# Fuse Maximum Voltage

## DC Operation of a Fuse.

The rated voltage of a fuse-link must be greater than or equal to the operating voltage of the equipment which is to be protected.

The difference between an applied AC voltage or applied DC voltage to a fuse; The fuse-links are on principle suitable for use at alternating and direct voltage. The breaking capacity at direct-voltage is however considerably lower than the one at alternating voltage. The performance of the fuse-link at direct-voltage mainly depends on the size of the time-constant = L/R of the load circuit.

Glass body fuses seem to have a DC voltage rating half that of a maximum AC voltage rating [in general]. While Ceramic cases appear to have the same maximum voltage rating regardless of the operating voltage, AC or DC.

The Military standard for Fuses; MIL-PRF-23419 does not seem to differentiate between AC or DC voltages for low voltage fuses.

-- From Military sources
Voltage rating. The voltage rating is the maximum dc or ac root mean square (rms) voltage for which a fuse is designed...... Fuse selection: The following steps should apply in the selection of a fuse for any application: Step 1: Select a fuse with a voltage rating equal to or in excess of the circuit voltage.

-- From Ferraz Shawmut
Voltage Rating: The maximum voltage at which a fuse is designed to operate. Exceeding the voltage rating of a fuse impairs its ability to clear an overload or short circuit safely. Fuse can be used at any voltage below the fuse voltage rating; a 250V fuse can be used in 125V circuits. Voltage ratings are assumed to be for AC unless specifically labeled as DC.

-- From Cooperbussmann
Voltage Ratings: The voltage rating of the fuse must be greater than or equal to the circuit voltage. Because the fuse has such low resistance, the voltage rating becomes critical only when the fuse is trying to open. The fuse must be able to open quickly, extinguish the arc after the fuse element has melted and prevent the system open-circuit voltage from restriking across the open fuse element.

### Fuse Aching at DC

The real issue that prompts the question of should a fuse be operated at DC is what extinguishes the arc. As the current increases in a fuse, the fuse link begins to heat up. At some point the fuse link melts and there is a momentary arc between the two end-points of the link. When an AC voltage is applied across a fuse the arc is assumed to extinguish as soon as the polarity of the voltage changes. That is, the arc remains for a maximum amount of time corresponding to half the frequency of the sine wave. In fact the arc should begin to dissipate some time during which the AC voltage is decreasing back to zero, or less than half the frequency. Once the sine wave reaches zero volts there is no potential difference across the fuse to cause an arc.

The argument against DC operation is that there is no decay to zero volts to break the arc. So once an over-voltage or over-current occurs and the fuse link melts, what extinguishes the arc. It's assumed that the arcing voltage remains long enough to make the argument valid, otherwise the arc dissipates on its own soon after it occurs.

### Fuse Derating

Fuses are specified to interrupt within a maximum of 5 seconds when driven at 200% of their rated current for nominal ratings up to and including 10 amperes. A fuse with a nominal rating of 15 amperes is specified to interrupt within a maximum of 10 seconds when driven at 200% of its rated current. The power supply shall be capable of delivering appropriate levels of current to achieve short fusing times. There is an additional derating of 0.2%/°C for an increase in the temperature of fuse body.
Also refer to the topic on How to derate a Fuse which is a closely related topic, but does not attempt to answer the question regarding will a fuse operate the same in a DC circuit or AC circuit.

Derating guidelines for other components; Guideline for Derating Electronic Components.

MIL-PRF-23419 Fuse

Related manufacturers;
Companies making Fuses
Companies making Fuse Holders

Modified 3/17/12