Discrimation and Fuses

Discrimation

A circuit fed from a distribution board may be fed through three or even four fuses or circuit breakers e.g. a heating circuit may be connected to a 15amp fuse in a fuse box fed from a section box in turn from a 500A circuit breaker on the main board. Discrimination occurs when the fuses nearest to the fault operates leaving all the other fuses or protective devices intact. Discrimination may be required between fuse and fuse, or between fuse and overcurrent device such as a circuit breaker.

Fuses

A fuse is a protective device which is there to prevent overloading. If too heavy a fuse or if the fuse is overridden then there is a possibility of overheating, deterioration of insulation and failure. Materials used are; Tin, Lead, or silver having low melting points. Use of copper or iron is dangerous, though tinned copper may be used.
Unlike some other forms of circuit protection devices (oil switches for instance), which are suitable for a.c. only, solid filled cartridge fuses have an approximately equal breaking capacity for D.C. and A.C. and the action of the fuse does not depend on breaking circuit at the zero point on the current cyclic wave

Requirements of a fuse;
• Have a high cut off speed on short circuit
• Operate on a sustained overload
• Make a complete break after operation with high insulation resistance between terminals
• Have a maximum temperature rise on continuous full load rating such that it will not be injurious to rudder cables connected to its terminals
• Be mechanically robust and capable of withstanding the large magnetic and gaseous forces exerted during operation under short circuit conditions

A fuse must not;
• Operate on continuous full load
• operate on momentary overload
• Deteriorate under continuous operation at full load
• Burst or emit flame or otherwise damage the fuse carrier and base when it operates

Regulations

I.E.E. and classification society rules now specify high breaking capacity (high category) fuses on main switch boards where the total normal generator capacity exceeds 400kW at 200v, this is for short circuit or low resistance protection of the very high currents that can be generated in these conditions.

In addition;
• Fuses on shipboard must be made to approved standards.
• Breaking capacity not less than prospective short circuit current at point of installation
• Fuses over 300A are not used for overload protection but may be used for short circuit protection
• Cartridge fuses only on tankers

To control the extent of heavy fault currents on large installations the protective device must have a very high speed performance or High Rupture Capacity (H.R.C.) H.R.C. fuses will operate quickly before the short circuit current exceeds 3 times the full load current.

Cartridge fuses

Are capable of handling large short circuits. Because of standardisation of manufacture they have very consistant time/current fusing characteristics making them accurate, dependable and non-deteriorating in service. Suitable filling powders such as silicon sand are used in cartridge fuses having the property of quenching the arc of the fused element.

Enclosed fuses

The element usually made of silver is much smaller than the tinned copper used in semi-enclosed fuses so that the amount of vaporised metal is less and this contributes to a better performance. The enclosed casing and use of silver ensures no degradation due to oxidation. After the silver element has fused the indicator wire will heat up sufficiently to ignite the indicator powder and the fuse will be shown to be blown.
Except in the lowest ratings there are two or more elements in parallel which increase the contact area in contact with the filler, and this increases the breaking capacity. The ends of the element are reinforced by larger wires to reduce resistance and therefore heat losses.
The indicator type should in the construction below consists of an indicator wire which ignites an explosive powder which chars the indicator paper. On other designs the indicator wire releases a spring and pop up indicator

Semi-enclosed fuses

Tinned copper fuse wire exposed to the atmosphere tends to deteriorate and will vary in performance after long periods in service. Also there is a temptation to increase the gauge of the wire, or the number of wires after a fuse has blown. However, rewireable fuses are cheap, easily replaceable, blown fuses are easily detected and within reason if the circuit is uprated slightly no new fuse holders are required.
Tin-fast heating and failure (expensive)
Copper-Slow heating and failure (cheaper)
On overload the tin will fail rapidly increasing the current through the core speeding up its failure.

Rating

Is that current the fuse will carry continuously e.g. for a circuit rated at 30 amp, a 30 amp fuse will be appropriate. Fuses and circuit breakers on switchboards and distribution boards are intended primarily for the protection of the cables and not the apparatus. Overload protection of the apparatus usually provided at the motor starter. The fusing factor = Minimum fusing current/ Current rating

There are three standards

Class P	Fuses protect against relatively small but sustained overloads with fusing factor of 1.25 (25% overload rating)
Class Q	Fuses where protection against relatively small overcurrents is not required, with a fuse factor not exceeding 1.5 for cartridge and 1.8 for semi-enclosed fuses. Motor overload protection to back up motor starter protection
Class R	Fuses require for protection against relatively large overcurrents (e.g. short circuit protection) 3 x Full load current

Minimum fusing current

is affected by length of fuse element. A short element with large terminals, or with special graded construction of wire will have its fusing current raised because of heat conduction away from the element. Alternately, the cross section of element can be reduced for a fuse of a given rating. This method is adopted in some designs of semi-enclosed and filled cartridge fuses.