longer element strips to interrupt the high voltage arching when the fuse element opens. The element strip for this fuse could be as long as 1 meter and is wound around a star-shaped ceramic core with the distance of each turn carefully taken into consideration to mitigate possible flash over between them. The wound ceramic core is then then fitted to fuse cartridge long enough to accommodate it.


Non-Current Limiting Fuses

High voltage fuses have two classes classified under international standards. They are:

1. Expulsion fuses - These fuses are for outdoor applications - only - for the protection of high voltage transmission and distribution system. This fuse has a thin element in series with a flexible braid in the fuse cartridge. The fuse is mechanically designed so that the braid is under constant spring tension. When the fuse element melts under heavy fault current, the braid is released activating a latch that will cause the fuse to disengage and fall downward. This mechanical action causes the arc to fully extinguish effectively preventing a restrike.

2. Liquid Fuse Link - The strip element of these fuses is fitted in a fuse cartridge filled with high dielectric liquid with a spring or a strain wire that secures the element under tension. The spring provides immediate separation when the fuse element melts. The high dielectric liquid provides the immediate quenching action. These fuses are the early type of non-current limiting fuses

Power Circuit Breakers

Circuit breakers for medium and high voltage applications are:

1. Oil Circuit Breakers - Arching between the contacts of the circuit breaker is quenched by mineral oil. Arc chambers are use to quench the arc at the shortest time possible. Arc duration last from 0.02 - 0.05 sec.

2. Vacuum Circuit Breakers - The contacts of this circuit breaker are hermetically sealed in a vacuum container with contacts separated by bellows. The arc is produced by metallic vapor boiled from the electrodes and extinguished when the vapor particles condense on solid surfaces.

3.Sulfur hexafluoride or SF6 Gas Circuit Breakers - Circuit breaker technology has greatly changed from 1950's to current time. The SF6 circuit breaker contacts are separated by the SF6 gas. Arc caused by contact separation is interrupted and cooled by the gas. The SF6 gas in the envelop should have the correct pressure in order to achieve this. It is for this reason that SF6 gas in the envelop should be constantly monitored for correct pressure. Ring Main Units are good example of medium voltage SF6 switchgears.

Circuit Breaker Interrupting Rating and Interrupting Capacity

Interrupting Rating

It is the circuit breaker’s maximum short circuit current that can be safely interrupted under test conditions. Under test conditions simply mean it is the breaker’s tested condition.

Properly selecting the interrupting rating of a circuit breaker can bring so many concerns to one specifying it, since there are so many current values to evaluate and consider. However, knowing the RMS (root mean square) symmetrical rating of the system together with percent power factor (%pf) or, alternately X/R ratio and the circuit breaker’s interrupting rating will suffice. Asymmetrical RMS, peak asymmetrical currents are reliant on %pf or X/R ratio and thus can be neglected.

For more information please see Determining Power Circuit Breaker Interrupting Ratings - Eaton Determining Power Circuit Breaker Interrupting Ratings - Eaton


Interrupting Capacity

As defined by IEEE Standard Dictionary of Electrical and Electronic Terms, interrupting capacity is the highest current at rated voltage that the device can interrupt.

Capacitor Trip Device

Pole Mounted Fuse Cutout

Typical installation of Capacitor Trip Device. Source: Time Mark Capacitor Trip Devices .pdf

Capacitor Trip Device

Our topic on circuit breakers would be incomplete without mentioning Capacitor Trip Devices (CTDs), which are DC voltage storage devices used to trip a medium voltage circuit breaker when the control transformer, such as 34.5kV/240V or 4.16kV/240V, etc., connected to the AC primary power supply is loss during a fault, unavailable, or may become lower than the normal voltage level needed to effect tripping of the circuit breaker. CTDs provide a good alternative, since circuit breaker tripping elements are impulse loads that require only a brief electrical pulse for it to actuate. A CTD is composed of series resistors and half-wave rectifier diodes connected to the secondary of a control transformer to the capacitor with the resistors acting as charging current limiters. Capacitor type used is electrolytic type.

The voltage stored in the capacitor is maintained at about 300VDC when connected to a 220VAC secondary of a control transformer, and discharges slowly when the AC power supply is loss. When a tripping signal is received at this stage, the capacitor stored voltage is discharged to the circuit breaker tripping elements. To accomplish this, CTD design must still have sufficient capacity when control voltage falls to its minimum voltage range as defined in ANSI C37.06, and must be capable of effecting a trip 2 seconds after the power supply is removed. CTD manufacturers exceeds this requirement by a good margin by testing their CTDs from 2 to 10 seconds. Some CTDs now have backup batteries. CTDs are also used to actuate the breaker lockout relay Device #86 that's also supplied by the control transformer. This, however would require two CTDs, one for tripping the circuit breaker, and one for locking-out the circuit breaker, since CTDs do not allow multiple loads.

The advantages of CTDs include economy of installation for small MV/LV substations in isolated or remotely located areas where the battery becomes a costly option in terms of initial cost and maintenance and its suitability for outdoor installation. Its disadvantages are: no breaker status indicating lights, no DC power supply for communication equipment; uneconomical for use in large substations; and uses limited life electrolytic capacitors that changes in value over time. For this reason, CTD’s must be checked from time to time, with care, by discharging the capacitor using 500ohms 5 to10 watt resistor across its terminals.


Electric Power System Protection and Coordination by: Michael Anthony - University of Michigan

Applying Interrupting Rating: Circuit Breakers - Cooper Bussman

High Voltage Fuse Technology - Cooper Bussman

Short Circuit Analysis Program ANSI/IEC/IEEE and Protective Device Evaluation - Power Analytics Corporation

Fuse Classes reference: Definitions from Bussman catalog Circuit Protection Solutions 10-00-50M