Is a miniature fuse suitable for both AC and DC operation?

When a short-circuit current flows through a fuse, the internal fuse element melts. Depending on the applied voltage and the strength of the short-circuit current, an arc occurs and the current flow continues – and the short circuit thus survives longer than it should. With alternating current (AC) applications, the arc is extinguished again with the zero crossing of the current.

In a direct current application (DC), however, there is no zero crossing. As a result, if the permissible DC rated voltage is exceeded, the arc can continue between the two fuse caps, burn holes in them or even cause the fuse body to burst. Metallised material then escapes from inside the fuse link. In a serious case, this can cause new short circuits and thus charring on the circuit board, and, in the worst case, a fire.

The actual behaviour of fuse links in disconnecting direct current depends on their construction and characteristics.

• For use with low voltages (up to approx. 65V DC), a wide choice of fuses is available. This is because, in this voltage range, only low-energy arcs form, allowing for safe working with very small builds.
• For voltages higher than 100V, fuses generally come with a ceramic body and corresponding filling. The voltage at which they correctly disconnect depends on the size, material, rated current and characteristics.
• Very quick acting fuses, because they are designed purely for short-circuit protection, are often suitable for use with high DC voltages.
• Time-lag fuses with the same build, however, are unable to even approach these values.

Magnitude of the DC voltage:

With some products, the DC voltage rating will be listed with a higher value than the AC rating. Why? A DC compatible fuse link can extinguish the arc, but will be loaded with the peak value in an AC application. The peak voltage values thus play a major role.

Time constant of the short-circuited circuit:
The greater the time constant, the lower the value of the DC voltage to be used with the fuse link. The time constant is derived from L/R, taking into account the reactances of the components in the short-circuit section. In most applications, the time constant is no more than 10ms, in battery circuits less than 3ms.
• Time constants of around 100ms can therefore lead to the rated AC voltage value having to be reduced by more than 50% for use in the DC voltage circuit.
• The maximum DC voltage, by contrast, can be higher than the rated AC voltage value of the fuse link due to the oscillation behaviour in the DC link of a frequency converter.

The performance characteristics of the fuse links from manufacturer SIBA vary greatly from type to type. Measuring 6.3 x 32mm, they can be used for short-circuit protection at up to 1000V DC, even at low rated currents.

Fuse links are typically used in the DC circuits of emergency power supplies and emergency lighting systems. While the systems, in normal operation, run with mains voltage of 230V AC, a fuse that can safely disconnect direct currents is required for emergency power operation, when power is supplied by a powerful central battery. As this fuse cannot simply be swapped out when necessary, it must be designed for both DC and AC operation.

In addition, to avoid damage, the level of the possible short-circuit current must be taken into account – this can amount to 6000A or more. Not every fuse is capable of properly disconnecting such a high DC current.

At Schukat electronic, we offer a wide range of reliable, high-quality miniature fuse links from SIBA direct from stock.

For further information and questions, please contact our sales team.