Protection of cables

Low voltage fuselinks with standardised gG characteristics are used to protect cables. The rules for the selection and overcurrent protection of cables have been drawn up and included in national or international wiring rules or regulations. IEC Publication 60364 deals with Electrical Installations in Buildings.

In these regulations, the term "overcurrent" covers both short circuit currents and overloads, an overload being defined as an overcurrent 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 insulation materials 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 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).

To allow the maximum sustained current to flow, the fuse must have an equal or higher rated current. In and to provide adequate protection the fuse rating should not exceed the current carrying capacity of the cable.

A cable can carry currents above its current carrying capacity Iz for limited periods. 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. 1.45 Iz).

In order to verify that gG fuselinks are capable of protecting cables against overload, a conventional cable overload protection test has been introduced into the fuse standard IEC 60269-1.

When the fuselinks are selected on the above basis, the shape of the gG time current characteristics required in IEC 60269-1 ensures that the cables are adequately protected at higher overcurrents.

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 with let through I2t values lower than those which can be withstood by the cables. For fault durations of 5 s or less the I2t withstand of cables may be determined from the expression:

I2t = K2 a2

In which a is the cross sectional area of the cable conductor in square millimetres and K is 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 regulations. The values range from 76 for aluminium conductors insulated with PVC material to 143 for copper conductors with 900C 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 I2t value associated with interruption in 5 s is lower than the cable withstand value.