Short Circuit Current Rating and Follow Current Interrupt Rating

Short Circuit Current Rating and Follow Current Interrupt Rating

Short circuit withstand capability (short circuit current rating Isccr):

During the normal operation of overvoltage protectiove devices, the SPD provides a high impedance at nominal system voltage and rated frequency. In case an SPD reaches its end-of-life in a low impedance state, the resulting short-circuit current must be interrupted. This interruption may be provided by an SPD internal disconnector or in conjunction with an external disconnector, e.g. a fuse.

When the SPD manufacturer provides information about a maximum allowed backup fuse rating, any alternative overcurrent protective device, like e.g. MCBs or circuit breakers, must be considered very carefully, because such devices may not provide the required impulse withstand, specifically in applications where type 1 SPDs are required and partial lightning currents are to be expected.

If other kinds overcurrent protective devices than the ones recommended by the SPD manufacturer are used, this is under the full responsibility of the installer. Furthermore the higher internal impedance of such other devices compared to a fuse may add to the voltage drop under surge conditions and may therefore worsen the effective voltage protection level for the installation and equipment.

Follow current interrupt rating Ifi:

A special characteristic for spark gaps is known as the follow current interrupt rating. If a spark gap is ignited by surge voltage, it forms a sort of short circuit for the connected supply network through which the current is driven.

The spark gap must therefore be in the position to suppress or interrupt the line current automatically after the discharge process, without triggering the upstream overcurrent protective device. The follow current interrupt rating indicates the maximum prospective short-circuit current at which this is guaranteed at the installation location. Modern spark gaps must therefore fulfill two functions:

• Discharging high-energy lightning currents
• Suppressing follow currents for powerful supply networks

In the case of lightning currents, the impedance of the spark gap is ideally very low, in order to keep the energy input as low as possible and minimize wear. In the case of follow currents,however, the impedance must be as high as possible in order to ensure fast elimination.

Follow current interrupt rating only exists in the IEC 61643-11 and relates to SPD constructions, which generally cause a follow current from the power supply after discharge current flow, and describes the ability of such SPDs to self-extinguish such follow current without operation or alteration of any disconnector.

Important for correct understanding is, that this parameter does not provide a real current value that gets interrupted by the SPD, but the maximum prospective short circuit current that may be available at the SPD's point of installation, at which any expected follow current will be self-extinguished by the SPD.

While IEC 61643-11 allows this follow current interrupt rating Ifi to be lower than the short-circuit current rating Isccr, EN 61643-11 requires this rating to be equal to the short-circuit current rating Isccr. But both installation rules, IEC 60364-5-534 as well as HD 60364-5-534, require that the follow current interrupt rating must be equal or higher than the maximum available short circuit current from the power system at the SPD's point of installation.