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Ck The various current values at the fault point are calculated using: c The equations provided c A summing law for the currents flowing in the branches connected to the node: v I’’k (see Fig. C6 for I’’k calculation, where voltage factor c is defined by the standard; geometric or algebraic summing) v Ip = κ x 2 x I’’k, where κ is less than 2, depending on the R/X ratio of the positive sequence impedance for the given branch; peak summing v Ib = µ x q x I’’k, where µ and q are less than 1, depending on the generators and motors, and the minimum current interruption delay; algebraic summing v Ik = I’’k, when the fault is far from the generator v Ik = λ x Ir, for a generator, where Ir is the rated generator current and λ is a factor depending on its saturation inductance; algebraic summing.

This type of HV service is very common in rural areas. Protection and switching devices are remote from the transformer, and generally control a main overhead-line, from which a number of these elementary service lines are tapped. Ba ck Fig. C11 : Single-line service Ring-main service Underground cable ring main Fig. C12 : Ring-main service (1) Copper is cathodic to most other metals and therefore resists corrosion. (2) A ring main is a continuous distributor in the form of a closed loop, which originates and terminates on one set of busbars.

The equipotential criterion to be respected is that which is mentioned in Chapter F dealing with protection against electric shock by indirect contact, namely: that the potential between any two exposed metal parts which can be touched simultaneously by any parts the body must never, under any circumstances, exceed 50 V in dry conditions, or 25 V in wet conditions. Special care should be taken at the boundaries of equipotential areas to avoid steep potential gradients on the surface of the ground which give rise to dangerous “step potentials”.