Current-Limiting Fuse Selection for Medium Voltage Capacitors 

Introduction
Metal-Enclosed Capacitor Banks and Harmonic Filter Banks utilize current limiting fuses, sometimes called Silver-Sand Fuses, for their protection.  Current limiting fuses (as opposed to expulsion fuses) are required and are utilized, as they do not emit ionized gases during operation.  Ionized gases are undesirable because they can cause bushing and insulator flashovers that result in additional damage.

This document provides information on the selection of Medium Voltage Current Limiting Fuses for the protection of Single-Phase Wye-Connected Capacitors utilized in Metal Enclosed Capacitor Banks.

Background
When selecting current limiting fuses for the protection of Single-Phase Wye-Connected Capacitors it is important to account for, and to consider the following:

• Maximum continuous current the fuse will see during normal operation. Factors such as capacitor tolerances, harmonic currents, system operating voltage, and blown fuses within a bank can all lead to higher than normal operating current.

• Inrush and outrush currents associated with capacitor bank energization.

Based on the above information it is important that the design engineer select a fuse that is small enough (or sensitive enough) to prevent case rupture, yet large enough to prevent spurious or false fuse operation due to normal operating conditions.

Fuse Selection Table
Table 1 below shows the recommended current-limiting fuse voltage and current rating for industry standard capacitors. The table should only be used for guidance and is based upon the following assumptions:

• The current limiting fuse type is full range

• The capacitor neutral voltage will not significantly shift away from 0-Volts

• The ambient temperature within the enclosure does not exceed 25 Degrees Celsius

• The capacitor is operated at its voltage rating

Depending upon the fuse manufacturer, temperature derating may be required.  The following table does not account for temperature derating. For banks in which the neutral voltage can shift away from 0 volts, the fuse voltage should be equal to the system line-to-line voltage. Such applications typically involve small ungrounded banks and harmonic filter banks.

 

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