Short note: Low voltage HRC fuse

A low-voltage HRC fuse is an enclosed cartridge fuse for circuits up to 1000 V AC (commonly 415 V) that can safely interrupt very high prospective fault currents (typically 50–80 kA, up to 120 kA) with current limitation.

Construction: a ceramic tube filled with pure quartz sand, containing one or more silver strip elements with notches, welded to brass end caps with knife-blade or bolted contacts; a tin bead (M-effect) gives time delay on overload; an indicator or striker shows operation.

Types:

• Knife-blade (NH) type: fitted into fuse bases with a handle puller; ratings up to about 1250 A.
• Bolted / tag type: bolted to the fuse carrier.
• Cylindrical (ferrule) type: small ratings for control circuits.
• Categories: gG (general purpose, cables), aM (motor back-up), gR (semiconductors).

Operation: on short circuit the notches melt within milliseconds, multiple arcs form, and the sand cools them and forms fulgurite, forcing current to zero before the first peak (cut-off).

Advantages: high breaking capacity, current limiting, fast and reliable, no maintenance, consistent characteristics, cheap compared with breakers.

Applications: LV distribution boards, transformer LV side, motor feeders (back-up to contactors), cable protection and semiconductor protection.