Semiconductor fuses (aR/gR) — I²t coordination to protect thyristors and IGBTs (IEC 60269-4)

The guide on fuses — Diazed, Neozed and the NH system covers the common fuse classes used for cable and installation protection (gG, aM). This article covers a specialised category that differs substantially: the semiconductor fuse, standardised in IEC 60269-4, designed to protect power-electronic components such as thyristors, diodes and IGBTs against short-circuit current.

Why an ordinary fuse doesn't protect a semiconductor fast enough

A power semiconductor has an extremely small, thin silicon chip with negligible thermal mass compared to a copper cable conductor. During a short circuit, chip temperature can reach destructive levels within a fraction of a mains cycle — long before an ordinary gG or aM fuse, let alone a circuit breaker, could interrupt the current. A semiconductor fuse is therefore designed to melt and clear extremely fast, with a much lower let-through I²t than a comparably rated standard fuse.

I²t: the common language between fuse and semiconductor

Both the fuse and the semiconductor element being protected are characterised by an I²t value (the time-integrated square of the current, a measure of the thermal energy dissipated during a short circuit):

The semiconductor manufacturer specifies a maximum I²t withstand value that the chip can tolerate without permanent damage (often expressed for a half mains cycle of 10 ms at 50 Hz).
The fuse manufacturer specifies the fuse's own let-through I²t, split into a pre-arcing I²t (up to the moment of melting) and a total (arcing) I²t (up to full current interruption).

For correct protection the following must hold:

I²t(fuse, total) < I²t(semiconductor, withstand)

with a practical safety margin, since manufacturing tolerances and the temperature dependence of both components cause the actual values to vary around their rated figures.

aR versus gR — the difference that's often overlooked

Among other categories, IEC 60269-4 defines two well-known utilisation categories whose names look similar but which differ functionally in an important way:

aR (partial-range): this fuse is designed exclusively to interrupt short-circuit currents, well above rated current. An aR fuse provides no reliable overload protection at currents just above rated value, and must therefore always be combined with a separate overload protection device (for example a thermal relay or the power-electronic controller's own current limitation).
gR (full-range): this fuse covers both overload and short- circuit currents across the full range, similar to an ordinary gG fuse but with the much faster response time and lower let-through I²t a semiconductor element requires.

Note: swapping aR for gR (or vice versa) based only on rated current and voltage, without checking the utilisation category, can leave a circuit running without working overload protection — the fuse itself will then only clear on a short circuit, not on a sustained, damaging overload.

Peak let-through current (Ip) as a second coordination quantity

Besides the I²t value, a semiconductor fuse also limits the peak let-through current (Ip) that still flows through the circuit while melting and clearing, before the current is fully brought to zero. This peak value must likewise stay below the semiconductor element's peak-current withstand — a fuse with a low I²t but too high an Ip could still expose the chip to a destructive current spike, even if the total thermal energy is limited.

Practical relevance

When specifying or replacing a semiconductor fuse in a variable- frequency drive, a rectifier bridge, or another power-electronic system, it is not enough to select purely on rated voltage and current as with an ordinary installation fuse: the fuse's let-through I²t and peak current (Ip) must be explicitly compared against the I²t and peak-current withstand quoted by the manufacturer of the protected semiconductor element, and the utilisation category (aR or gR) must match whether separate overload protection is already present.

Common mistakes

Replacing a semiconductor fuse with an ordinary gG or aM fuse of the same rated current — a standard fuse's let-through I²t is typically well above what a semiconductor element can withstand.
Applying an aR fuse without separate overload protection, assuming the fuse alone covers every protection function — aR only protects against short circuits.
Selecting purely on rated current without comparing the fuse's I²t and Ip values against the semiconductor manufacturer's data, while two fuses with the same rated current can differ substantially in let-through I²t.
Forgetting that the semiconductor element's I²t withstand is temperature-dependent — a chip already warm from normal operation has less thermal margin left for a subsequent short circuit than the datasheet value at ambient temperature suggests.