IEC 60269 is the overall standard for low voltage fuses. The fuses are available in various designs, for example
Cylindrical fuses
Regardless of design the time/current characteristics are equivalent. It is possible to replace an NH fuse with a BS88 fuse, provided that the fuse rated voltage and breaking capacity are higher than in the circuit. The operating class is denoted with two letters, the first refers to usage class, and the second to the object to be protected.
Current limiting fuse designed for cable, line and general protection, full range protection. The fuse has a time lag in the lower overcurrent range and is quick acting in the short circuit range. Replaces older gL fuses.
Current limiting fuse designed for motor short circuit protection, partial range protection. The motor circuit must have an additional protective device for the lower overcurrent range, for example a thermal overload relay.
Current limiting fuse designed for motor protection, full range protection. Only available designed according to BS 88-2 and BS 88-6. Rated current is stated in dual values, thermal current rating and a higher value that relates to the fuse’s ability to withstand starting currents. A fuse with rated current 63M100 can carry up to 63 A continuously and withstand starting
currents corresponding to a 100 A class gG fuse.
Current limiting fuse designed for overload protection of power transformers and short circuit protection of bus bar systems. The fuse is coordinated for discrimination with the high voltage fuses of the power transformer.
Current limiting fast acting fuse. Mainly intended for utility network protection in rural areas based on aluminium cable runs.
Discrimination in a distribution system means that fuses installed in series are coordinated in a way that only the fuse immediately upstream the fault trips. The condition for discrimination is that clearing time (melting+arcing time) of the smaller fuse is shorter compared to the melting time of the larger fuse further upstream. Operating class gG fuses offer discrimination to other gG fuses with a ratio 1,6:1 ie rated current for upstream fuse must be 1,6 times larger.
Compared to AC circuits, the absence of natural voltage zero in a DC circuit makes it more difficult for a fuse to interrupt the current at fault. It is especially notable in circuits with high time constant. Most fuses can be used in DC circuits, see technical data. Contact CHS Controls for application assistance.
