Industrial Power-System Protection--Fuses (part 1)

Historical

Fuse is the most common and widely used protective device in electrical circuits. Though 'fuseless' concept had been catching on for quite some time, still quite a lot of low voltage distribution circuits are protected with fuses. Further fuses form a major backup protection in medium-voltage and high-voltage distribution to 11 kV, where switches and contactors with limited short-circuit capacities are used.

In 1881, Edison patented his 'lead safety wire', which was officially recognized as the first fuse. However, it was also said that Swan actually used this device in late 1880 in the lighting circuits of Lord Armstrong's house. He used strips of tin-foil jammed between brass blocks by plugs of woods. The application of the fuse in those days was not to protect the wires and system against short circuit, but to protect the lights which cost 25 shillings a time (a fortune in those days). Later, as electrical distribution systems grew, it was found that after short circuits, certain conductors failed. This was due to the copper conductors, not being accurately drawn out (extruded) to a constant diameter throughout the cable length; faults always occurring at the smallest cross-sectional area.

Fuses were often considered as casual devices until not so long ago. The open tin-foil (rewireable) sometimes came in for a lot of abuse. If it blew constantly, then the new fuse was just increased until it stayed in permanently. Sometimes hairpins were used. Greater precision only became possible with the introduction of the Cartridge fuse.

Current-Limiting vs. Expulsion

Current-Limiting Fuses: A current limiting fuse is a fuse that, when its current responsive element is melted by a current within the fuse’s specified current limiting range, abruptly introduces a high resistance to reduce current magnitude and duration, resulting in subsequent current interruption.

Expulsion Fuses: An expulsion fuse is a vented fuse in which the expulsion effect of the gases produced by internal arcing, either alone or aided by other mechanisms, results in current interruption. An expulsion fuse is not current limiting and, as a result, limits the duration of a fault on the electrical system, not the magnitude.

Re-wireable type

As the name indicates the fuse can be replaced or 'rewired' once it fails. Fusible wire used to be contained in an asbestos tube to prevent splashing of volatile metal.

Disadvantages:

1. Open to abuse due to incorrect rating of replacement elements hence affording incorrect protection.

2. Deterioration of element as it’s open to the atmosphere.

Cartridge type

Silver element, specially shaped, enclosed in a barrel of insulating material, filled with quartz. Silver and quartz combine to give a very good insulator and prevent arc from re-striking.

++++ Sectional view of a typical class - GP type 5 Cartridge fuse - link

Advantages:

1. Correct rating and characteristic fuse always fitted to a circuit-not open to abuse as re-wireable type.

2. Arc and fault energy contained within insulating tube-prevents damage.

3. Normally sealed therefore not affected by atmosphere hence gives more stable characteristic-reliable grading.

4. Can operate considerably faster, suitable for higher short-circuit duty:

- Cartridge type can handle 100,000 A

- Semi-open type can handle 4000 A.

Normal currents carried continuously are much closer to fusing current due to special design of element. These fuses are most widely used in electrical systems and are named as HRC (high rupturing capacity) fuses, with the name synonymous with their short circuit current breaking capacity.

Operating characteristics

All fuses irrespective of the type have inverse characteristic ---- graph that follows. Inverse means that they can withstand their nominal current rating almost indefinitely but as the currents increases their withstanding time starts decreasing making them 'blow'. The blowing time decreases as the flowing currents increase. The thermal characteristic or withstand capacity of a fuse is indicated in terms of 'I^2 t' where I is the current and t is the withstand time. The prospective current is the I_rms that would flow on the making of a circuit when the circuit is equipped for insertion of a fuse but that the fuse is replaced with a solid link. The curves are very important when determining the application of fuses as they allow the correct ratings to be chosen to give grading.

++++ Inverse characteristic of fuse

U.S. standards

This standard lays down definite limits of: (a) Temperature rise (b) Fusing factor = minimum fusing current/current rating = 1.4 (c) Breaking capacity.

These are all dependent on one another and by careful balancing of factors a really good fuse can be produced. For example, a cool working fuse may be obtained at the expense of breaking capacity. Alternatively, too low a fusing factor may result in too high a temperature, therefore too close protection and possibilities of blowing are more.

Energy "let through"

Fuses operate very quickly and can cut-off fault current long before it reaches its first peak. Energy (I^2 t) let through by fault of one cycle duration Time HRC Fuse-link cut-off HRC fuse-link duration Fault current one full cycle (0.02 s) Energy (I^2 t) let through by HRC fuse-link I (rms) I (rms) Peak

++++ Energy “let through”: If a fuse cuts off in the first quarter cycle, then the power let-through is I²t. By comparison, circuit breakers can clear faults in any time up to 10 cycles and in this case the power let-through is the summation of I^2 for 10 cycles. The energy released at the fault is therefore colossal compared with that let through by a fuse. Damage is therefore extensive. In addition, all apparatus carrying this fault current (transformers, etc.) is subjected to high magnetic forces proportional to the fault current squared (IF 2 )!