Principles of fault-current switching

Speaking very generally, a faulty power system is fed by a power source (Fig. 12.5) with an internal impedance Zl, connected to a load, e.g. a motor with impedance Z2. The point of fault shows an impedance (in most cases 

Z1 internal grid impedance Z2 load impedance RF fault resistance T1, T2 protective devices Q1, Q2 isolating switches K1, K2 short-circuiting devices R1, R2 resistances ofK1, K2 11 fault current generated by the grid I2 fault current generated by a reactive load. 

In case of a fault, the current II - from the power source - feeds RF. It should be emphasized that an additional current 12 flows from the (reactive) load to Rp, due to the stored energy at the load side. This aspect will become important if cable protection is achieved. For other purposes, 12 can be neglected. 

Apart from cable protection, only one protective unit Tl, composed of K1 (short-circuit device) and Q1 (isolating device) can meet the requirements as stated above. 

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