Consideration for selecting fuse links
Coordination between fuses and other devices
a) In this case the current limiting fuse is used as a back-up for the other device. Thus in the event of heavy faults, only the current limiting fuse is required to operate while only the associated device is required to operate in the event of overloads or small faults. This is achieved by choosing characteristics for the fuselink and the other device, so that they produce a composite characteristic of the form shown and this clearly must give sufficient rapid clearance at all current levels to protect the associated circuit adequately.
a = other device
b = fuse
The other criteria which must be satisfied are as follows:
b) The take-over point (A) at which the curves intersect must be at a current level below the breaking capacity of the other device.
c) To deal with cases where the current limiting fuse clears the circuit, the other device must be able to carry the maximum fault current safely and where it may have to close on to a fault, it must have a making capacity adequate for the cut-off and I2t let-through values of the fuselink.
Discrimination and coordination
Overcurrent discrimination is defined as: "co-ordination of the relevant characteristics of two or more overcurrent protective devices such that, on the occurrence of overcurrents within stated limits, the device intended to operate within these limits does so, while the other(s) do(es) not".
Most circuits contain several protective devices and some of these are effectively in series. They must all be co-ordinated so that correct discrimination is achieved under all fault conditions and only the minimum of interruption should occur to clear any fault condition.
The co-ordination of the circuit protective devices PD’s, is affected by their operating characteristics and there are several possibilities which may arise; for example, a network may contain a considerable number of fuses which must be chosen to discriminate or alternatively a fuse may have to operate in series with a circuit breaker which is tripped by a protective relay. These situations are considered separately using the simple network shown below.
Protection by fuses
It is very common to employ the radial system shown and to use a major or upstream fuse in the supply connection (PD4) and minor or down-stream fuses in the individual load circuits (PD1, 2 and 3).
Clearly, each minor fuse must have the time/current characteristic needed to protect its load circuit and a fault on a particular load should only cause its associated minor fuse to operate. The major or upstream fuse (PD4) will also carry the fault current but it must not operate or be impaired.
For faults which cause relatively small currents to flow, the arcing times, as proportions of the pre-arcing times, are small and consequently discrimination can be predicted by comparing the time current curves of the major and minor fuses. Provided that the curves for the minor fuses are to the left of that for the major fuse, i.e. the minor fuses operate more quickly, then discrimination should be obtained.
At higher fault current levels which will result in melting of the minor fuse in less than 100 ms, the arcing time of the minor fuse must be taken into account. This is done not by considering the actual values of time, but by using the I2t values. The requirement is that the pre-arcing I2t of the major fuse shall exceed the total operating I2t of the minor fuse by a reasonable margin (say 40%).
An integrated system protected by fuselinks excel in this application, giving minimum disruption in the system. The standardisation of gG fuselink characteristics ensures that discrimination between fuselinks can be achieved on a 1.6:1 ratio of current ratings for most practical situations. The 1.6:1 ratio represents two steps in the R10 series of ratings, i.e. a 100A (downstream) fuselink will discriminate with a 160A (upstream) fuselink.
The particular case which arises when discrimination has to be achieved between fuses on the two sides of a transformer, the effective transformation ratio needs to be taken into account.
Protection by fuses and other devices
Here the general requirement is similar to that for discrimination between two fuselinks, in that only the downstream device is required to operate. It is this latter device which has to be chosen first, because its time current characteristics must provide the necessary protection for its associated circuit. Thereafter the upstream device must have a characteristic which will ensure discrimination.
In practice, two alternative arrangements are encountered. One in which the upstream device may be a fuse whilst the downstream devices may be small or miniature circuit breakers incorporating overcurrent protective features, and the other in which an upstream circuit breaker and downstream fuses are used.
With the first arrangement there is always an actual or potential upper limit to the fault current at which discrimination can be obtained. This is because the circuit breaker or other downstream device always has a definite minimum operating time resulting from the delays in the overcurrent detection equipment and the circuit breaker itself, plus its own arcing time, of which the latter is not likely to be less than the duration of one half cycle. The operating time of the upstream fuse, on the other hand, decreases continually with increase in current giving an upper current limit at which discrimination can be achieved.
With the second arrangement, there is usually little difficulty in choosing characteristics which enable full discrimination to be obtained.
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.
Protection of motor circuits
Low voltage current limiting cartridge fuselinks are used normally in conjunction with air break contactors to protect 3-phase ac induction motors. The fuses provide the protection against short circuits and must therefore have adequate capacities. The lower currents are cleared by the overload protection associated with the contactors. In these circumstances the rated current of the fuselinks 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 gG fuselinks 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.
The surges are not so great when other methods of starting are employed, and therefore fuses with lower current ratings are used, but these levels again are exceeded during motor starts. Allowance may also have to be made for the high transient currents which flow, with some methods of starting, when transitions are made from one connection to a succeeding one, as occurs, for example, when a star-delta or a rotor-resistor starter is used.
As stated earlier, the fuselink provides short circuit protection and does not provide low overcurrent protection. Compact aM back-up fuses or full range gM fuselinks are thus supplied. These also give economies of size in the associated equipment and are therefore commonly used where large numbers of motor starters, or motor control centres are installed.
For low voltage applications the requirements for contactors and motor starters are given in IEC 60 947-4-1. This includes the co-ordination requirements with short circuit protective devices (SCPD’s). The rated conditional short circuit currents of the contactors and starters backed up by the short circuit protective devices are specified. Two types of co-ordination are permissible: Type ‘1’ and type ‘2’.
Type ‘1’ co-ordination requires that, under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts.
Type ‘2’ co-ordination requires that, under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognised, in which case the manufacturer shall indicate the measures to be taken as regards the maintenance of the equipment.
Clearly type ‘2’ is the preferred co-ordination. In the past decade there have been developments in contactors and motor starters which have required the short circuit protection device to have relatively low values of let-through I2t and cut-off current characteristics. Co-ordination recommendations are made by the manufacturers of motor starters in accordance with IEC 60947-4-1.
The low voltage fuse standards committee of the IEC has undertaken a study into the co-ordination of fuselinks with motor starters and contactors and has found from a survey of tests, that type ‘2’ co-ordination is achieved by using aM, gG or gM fuselinks which have pre-arcing I2t values towards the bottom of limits specified in IEC 60269-1.
This IEC committee produced a technical report IEC 61459 on an Application Guide on co-ordination between fuses and contactors/motor starters. The figure shows the important parameters in the region of the crossover current for successful co-ordination between fuses, overload relay and contactors.
The strong current limiting feature of the fuselink prevents thermal damage to the contactor and its associated overload relay. It also prevents mechanical damage because of the low peak let-through, or cut-off current, of the fuselink. If the contactor/relay were not protected by the fuselink, the electromagnetic forces associated with the fault current would be more likely to cause damage and possible welding of the contacts. The motor starter manufacturers recommend suitable gG or gM fuselinks that provide type ‘2’ co-ordination.
