Fuses are common protection devices in low-voltage distribution systems, primarily providing short-circuit protection and overload protection. A fuse consists of a fuse link and a housing, connected in series with the protected circuit. When the current through the fuse link exceeds its tripping threshold, the fuse link melts, disconnecting the power supply to protect the equipment or circuit.

Understanding aM and gG Fuses

aM-class fuses: Designed for motor protection with partial-range breaking capacity. They can interrupt currents from a minimum value (typically 6.3× the fuse’s rated current, Iₙ) to their rated breaking capacity.

gG-class fuses: Designed for general-purpose protection with full-range breaking capacity, covering all currents from the melting threshold to the rated breaking capacity.

Key Differences

Selection Guidelines

For Motor Protection (aM Fuses):

Set minimum breaking current to 6.3×Iₙ.

Choose Iₙ ≥ 1.1×Iᴍ to avoid nuisance tripping during startup.

Use thermal relays for overload protection.

For General Protection (gG Fuses):

Select Iₙ = 1.5–2.3×Iᴍ (lower values for light loads, higher for heavy loads).

Provides both overload and short-circuit protection.

Example

A 380V three-phase induction motor (Iᴍ = 34A, light-load startup):

aM Fuse: Iₙ ≥ 1.1×34A = 37.4A → Choose 40A fuse.

gG Fuse: Iₙ = 1.8×34A = 61.2A → Choose 63A fuse.

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