The 170M series dominates high-power semiconductor protection — but it isn't the only answer. Bussmann's FW family covers lower currents, compact spaces, and retrofit applications where the square-body 170M won't fit.

This guide covers the FW series (FWH, FWP, FWE, FWJ, FWS, FWX), their formats, ratings, and how to choose between them and the 170M.

The FW Family at a Glance

FWH Series — The Workhorse Cylindrical

FWH is the most widely used FW-series fuse. Its cylindrical body fits standard holders and offers high-speed performance across a broad current range.

FWH features: - aR-class semiconductor protection - Cylindrical ceramic body with quartz sand fill - Available with or without end caps (blade versions for larger currents) - Holder systems: CH series cylindrical holders

FWH Selection Example

Application: 100kVA UPS, 480V, rectifier DC bus - DC bus current: 180A - IGBT I²t withstand: 80,000 A²s at 700V - Selected: FWH-200A — total I²t ≈ 25,000 A²s < 80,000 ✓

FWP Series — Compact Ferrule Fuses

FWP uses a ferrule (end-cap) format for compact installation, common in PCB-mounted power supplies and small drives.

FWP is often specified where space is at an absolute premium — inside servo drives, compact VFDs, and power supplies.

FWE Series — 1000VDC Semiconductor Protection

FWE is Bussmann's cylindrical high-speed fuse rated for 1000VDC — for solar inverters, energy storage PCS, and high-voltage DC power supplies.

FWE's niche: Compact DC protection for auxiliary circuits in PV inverters and ESS PCS, where 170M is too large and a standard fuse lacks the DC high-speed characteristic.

FWJ Series — Very High Current

FWJ extends the cylindrical format to very high currents:

FWJ application: Large rectifiers, electroplating power supplies, induction heating, and DC traction auxiliary systems where 170M may not be available in the required rating or voltage.

FWS Series — Small Semiconductor

FWS application: Gate drive protection, small thyristor protection, and auxiliary semiconductor circuits.

FWX Series — Compact Fast-Acting

FWX application: Small, fast protection where a semiconductor device has minimal I²t tolerance.

FW vs. 170M: How to Choose

Rule of thumb: - Under 100A and space-constrained → FWH / FWP / FWE - Over 100A in new designs → 170M - 1000VDC small current → FWE - Retrofit into cylindrical holders → FWH

I²t Coordination for FW Fuses

The same I²t rule applies:

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

Example: A 30A thyristor with I²t withstand of 12,000 A²s at 600V.

Selection: FWH-50A (load 30A, fuse 50A, I²t 5,200 < 12,000) — leaves margin for load growth.

Application Examples

Variable Frequency Drive, 22kW, 400V

• Rated output: 45A
• DC bus current: ~40A
• Selected: FWH-50A14F (aR, 500VAC) on the DC bus

UPS Rectifier Input, 200kVA, 480V

• Input current: 240A
• Selected: FWH-250A × 1 per phase, or FWJ-300A
• Pair with 170M on the DC link for full protection

Solar Inverter Auxiliary Supply, 1000VDC

• Aux current: 35A
• Selected: FWE-40A (1000VDC, 40mm body)

Servo Drive, 5kW, 400V

• DC link current: 12A
• Selected: FWP-15A14F — compact, fits the drive's internal layout

Holder Compatibility

The ferrule form factor

Both FWH and FWP fuses use a cylindrical body with ferrule (cap) terminals -- the same form factor as standard IEC 60269-4 semiconductor fuses. This cylindrical ferrule design offers practical advantages: compatibility with standard DIN rail clips and busbar-mounted fuse holders; uniform heat dissipation without hot spots; proven automotive vibration and thermal cycling performance meeting ISO 16750 requirements; and field replaceability by trained technicians without specialized tools.

FWH Series -- 500 Vdc, 50 kA

FWP Series -- 700 Vdc, 50 kA

FWJ Series -- 800 Vdc, 50 kA (Imperial Ferrule)

Voltage Selection: FWH vs FWP vs FWJ

Select the lowest voltage family that provides adequate margin above your maximum system voltage. For a 400V nominal system (approximately 460V peak at full charge), FWH at 500 Vdc provides 8.7% voltage margin -- acceptable for most applications. For a 600V nominal system (approximately 690V peak), FWP at 700 Vdc provides only 1.4% margin -- marginal for some applications. FWJ at 800 Vdc provides 15.9% margin for the same 600V system, often preferred for robust protection. The FWJ uses imperial (inch-based) ferrule dimensions, which may affect holder compatibility compared to the metric FWH-C and FWP-C series. Verify holder compatibility before specifying.

FWH/FWP/FWJ Applications in EV and ESS

 In EV: FWH-C is used for OBC AC input protection (500V rating provides adequate margin above 400Vac input), DC-DC converter input/output protection, and low-voltage battery pack protection in 200-400V architectures. In ESS: FWH-C for battery rack combiner protection (500V tier) and PCS DC side protection for 500V-class BESS. FWP for 600-700V battery bus protection where FWH's 500V rating is insufficient. FWJ for 800V battery bus protection in a ferrule form factor where the 170M square-body size is too large.

Common Questions

Q: Is FWH as fast as 170M? A: Comparable at equivalent ratings. Both are aR-class high-speed fuses. The choice is usually driven by format (cylindrical vs. square) and available holder, not by speed.

Q: Can FWH replace a 170M in an existing design? A: Only if the current is within FWH's range and the time-current characteristics match the application. Never substitute without re-verifying I²t coordination with the semiconductor.

Q: Why choose FWE over a standard 1000VDC fuse? A: FWE is a high-speed (aR) fuse. A standard 1000VDC fuse (e.g., a gPV fuse) is not designed for semiconductor I²t coordination. FWE protects the semiconductor; a standard fuse protects the cable.

Q: Do FW fuses need holders? A: Yes — cylindrical fuses require holders or clips. The CH series provides DIN-rail mountable options for 10×38mm, 14×51mm, and 22×58mm bodies.

Written by the asbeam.com technical team. asbeam stocks FWH, FWP, FWE, FWJ, FWS, and FWX fuses with matching CH-series holders. Contact us for semiconductor protection coordination.

Further reading: - 170M High-Speed Fuse Guide - Fuse Holders, Bases & Accessories Guide - Railway & Traction Fuse Selection