Discrimination Between Fuse Links:

Discrimination is achieved between major (upstream) and minor fuses if the major fuse remains unaffected when the minor fuse operates under maximum fault conditions.

To achieve this, the total I²t of the minor fuse must be less than the pre-arcing I²t of the major fuse.

Eaton MEM fuse links to BS 88: Part 2: 1988 will discriminate on a 2:1 ratio between major and minor current ratings. In practical terms this ratio will ensure adequate discrimination at all levels of fault current, but characteristic curves may be used to calculate closer ratio, where necessary, when the fault current is small and results in the fuse operating in more than approximately 0.02 seconds.

For larger short-circuit currents, which involve the fuse operating in less than 0.01 seconds (1/2 cycle) appropriate total and pre-arcing I²t figures can be established from the characteristic curves.

Back-up Protection:

Suitable S-Type Eaton MEM fuse links to provide back-up protection for Memshield 2 moulded case circuit breakers are detailed in the moulded case circuit breaker section of this brochure.

Example:

By installing BS 88 Eaton MEM S-Type HRC fuse (80kA breaking capacity) at the upstream end of the installation and with an Isc of 40kA on the busbars it is possible to install Memshield 2 Type B, C or D, characteristic 1 - 63A MCBs (10kA breaking capacity) on the outgoing lines.