NH Fuse Selection: gG and aM Characteristics and Size Classes
NH fuse selection: A gG NH fuse protects against both overload and short circuit and suits distribution circuits; an aM fuse protects only against short circuit, withstands motor starting current and must be combined with a thermal relay for overload protection.
Contents
- Construction and operating principle of an NH fuse
- The difference between gG and aM characteristics
- Size classes, base selection and derating
- I²t, breaking capacity and cable protection
- Fuse or circuit breaker? The operation and maintenance view
- Key takeaways
- Frequently asked questions
- Related articles
NH knife-blade fuses still belong to the most common protective devices in main and sub-distribution because they offer very high breaking capacity in a small volume. However, selecting them by rated current alone leads to nuisance operation on motor circuits or to unprotected cables in distribution circuits. A correct choice combines the characteristic class, the size, the base temperature and the I²t value the circuit can tolerate.
Construction and operating principle of an NH fuse
An NH fuse consists of silver fuse elements stretched inside a ceramic body and surrounded by quartz sand. Under overcurrent the element melts at its narrowed sections, the resulting arc is cooled and quenched in the sand, and the current is limited before the natural zero crossing. This current-limiting behaviour reduces the energy that reaches the cable and busbar downstream. The indicator pin on the body allows a blown fuse to be identified quickly in the de-energised state.
The difference between gG and aM characteristics
The gG characteristic provides full-range protection: defined operating times exist from small overloads up to short circuit, and cable protection is built on this characteristic. The aM characteristic is partial-range: it deliberately does not operate up to several times the rated current, because its duty is to tolerate motor starting current and clear only short circuits. Therefore, in a motor circuit using aM fuses, overload protection must be provided separately by a thermal relay or an electronic motor protection device.
Size classes, base selection and derating
NH fuses are produced in NH000, NH00, NH1, NH2, NH3 and NH4 sizes; as the size grows, both the rated current and the base dimensions increase. Base selection is driven not only by current but also by the ambient temperature inside the enclosure and the spacing between adjacent bases. In a closed compartment with bases mounted side by side, temperature rise accelerates, and if the catalogue derating factor is ignored the fuse is thermally stressed even below its rated current.
I²t, breaking capacity and cable protection
| Characteristic / size | Typical rating | Key property | Application |
|---|---|---|---|
| gG · NH000 / NH00 | 16 – 160 A | Overload and short-circuit protection | Sub-distribution and cable protection |
| gG · NH1 – NH3 | 100 – 630 A | High breaking capacity | Main distribution and transformer outgoing |
| aM · NH00 – NH3 | 16 – 630 A | Withstands starting current, no overload protection | Motor feeders with a thermal relay |
| gR / gS | 16 – 400 A | Very fast operation, low I²t | Semiconductor protection for drives and rectifiers |
Whether a fuse actually protects cables and equipment is judged by the total energy it lets through, that is its I²t value. The operating I²t of the fuse must stay below the permissible k²S² limit of the cable being protected. On the breaking side, NH fuses typically reach 100 kA and above, which offers a compact solution at transformer outgoing ways where prospective short-circuit current is high. The table below summarises how characteristic and size relate to the application.
Fuse or circuit breaker? The operation and maintenance view
A fuse is inexpensive, compact and has very high breaking capacity, but it must be replaced after every operation, and single-phase clearing can leave three-phase loads running on two phases. A circuit breaker can be reclosed and offers adjustable settings and auxiliary contacts. On critical motor and transformer feeders, fuse switch-disconnectors and breakers are compared with restoration time after a fault and spare-part management in mind.
Key takeaways: NH fuse selection
- Characteristic decision — gG for distribution circuits, aM for motor feeders and gR for semiconductor protection.
- Cable compatibility — The operating I²t of the fuse must remain below the k²S² limit of the cable.
- Thermal stress — In closed compartments with adjacent bases, the catalogue derating factor must be applied.
- Operational view — A blown fuse is read from the indicator pin; spare stock and size variety are planned early.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can gG and aM fuses be used interchangeably?
No. An aM fuse deliberately does not operate in the overload range. A gG fuse on a motor circuit causes nuisance operation at starting, while an aM fuse in a distribution circuit leaves the cable unprotected against overload.
How is the NH fuse size determined?
First the continuous current of the circuit and the characteristic class are defined, then the smallest size that can carry this current after derating at the actual ambient temperature is selected. Base dimensions, busbar connection and internal layout also influence the size.
Is a thermal relay still required when an aM fuse is used on a motor?
Yes. An aM fuse only covers short-circuit protection, so overload, locked rotor and phase unbalance must be handled by a thermal relay or an electronic motor protection device.
Related articles
- Switch-Disconnector, Disconnector and Fuse Switch: Differences and Selection
- What Is Selectivity? Protection Coordination in Switchgear
- Cable Sizing and Voltage Drop Calculation
- All technical articles
Let us verify fuse characteristic, size and base selection together with your cable cross-sections and short-circuit figures; the Devpan engineering team calculates protection coordination project by project. Related Devpan solution: DevBox sub-distribution board.
