A fuse can be used in an electrical circuit for the following purposes:
Protection of cables
Low-voltage type ‘gG’ fuses are used to protect cables and in this role, they are required to operate over the whole range of over-current conditions. It is clearly desirable that their performance and characteristics should ensure that the cables will not be damaged because of overloading or faults in the circuits they feed and, to this end, rules for the selection and over-current protection of cables have been drawn up and included in national wiring rules or regulations.
The first important factor which must be considered is the current-carrying capacity of the cables to be protected. This is clearly dependent on the conductor and insulation materials and dimensions. In addition, it is affected by the ambient temperature of the environment in which the cables will operate and on the installation arrangements, including the spacing and adequacy of air circulation. The current-carrying capacities of cables under a range of operating conditions have been determined and they are tabulated in the wiring regulations.
Protection of motors
High-voltage and low-voltage current-limiting fuses are used in conjunction with either air-break or vacuum contactors in many countries to protect three-phase AC induction motors up to 2 MW rating operating at line voltages up to 11 kV.
The fuses, as stated earlier, provide protection against short circuits and must therefore have adequate capacities. The lower currents are cleared by the overload protection in the motor starters. In these circumstances, the rated current of the fuses does not need to correspond to the motor rating, and certainly when motors with direct-on-line starting are to be protected the choice of fuse-current rating is dictated by its ability to withstand the motor starting-current surge, typically 5–6 times the full-load current. This usually results in the use of fuses with rated currents up to twice the motor full load current.
Such fuses thus carry up to about three times their rated current during starting periods. Consequently, and because of the low thermal inertia of the fuse elements, they reach temperatures considerably higher than those caused by continuous operation at their rated current. The resulting expansion and contraction would tend to lead to mechanical failures in long fragile elements, after a number of motor starts, and therefore the elements of high-voltage motor fuses are corrugated, to minimize this effect and avoid the necessity of choosing fuses with even higher ratings. The elements of low-voltage fuses tend to be more robust and corrugation has not been found to be necessary.
Protection of soft starters
AC induction motors are frequently used at fixed speeds and traditionally these have been started at full rated voltage, direct online (DOL). This is still very popular up to 7.5 kW. However, DOL starting gives far higher torque than is delivered at full speed and creates a ‘jolt’ in the motor that can result in wear and mechanical damage to the motor, gearboxes, clutches, couplings, transmission equipment, and the load including goods being handled. The high associated start-up currents can also cause significant line voltage dips affecting the power quality of the system. There are other methods of assisted starting, including star–delta, auto-transformer, and pole changing motors. With the advent of economical and reliable power semiconductors, there has been an ever-increasing use of ‘electronic soft starters’. The modern soft starter usually consists of six thyristors arranged in an anti-phase parallel configuration. This is the most common connection method, however for large motors, up to 1000 kW, the thyristors are sometimes connected in the delta circuit thus reducing the thyristor current to 58 percent.
When the soft starter is activated, the thyristors will switch out large parts of the supply voltage, gradually less and less of the supply voltage is switched out until the full voltage is supplied to the motor. Slow ramping up the voltage avoids both current surges and torque transients. The soft starter has adjustments for limiting the starting current and setting the ramp-up time. Typically starting current is reduced to 300 percent of the full load current with ramp-up times approaching 30 s.
Soft starters can also cover torque control starting, low-speed jogging, kick-starting, soft stopping, and breaking. In addition, they can provide remote communication. In some designs, the thyristors are bypassed via a contactor when the motor is run up to speed so that the mains voltage is applied directly to the motor and the thyristors are not in continuous operation reducing the size and increasing efficiency.
Like their electromechanical counterparts, electronic soft starters need to be protected against high over-currents with a fuse. Power semiconductors can be damaged by high over-currents and special fast-acting fuses are required.
The fuses are normally fitted in the input supply lines. The current rating of the fuse selected is often governed by the repetitive duty of the motor, taking into account the cyclic loading factor for the fuse.
Protection of power transformers
Both step-up and step-down power transformers are used in power systems but almost all of the former types are used in conjunction with the alternators in generating stations to form generator–transformer units or they are used to interconnect main transmission networks. In both these applications, differential-type relay protective schemes are employed because of the VA levels involved. Step-down transformers are much more widely used and there are many in the distribution networks which rely on fuse protection on both the high-voltage (supply) side and low-voltage (load) side.
The normal purpose of the fuses on the lower-voltage side of a transformer is to protect the load circuits connected to the secondary windings and in these circumstances, the characteristics of the fuses have to be chosen suitably to match those of the loads and connecting cables. If, however, there is an interconnection in the lower-voltage network because of the parallel connection of transformers, then the secondary-circuit fuses may carry currents being fed back into a transformer in the event of a fault within it, and fuses capable of operating under such conditions, as well as providing protection for the loads, must be chosen.
The fuses on the higher-voltage side of a transformer must isolate it if a fault occurs within it, and this must be done with the minimum disturbance to the system and without causing unnecessary loss of supply to the healthy parts of the system.
Protection of capacitors
For low-voltage power-factor-correction applications, it is usual to install a single capacitor in each phase. Simple recommendations, associated with the protection of these capacitors, are usually provided by fuse manufacturers. These are based on service experience and take into account the high transient inrush currents, the possible harmonic content of the currents, and the capacitor tolerances.
In large installations at higher voltages, capacitor banks are made up of individual capacitors connected to form a number of separate units. For three-phase applications up to 11 kV (line) and 1 MVAr, the phases may be star- or delta-connected and the units are connected in parallel. For higher-voltage systems the phases are star-connected and the units in each are connected in parallel.
A practice widely adopted is to fuse each individual capacitor element in the units. The fuses used for this purpose contain simple wire elements with the appropriate low-current rating and breaking capacity.
The alternative practice is to fuse each unit as a whole, although frequently a line fuse for each phase is also included for the smaller banks used at voltages up to 11 kV. A unit fuse should operate if its associated unit becomes faulty, leaving the remainder of the bank in service.
As with other applications, the requirements are that a fuse should operate as quickly as possible in the event of a fault but also be able to carry load current and transient overcurrents. The latter arise in the event of a sudden change in the voltage across a bank, a situation that arises on connection to the supply, or in the event of a system fault which affects the network voltages. To prevent operation under this condition, it is usually necessary to use fuses with a current rating considerably higher than the normal capacitor current.
If a unit develops a short circuit, a discharge will occur within it and current will flow into the unit from the supply and from other healthy units. Clearly only the fuse associated with the faulty unit should operate in these circumstances.
Protection of semiconductor devices
Semiconductor power diodes were first marketed in 1953. It was realized from the outset that these devices had very limited overload capacities and, as they were expensive, the fuse manufacturers attempted to produce fuses that were more sensitive to overloads and which would operate more quickly than their conventional designs. As a result, the first applications were filed in 1955 for patents on fuses specifically designed to protect semiconductor rectifiers (e.g. US Patent 2 921 250, 13 June 1955). The invention of the thyristor and the subsequent rapid expansion of the power electronics industry which it initiated, made the need for semiconductor fuses even more apparent.
Today, semiconductor devices are being manufactured with maximum continuous current ratings up to 15 kA and peak inverse voltages of 7 kV. Unfortunately, the devices still have poor overload capacities and continue to need sensitive and fast-acting protection.
The introduction of the semiconductor devices presented a new situation to the fuse manufacturers in that previously the fuse elements were used to protect metallic conductors, as for example the winding of a motor, and these had similar properties to the metallic elements making it relatively easy to match the characteristics.