In addition to the protection devices specific to each capacitor (internal fuses or superchargers), an additional external protection device must be incorporated in the bank.

The most commonly used external protection devices are:

• H.R.C. fuses

• unbalance or differential protection

H.R.C. fuses

Protection with H.R.C fuses incorporated in the bank is suitable (for technical and economic reasons) for capacitor banks with the following characteristics:

• low power (< 1000 kvar)
• equipped with three-phase connection
   capacitors (fig. 1)
• network voltage less than 12 kV

The H.R.C. fuse rating will be selected at a value between 1.7 and 2.2 times the nominal bank current.
H.R.C. fuse blow-outs are generally caused by a direct short-circuit inside the capacitor.

Unbalance or differential protection

This protection generally applies for banks with the following characteristics:

• medium or high power (> 1000 kvar)
• equipped with single-phase connection
   capacitors (fig. 2 or 3)
• network voltage greater than 12 kV

Unbalance or differential protection is sensitive, capable of detecting and reacting to a partial fault in a capacitor.
It is composed of a current transformer connected between electrically balanced points linked to a current relay. A fault in a capacitor produces an unbalance and, therefore, flow of current in the current transformer which, by means of the relay, opens the bank control device (circuit breaker, switch, contactor, etc).