How to size a fuse: gG vs aR fuse characteristics, rated current and breaking capacity, and how to choose cylindrical, blade or screw-type forms.
A fuse is the oldest protective device on a panel and still the one engineers reach for when they need a definite breaking capacity at the lowest cost. Where a miniature circuit breaker is resettable and adjustable, a fuse is a single-use, precisely rated weak link that opens faster and interrupts a higher fault current for its size. This guide covers the choices that actually change the part number: characteristic, current and voltage rating, breaking capacity, and physical form.
Both protect against overcurrent, but they suit different points in the installation. A fuse is cheaper, clears very high faults faster, and has no mechanism to age or stick — which is why it still backs up the most expensive equipment and why a surge protective device must sit behind one. A circuit breaker is resettable and gives adjustable, selective protection. Use a breaker for circuits you switch and reset often; use a fuse where you want a sealed, maintenance-free, high-breaking-capacity link, or as the mandatory backup device for an SPD or a semiconductor.
| Fuse | Miniature circuit breaker | |
|---|---|---|
| Reset | No — replace the element | Yes |
| Breaking capacity per size | Very high | Limited by frame |
| Cost | Low (body reused across types) | Higher |
| Selective coordination | Good with gG/gR tables | Good with type tables |
| Best for | SPD backup, semiconductors, motor branch, cheap main protection | Final circuits, frequently switched loads |
The letter code states how the fuse reacts to an overcurrent — its time-current characteristic.
| Type | Protects | Typical use |
|---|---|---|
| gG (general purpose) | Cables and equipment against overload and short circuit | General distribution, motor branches, the default choice |
| gR / aR (semiconductor) | Semiconductors and rectifiers against very fast faults | Drives, rectifier bridges, soft starters, UPS |
| aM (motor) | Short circuit only, with a contactor and overload relay doing the overload job | Motor circuits coordinated with a thermal relay |
For almost any distribution or motor branch, gG is the part to quote unless you are protecting a semiconductor, where an aR or gR fuse clears a fault in milliseconds before the device is destroyed. Using a gG fuse on a semiconductor leaves it exposed; using an aR fuse on a cable leaves the cable unprotected against overload.
The form decides the holder and how you change the element.
| Form | Examples | Holder | Use |
|---|---|---|---|
| Cylindrical (ferrule end) | RT14, RT18, RT19 10x38 / 14x51 | DIN-rail fuse base | Control circuits, small distribution, instruments |
| Blade / NH (strip) | NH00–NH3, NT H.R.C. | NH fuse switch-disconnector | Main incomers, high current, visible disconnect |
| Screw-type / bolted | RL6, RL1 series | Screw-in or bolted tag | Legacy panels, transformer and capacitor protection |
Cylindrical fuses are the default for panel builders because the base clips onto the 35 mm rail next to the breaker. Blade (NH/NT) fuses are chosen where you also want a visible disconnecting point. Match the element to the exact holder — a 14x51 will not seat in a 10x38 base.
A fuse is only as good as the holder and the devices around it. Confirm the fuse base is rated for the same current and breaking capacity, that the fuse is coordinated with the upstream breaker (so a downstream fault blows the fuse, not the main), and — for an SPD — that the fuse is the manufacturer-approved backup listed in the coordination table. Where the fuse protects a motor, pair a gG or aM fuse with the overload relay rather than relying on the fuse alone for overload.
ELEHPD supplies cylindrical (RT14/RT18/RT19), screw-type (RL6/RL1) and blade (NH/NT H.R.C.) fuses in gG, aR, gR and aM characteristics, with the DIN-rail bases, NH switch-disconnectors and holders needed for a complete fused protection scheme.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.