8. System Protection - General

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8.1 Throughout the installation every circuit breaker and fuse shall be suitable for operation at the maximum short-circuit current which can pass through it, except that a circuit breaker or fuse of a breaking capacity less than the prospective short-circuit current at the point of application where it is installed may be used, provided that it is backed up by:

1. a calibrated fuse of suitable category of duty; or
2. a circuit breaker with an instantaneous trip setting of not more than 90% of the breaking capacity of the device protected and the back-up protective device shall have a breaking capacity not less than the prospective short-circuit current at the point where it is installed and the time-current characteristics shall be suitably co-ordinated.

8.2 The making capacity of every circuit-breaker or switch intended to be capable of being closed, if necessary, on short-circuit, is to be not less than the maximum value of the short-circuit current at the point of installation; on alternating current this maximum value corresponds to the peak value allowing for maximum asymmetry.

8.3 Overload and short-circuit protection shall be so arranged that effective discrimination is achieved for all essential services.

8.4 For the evaluation of the prospective short-circuit current in ac systems, the equivalent system impedance shall be considered, seen from the point of fault.

8.5 The maximum asymmetrical RMS short-circuit current shall be assumed to be equal to the sum of the generator contribution based on circuit impedances including direct axis sub-transient reactances of generators, and the motor contribution based on 4 times the rated current of motors.

8.6 The average asymmetrical RMS short-circuit current shall be assumed to be equal to the sum of the generator contribution based on circuit impedances including the direct axis sub-transient reactances of generators and the motor contribution based on 3.5 times the rated current of motors.

8.7 For ac systems, the maximum available short-circuit current shall be determined from the aggregate contribution of all generators that can be simultaneously operated in parallel and the maximum number of motors which will be in operation simultaneously with a three-phase fault on the load terminals of the protective device; under these conditions, three-pole breakers shall be selected on the basis of the average asymmetrical RMS value of the currents in the three phases; fuses shall be selected on the basis of the maximum asymmetrical RMS value of the current occurring in any one of the three phases; the asymmetrical RMS values of current can be obtained by applying the K1 and K2 factors of Figure 8-1 to the symmetrical values or by using the values of Table 8-1; the X/R ratio of Figure. 8-1 is determined from the inductive reactance (X) and the resistance (R) of the circuit under consideration; for a preliminary estimate of short-circuit currents, pending the availability of generator reactances, the following may be used for estimating the generator contribution:

1. maximum asymmetrical RMS current, 10 times generator full load current;
2. average asymmetrical RMS current, 8.5 times generator full load current; and
3. these values for estimating generator contribution shall not be used where unusually stringent transient voltage dip limitations have been specified for the generator.

8.8 For the evaluation of the maximum short-circuit current on dc systems, it shall be assumed that each generator which can be simultaneously operated in parallel will, if limited only by internal resistance, contribute ten times the normal rated current and that all motors which may be in operation simultaneously will contribute six times their combined normal ratings.

8.9 Where provision is made to obtain an external supply from on-shore or elsewhere, the interrupting capacity of the shore connection protective device shall be based on the maximum short-circuit current available from the external source.

8.10 Fuses shall be so applied that single-phase operation of any three-phase connected ac motor will be precluded; where overload protection is not designed to provide single-phase protection additional means of single-phase protection is required.

Table 8-1: Asymmetrical Factors

Figure 8-1: Fault Current Decreement Converstion Factors

X/R ratio

K₁ Ratio of the average asymmetrical RMS current in the three phases at one half cycle to the RMS value of the symmetrical current.

K₂ Ratio of the maximum RMS asymmetrical current in one phase at one-half cycle to the RMS value of the symmetrical current.

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