Miniature fuselinks are not generally called upon to protect cables. High-voltage fuselinks and those for the protection of semiconductor devices usually provide only back-up protection, i.e. they only clear high currents, when used to protect items of equipment and the associated cables. Low-voltage type ‘gG’ fuselinks are used
extensively, however, 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. IEC Publication 60364 deals with Electrical Installations in Buildings. In the UK, the Institution of Electrical Engineers (IEE) produces the Regulations for Electrical Installations. The 16th Edition, which was published in 1991, is based on IEC Publication 60364 and became BS 7671 in 1992.
The 16th Edition of the IEE Regulations includes the requirements for the over-current protection of cables and these will be met in Britain and in other countries which follow IEC practice. In these regulations, the term ‘over-current’ covers both short-circuit currents and overloads, an overload being defined as an over-current which flows in a circuit which is perfectly sound electrically. Clearly an overload can occur, for example, if a motor is stalled or caused to run slowly because of the torque required of it.
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 ambi-ent temperature of the environmambi-ent 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 referred to above.
To avoid damage it is essential that the maximum sustained current (IB)carried by a cable should be less than or equal to its current-carrying capacity (IZ), i.e.
IB≤ IZ
To allow the maximum sustained current to flow, the fuse must have an equal or higher rated current Inand to provide adequate protection the fuse rating should not exceed the current-carrying capacity of the cable; therefore:
IB≤ In≤ IZ
A cable can carry currents above its current-carrying capacity IZfor limited periods and to give ideal protection, the protecting fuselinks should operate, at any current level, within the period for which the cable can carry that current. Strictly, to elim-inate all possibility of damage, the minimum fusing current of a fuse should be just below the current-carrying capacity of the protected cables so that the time/current characteristics will nest as shown in Figure 7.11. This would require, for example, the installation of cables able to continuously carry currents up to 50 per cent more than the fuse rating, if the fuses had a fusing factor of 1·5 and, of course, as stated above, the maximum-sustained current would not have to exceed the fuse rating.
Such an arrangement could be unacceptably costly and, in practice, slight risks are taken. The limiting factor is the temperature reached by the insulation as a result of conductor heating during overloads of fairly long duration, of the order of 1 h or more, depending on the thermal-time constant of the cable. In general this period
time b
a
In Imfc
current
Figure 7.11 Cable and protective fuse characteristics Where
a cable-withstand characteristic b fuse-operating characteristic Imfc minimum fusing current of fuselink In rated current of fuselink
increases with cable size and current-carrying capacity. The regulations, which are intended to ensure that the life of the insulation is not significantly shortened, specify that the minimum-operating current of the protective devices should be equal to or less than 1·45 times the current-carrying capacity of the cable (i.e. 1·45IZ).
In order to verify that ‘gG’ fuselinks are capable of protecting cables against overload, a conventional cable-overload-protection test has been introduced into IEC 60269-1. The fuselink must operate when subjected to an overload current of 1·45 times the rated current of the cable it is to protect (1·45IZ), the value of IZbeing taken from a common installation condition. In practice, several fuselinks will be fitted into an enclosure so that the 1·45 IZcondition will be met even when the fuse and cable ratings are equal (In= IZ).
Semi-enclosed fuselinks require about twice their rated current to give operation in periods of one hour or more and therefore to meet the 1·45 condition:
In≤ 1·45 2 IZ
This means that larger cables may have to be used if semi-enclosed fuselinks are used for cable protection rather than cartridge fuselinks.
The wiring regulations are not framed to allow for frequent overloads because such over-currents could shorten the life of the cable dielectric. Advantage is not therefore to be taken of the overload capacity of cables and, if frequent overloads are likely to occur, then these currents should be regarded as the normal current of the circuit and correspondingly larger cables should be installed.
When the fuselinks are selected on the above 1·45IZ basis, the shape of the time/current characteristics ensures that the cables are adequately protected at higher over-currents.
In those applications where the low-voltage fuselinks are to provide back-up or short-circuit protection to the cables, then co-ordination must be ensured by providing fuselinks which let through I2tvalues lower than those which can be withstood by the cables. For fault durations of 5 s or less the I2twithstand of cables may be determined from the expression
I2t = K2a2
in which a is the cross-sectional area of the cable conductor in square millimetres and Kis a factor which depends on the conductor material and the limiting temperature which can be withstood by the insulation. Values of K for various conductor and insulator combinations are given in the current edition of the IEE Regulations. The values range from 76 for aluminium conductors insulated with PVC material to 143 for copper conductors with 90◦C thermosetting insulation.
Co-ordination is normally checked using the fuselink I2t value associated with operation in 5 s.
It will be noted that the I2t withstand of the cable is not affected by the duration of the short circuit. That of the fuselink does increase with operating time, however, and therefore correct operation can be assured by checking that the fuselink I2tvalue associated with interruption in 5 s is lower than the cable-withstand value.
The requirements for the selection of fuses for the protection of conductors are found in the North American wiring regulations and are summarised below and taken from IEC TR 61818.
(a) The voltage rating of the fuse is selected to be equal to or greater than the maximum system voltage.
(b) The load current is calculated and multiplied by 1·25 for continuous loads (continuous loads are those which are present for 3 h or more).
(c) The conductor size is selected from an ampacity (current-carrying capacity) table found in the wiring regulations.
(d) The general rule for selecting the fuse is to select a standard fuse current rating to coincide with the conductor ampacity. For conductor ampacity less than 800 A, if the conductor ampacity falls between two standard fuselink current ratings, the larger fuselink current rating is used. For conductor ampacities of 800 A and over, if the ampacity falls in between two standard fuselink current ratings, then the smaller fuselink current rating is used.
(e) The fuse is selected to protect the conductor under short-circuit conditions.
In practice, North American cable standards have been co-ordinated with fuse standards so that short-circuit protection is achieved. For other types of conduc-tors, short-circuit withstand ratings are compared with the fuse characteristics to make sure that conductor damage does not occur.