Page 685 - Industrial Power Engineering and Applications Handbook

19/650 Industrial Power Engineering and Applications Handbook
the contacts. The C of the contacts will change with the
travel of the contacts and the deionization of the arc
plasma. The influence of the inductance of the circuit L,
or V, = I,, J$ 2- I,, Z, can be reduced by introducing some capacitance or
resistance into the circuit. Capacitance can be introduced
We represent this through a circuit diagram (Figure 19.30) by installing a few p.f. correction capacitor banks across
where the motor or the transformer terminals and the resistance
is the surge impedance, Z,, L and C the circuit can be introduced temporarily across the moving contact
of the interrupting device through its closing mechanism.
constants of the interrupting circuit, as discussed in This is a practice adopted by leading manufacturers to
(Section 17.6.4). C represents the dielectric capacitance provide a resistance where the interrupter is required to
between the parting contacts of the interrupter. V, must operate under adverse switching conditions.
be prevented, as far as practicable, from reaching
dangerous levels with the use of surge arresters.
In( of equipment cannot be altered once it is manu-
factured. The surge voltage V,, that will appear across 19.9 Comparison of interrupting
the contacts therefore becomes a function of the induc- devices
tance, L, of the circuit being interrupted. Lis the inductance
of the motor or the transformer windings up to the
terminals of the interrupting device, including the To assist in making an easy selection of an interrupting
inductance of the interconnecting cables or the bus system. device for a particular application, we have provided a
The dielectric strength is the capacitance, C, between comparison between the various devices as in Table 19.1.