NH Fuse Operating Class: gG, aM or gR - Which One?
The operating class of an NH fuse decides which currents it interrupts and which it lets through. This guide explains the differences between gG, aM and gR to IEC 60269, their time-current curves and how to match them to cables, motors and semiconductors.
View NH fusesWhat do the letters gG, aM and gR mean?
Each operating class uses two letters under IEC 60269. The first (lower case) describes the breaking range: g means a full-range fuse that interrupts from the lowest melting current up to the rated breaking capacity, while a means a partial-range fuse that only clears above a multiple of the rated current.
The second letter (upper case) names the object protected: G = general use (cables and lines), M = motor circuits, R = semiconductors (rectifier). So gG is a full-range fuse for wiring, aM a partial-range fuse for motors, and gR a full-range fuse for semiconductors.
How to combine fuse, contactor and overload relay to standard.
Read the guideHow do the tripping curves differ?
The key difference is behaviour at small overcurrents. A gG fuse already operates in the overload range, whereas an aM fuse stays deliberately slow there and only reaches its melting range at roughly four to five times its rated current.
- gG: full-range protection, tolerates short inrush, trips on sustained overload.
- aM: permits high motor starting currents, clears only the short circuit - overload protection needed separately.
- gR and aR: extremely fast semiconductor fuses with very low I²t.
- The I²t (let-through energy) must stay below the value the component or cable can withstand.
Which class suits which load?
The choice follows the object protected. For cables and general loads gG is the standard because it covers overload and short circuit together. For motors choose aM so that the inrush current does not cause nuisance tripping, and add a motor overload relay for the overload case.
Sensitive semiconductors in variable-speed drives or rectifiers need gR or aR, because only their low I²t protects the junction of diodes and IGBTs fast enough. A normal gG fuse would be too slow here and would let the component fail.
Frequently asked questions
Can an aM fuse replace a gG fuse?
No. aM offers no overload protection and only clears at a multiple of rated current. Without a separate overload relay a cable would be unprotected against sustained overload.
What does the R in gR fuses stand for?
R comes from rectifier. gR and aR fuses protect semiconductors such as diodes, thyristors and IGBTs through very fast operation with a low I²t value.
Which NH size do I need for which current?
NH000 and NH00 usually cover up to 160 A, NH1 up to 250 A, NH2 up to 400 A, NH3 up to 630 A and NH4a up to 1250 A. The link must match the base size.
Is gL the same as gG?
Practically yes. gL was the older VDE designation, gG is the current IEC 60269 code. Both describe full-range fuses for line protection.
Looking for the right NH fuse?
We supply NH fuse links in gG, aM and gR across all sizes NH000 to NH4a - tested to IEC 60269 including matching fuse bases.
Standard tested
All links meet IEC 60269 (VDE 0636).
For every load
gG, aM and gR for line, motor and semiconductor.
Exact fit
Sizes NH000 to NH4a with matching bases.
Expert advice
Specialists help with selection and selectivity.