Medium voltage fuses

Medium voltage fuses generally have ratings that range from 2.5 kV to 38 kV and are designated under one of three ANSI/IEEE C37.40 defined classifications: 

General Purpose Current-Limiting: 

A fuse capable of interrupting all currents from the rated interrupting current down to the current that causes the fusible element to melt in one hour 

Back-up Current-Limiting: 

A fuse capable of interrupting all currents from the maximum rated interrupting current down to the rated minimum interrupting current 


A vented fuse in which the expulsion effect of gasses (produced by the arc and housing, either alone or aided by a spring) extinguish the arc 

The general purpose and back-up current-limiting fuses are constructed in a sealed, non-venting design that, when the element melts from a current within the fuse’s interrupting rating, produces arc voltages exceeding the system voltage which, in turn, forces the current to zero. The arc voltages are produced by a series of high resistance arcs within the fuse’s element to create a fuse that typically interrupts high fault currents within the first 1/2 cycle. The expulsion fuse, in contrast, depends on the interruption process being initiated by a single arc that acts as a catalyst to create and cause a de-ionizing gas to escape from its housing. The arc is then elongated, either by the gases’ force or a spring so that, at some point, the arc elongates sufficiently enough to prevent a restrike after the AC current cycle passes through zero volts and may take many cycles to clear. 


Many rules for applying expulsion and current-limiting fuses are the same, with some additional rules applied to current-limiting fuses because they operate much faster on high fault currents. The three basic factors to consider when applying any medium voltage fuse are: 

• Voltage rating 

• Continuous current carrying capacity 

• Interrupting rating 

Voltage rating 

As a rule, medium voltage fuses should be applied on systems as close to their voltage rating as possible (unlike low voltage fuses that can be applied on a system at or below their rating). This is particularly important with current-limiting fuses that function by creating multiple high resistance arcs that will drive up the fuse’s peak arcing voltage. The arcing voltage should never exceed the system basic insulation level (BIL) and create a safety hazard. 

Continuous current carrying capacity 

Continuous current values shown on the fuse label represent the continuous current the fuse can carry without exceeding the temperature rise specified in ANSI C37.46. An application that exposes the fuse to a current slightly above its continuous rating, but below its minimum interrupting rating, may cause damage to the fuse from excessive heat. This is the main reason motor circuit protection uses overload relays in series with back-up current-limiting fuses. 

Interrupting rating 

As with all fuses, medium voltage fuses need to have an interrupting rating equal to or greater than the available fault current.

Rules for medium voltage current-limiting fuses 

To ensure proper current-limiting fuse application, it’s important to apply the following: 

• As stated earlier, current-limiting fuses produce arc voltages that exceed the system voltage. Care must be taken to ensure the peak voltages do not exceed the system’s basic insulation level (BIL). If the fuse voltage rating is not permitted to exceed system voltage by 140%, there should not be a problem. This does not mean that a higher rated fuse cannot be used, but points out that one must ensure the system’s BIL will handle the peak arc voltage produced.

 • As with the expulsion fuse, current-limiting fuses must be properly coordinated with other system OCPDs. For this to happen, the rules for applying an expulsion fuse must be used at all currents that cause the fuse to interrupt in 0.01 second or greater. 

When other current-limiting OCPDs are on the system, it becomes necessary to use I2 t (the thermal energy required to melt a specific fuse element at rated current under test condition, expressed “current squared times seconds” or as A2 s “amps squared times seconds” values for coordination at currents causing the fuse to interrupt in less than 0.01 second. These may be supplied as minimum and maximum values, or minimum melting and total clearing I2 t curves. In either case, apply the following: 

• The fuse’s minimum melting I2 t should be greater than the downstream current-limiting device’s total clearing I2 t.

 • The fuse’s total clearing I2 t should be less than the upstream current-limiting device’s minimum melting I2 t.

Applying R-Rated fuses 

The current-limiting fuse should be selected so that the overload relay’s curve crosses the fuse’s minimum melting curve at a current greater than 110% of the motor’s locked rotor current. 

A preliminary choice is obtained through the following formula:

 6.6 x Full Load Current /100 = R rating of fuse  

This value is rounded up to the next R-Rated fuse size. 

Example: A 2300 V motor has a 100 amp full load current rating and 600 amps locked rotor current. The preliminary choice is

 6.6 x 100 /100= 6.6 

When rounded up to the next standard R-Rated size, it becomes a 9R fuse, but this must be checked against the appropriate time-current characteristics curves, shown in the figure below.

Special care must be taken to ensure the fuse amp rating is compatible with the motor’s overload relay.

Special care must be taken to ensure the fuse amp rating is compatible with the motor’s overload relay. The overload relay in this example has the time-current characteristic shown in the Figure above. To ensure the proper fuse is selected, one must plot 110% of the locked rotor current and the range (6R, 9R, 12R) of R-Rated fuses on the same graph as the overload relay. The selected fuse should be the smallest whose minimum melting characteristic crosses the overload relay at a current greater than 110% of the motor’s locked rotor current. 

In this example, it would be a 2400 V 9R fuse determined in the given formula. This agrees with the quick selection choice. Depending on the type of installation and starter being used, a JCK-9R, JCK-A-9R, or 2BCLS-9R would be the correct choice. 

Additional rules 

When choosing an expulsion fuse, it’s important that it be properly coordinated with other upstream and downstream OCPDs. To accomplish this, one must consider the devices’ melting and clearing characteristics. Two curves, the minimum melting and the total clearing curve, provide this information. To ensure proper coordination, the following should apply:

 • The total clearing curve for any downstream protective device must be below a curve representing 75% of the applied fuse’s minimum melting curve 

• The total clearing curve for the applied fuse must lie below a curve representing 75% of the minimum melting curve for any upstream protective device

R-Rated medium voltage fuses and motor circuits 

R-Rated medium voltage fuses are back-up current-limiting fuses used in junction with medium voltage motors and controllers. These fuses are only for short-circuit protection and do not protect themselves or other components during extended overloads. They offer a high level of fault current interruption in a self-contained, non-venting package that can be mounted indoors or in an enclosure. R-Rated fuses come with open fuse indication, with some available with a Hookeye option for use with a hookstick for non-loadbreak isolation. R-Rated fuses do not have “amp ratings,” but rather an “R” rating when they meet the following requirements: 

• The fuse will safely interrupt any current between its minimum and maximum interrupting rating 

• The fuse will melt in a range of 15 to 35 seconds at a value of 100 times the “R” number (ANSI C37.46). 


Medium voltage motors are efficiently protected by overload relays applied in conjunction with back-up current-limiting fuses that open the circuit under high fault conditions. The overload relay is chosen to interrupt currents below the fuse’s minimum interrupting rating. Since multiple devices are used to provide motor protection, it is very important that they be properly coordinated. The motor starter manufacturer typically designates the proper fuse R-rating, overload relay and contactor.

This article has been extracted from the "Overcurrent protection fundamentals"  by EATON, this free guide can be downloaded below.

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