How Extreme-Rapid Semiconductor Fuses Work: Application Notes
Catalog pages 9 to 20 explain how extreme-rapid semiconductor fuses are built and applied: silver elements in quartz sand, melting and arcing time, full-range gR versus partial-range aR protection, I²t coordination with the semiconductor, derating for ambient temperature, cooling and conductor size, DC and L/R effects, cyclic load, and fuse position in rectifier circuits.
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Semiconductor fuses are sized specifically for protecting semiconductor components. The catalog traces them to the first power diodes of 1950, which had very little thermal capacity for overloads and short circuits; combined with the high price of those devices, this led fuse makers to develop fuses that react faster than conventional types, called extreme-rapid fuses. Development continued after the thyristor appeared in 1956, and the introduction notes that power semiconductors and matching extreme-rapid fuses are now made for rated currents of 4 kA and rated voltages up to 6 kV.
A typical fuse has one or more fusing elements connected in parallel, embedded in quartz sand inside a ceramic body. The elements are pure silver strip (99.9%) with narrow sections (notches), welded to the internal terminals. Terminals are copper, tinned copper or silver-plated alloy, and their shape sets the mounting method: bolted, blade or bus-bar mounting. Fuses below 60A are made in the same form as conventional industrial fuse cartridges so they fit standard bases. The body is high-quality steatite ceramic, chosen for electrical insulation, mechanical strength and resistance to thermal shock during interruption. The cross-section of the notches depends on the rated current and the number of notches on the operating voltage; parallel elements must be identical so current divides evenly.
Under a fault, current density and heating are highest at the notches, which melt first. Molten silver fuses with the surrounding sand into a non-conducting fulgurite, and part of it vaporizes into the gaps between sand grains. An arc forms, and the arc (switching) voltage rises, possibly above the operating voltage, until the current is interrupted. This fast voltage rise limits both the peak current and the total clearing I²t. Arc voltage depends on the number of notches: an element for 440V typically has four, one for 220V only two. Manufacturers publish arc voltage against operating voltage as a curve, based on a test circuit with a 15% power factor.
Clearing time is the melting (pre-arcing) time plus the arcing time. Melting time depends on the fault current; arcing time is also affected by operating voltage, sand compaction and grain size, frequency and power factor. On AC, the current zero helps extinguish the arc; on DC there is no current zero. The catalog defines two classes. A gR fuse gives full-range protection, from rated current to short circuit, so its melting curve must lie ahead of the semiconductor's curve. An aR fuse gives partial-range, short-circuit-only protection and cannot clear overloads; overloads with times above 10 seconds are then handled more economically by overload relays (setting range 1.1 to 2 times the fuse rated current) or electronic current limiters.
What melts the element is the thermal effect of the fault current, expressed as the Joule integral I²t. Silver interrupts faster than copper: citing Professor Rüdenberg, the catalog states that the heat needed to vaporize silver is only 60% of that for copper of the same dimensions. Clearing I²t is the sum of melting I²t and arcing I²t, and because it rises with operating voltage, data tables give clearing I²t at several voltages. For coordination, the fuse's total clearing I²t must be lower than the limiting I²t of the semiconductor, comparing cold semiconductor with cold fuse, since fuse I²t falls faster with rising temperature than the semiconductor's.
Rated current is affected by ambient temperature, forced cooling and the cross-section of the connecting conductors. Fuse data refer to 25°C ambient unless stated; above that the rated current is reduced. The catalog's worked example is a 200A fuse at 65°C in still air, which may be loaded to only 160A. Mounting the fuse in the same forced airflow as the semiconductors can allow a smaller rating with lower I²t. Connecting conductors should be sized per IEC 269-1; smaller conductors require a lower current, and insulating materials near the fuse must tolerate its higher working temperature compared with general-purpose (gL) fuses.
On DC, a high L/R time constant makes the fault current rise slowly, so heat flows into the sand, melting time and melting I²t increase, and the missing current zero prolongs arcing. Total clearing I²t on DC can therefore be much higher than on AC. Lowering the operating voltage, for example from 660 VDC to 380 VDC, greatly improves interrupting behavior, and manufacturers give the permissible DC voltage against L/R for each series; the catalog's example is an extreme-rapid fuse for 500 VAC used at 300 VDC with an L/R of 40 ms. For cyclic loads, currents up to the rated current cause no permanent change, but repeated overloads can alter the silver structure and cause premature operation (fuse aging); experience shows this is avoided when the fuse's melting current for the overload duration is about twice the overload current.
In rectifier circuits a fuse can sit in the load (DC) circuit, in series with each rectifier arm, or at the AC input, and each position gives a different fuse rated current; the factors also hold when diodes are replaced by thyristors. A fuse in the load circuit is the lowest-cost option but does not detect internal diode faults, must be sized for the full DC current and has a higher clearing I²t. The other positions detect internal as well as external faults. Newer super-extremely-rapid designs bond the quartz sand with an inorganic binder so arc gases cannot diffuse, cutting clearing I²t by about 60% compared with loose sand. The catalog's selection summary: breaking capacity above the maximum prospective fault current, rated voltage at or above the operating voltage (reduced on DC), I²t checked against the semiconductor, rated current corrected for temperature, conductor area, forced cooling and cyclic load, and all fuses of a series or parallel group replaced after any one operates.
Specifications
| Parameter | Value |
|---|---|
| Element material | Pure silver strip, 99.9% |
| Body material | Steatite ceramic |
| Filler | Quartz sand |
| Terminal materials | Copper, tinned copper or silver-plated alloy |
| Reference ambient temperature | 25°C |
| Indicator operating voltage | 6V minimum; 0.4V to 0.6V for indicators that must show a single blown fuse in a parallel group |
| Parallel fuse matching | Same type and rating, resistance within 1% |
| Overload relay setting with aR fuses | 1.1 to 2 times fuse rated current |
Construction and options
- gR class: full-range protection from rated current to short circuit
- aR class: short-circuit (partial-range) protection only
- Indicator or microswitch to signal a blown fuse and trigger an alarm
- Bonded (solid) sand filling on super-extremely-rapid designs reduces clearing I²t by about 60%
Applications
- Semiconductor protection in rectifier circuits (diodes and thyristors)
Standards and equivalents
- All fuses in a series or parallel group to be replaced after operation, in agreement with IEC 282-1
- Connecting conductor cross-section defined in agreement with IEC 269-1
- Extreme-rapid fuses in AC and DC circuits in agreement with VDE 0636
Wording is the catalog’s own. “In accordance with” a standard is not the same as certified or listed to it.
Curves and drawings
The catalog prints the following for this series: arc voltage vs operating voltage (200a, 500v fuse), current vs clearing time, time-current curve, i²t vs operating voltage, rated current reduction vs ambient temperature, forced cooling and terminal temperature influence, current correction vs connection area, dc voltage reduction vs l/r time constant, i²t reduction with solid sand, rectifier fuse positions. This series is on pp.9–20 of the January 2025 DENCO catalog. Access the catalog or ask for the drawing or curve you need.
Technical review
Specifications on this page are quoted from DENCO’s product catalog. Confirm any rating against DENCO engineering before specifying.
Related
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- French standard semiconductor fuses
- Ferrule semiconductor protection fuses
- All fuse product families
- Cross-reference and discontinued replacement