Three-phase power capacitors terminals are directly connected with discharge resistor(2 nos. of resistors for 3-phases, e.i  R-Y-B ). Because when capacitors are off, after 60 seconds automatically capacitor is discharge by resistors.

WHY WE USED DISCHARGE RESISTORS?


When capacitor is switched off, within 60 seconds, capacitor should be discharge to below 50 volt.

  • *This is used to protect our human body from electric shock.
  • **Also used for re-switching the similar capacitor in APFC Panels.

**In a APFC panels, if a capacitor is re-switched without proper discharging, there may be of the voltage difference across the contactor shooting  up to 1 KV. The result is very high peak inrush current as the rate of change of very high voltage, i.e Ic = C dV/dt. Also reduces capacitors, contactors life. It causes power factor failure.

Note:- When capacitor re-switching by manual or automatically, it is mandatory to discharge the capacitor.

More faster discharge also achieved by replacing the existing resistors of  lesser resistance.

By this formula:    R < t / 3Cxlog(√2Vn/Vr).

Where, R= discharge resistance value in ohms.
t= discharging time.
C= capacitance/phase.
Vn= capacitor voltage.
Vr= permissible residual voltage.

***Above this formula applicable for only delta connected capacitors.

 Watt calculation of resistor = 1.3 x V²n / R.