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Properly fused - equipment protection fuses in DC applications: Is a device protection fuse suitable for both AC and DC operation? And what should be considered for overcurrent protection in DC applications?
The problem
When a short-circuit current flows through a fuse, the fusible conductor inside melts. Depending on the voltage applied and the level of the short-circuit current, an arc forms here, via which the current flow is maintained. As a result, the short circuit persists longer than intended. In alternating current (AC) applications, the arc is extinguished again with the help of the zero crossing of the current. In a direct current (DC) application, on the other hand, this zero-crossing is missing - the result is that the arc can stop between the two fuse caps if the permissible DC rated voltage is exceeded, burn holes in them or cause the fuse body to blow. Metallized material then escapes from inside the device protection fuse, which in an emergency can cause new short circuits and thus charring on the circuit board and, in the worst case, a fire.

Suitable for direct currents
The actual behavior of G-fuses when switching off DC currents depends on their design and characteristics.

- There is a wide selection for use with extra-low voltages up to about 65V DC. The reason is that in this voltage range only low energy arcs occur, so safe operation is possible even with very small designs.
- For voltages greater than 100V, fuses with a ceramic body and corresponding filling are usually used. At which driving voltage these properly switch off depends on the size, material, rated current and characteristics.
- Superfast (FF) fuses are often suitable for high DC voltages because they are designed for pure short-circuit protection.
- Inert (T) fuses of the same design do not come close to achieving these values.

Influence on breaking capacity
Level of DC voltage:
For certain products, the DC rated voltage is specified with a higher value than the AC rated voltage. Why? A DC rated G-fuse can extinguish the arc, but will be loaded with the peak value in an AC application. Thus, the peak voltage values play a major role.

Time constant of the short circuit:
The larger the time constant, the lower the value of the DC voltage for the use of the G-fuse link. The time constant is determined from L/R, taking into account the reactances of the components located in the short-circuit branch. In most applications, the time constant is no more than 10ms, and less than 3ms in battery circuits.
- Thus, time constants of about 100ms can result in the need to reduce the value of the rated AC voltage by more than 50% for use in the DC circuit.
- The maximum DC voltage, on the other hand, can turn out to be higher than the rated AC voltage (rms value) of the G-fuse link because of the oscillation behavior in the DC link of a frequency converter.

► Tip: Performance behavior of G-fuses
The performance of the equipment protection fuses from the manufacturer SIBA varies greatly depending on the design. In the dimensions 6.3x32mm, these can already be used at low rated currents for short-circuit protection at up to 1000V DC.
Application areas
G-fuses are typically used in the DC circuits of emergency power supplies and emergency lighting systems. While the systems are operated with a mains voltage of 230V AC in normal operation, a fuse that can safely switch off DC currents is required for emergency power operation and supply by a powerful central battery. Since this cannot be replaced when required, it must be designed for both DC and AC operation.

In addition, to prevent damage, the level of possible short-circuit current must be taken into account here, which can be 6,000A or more. Not every fuse is capable of properly disconnecting such a high DC current.

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