PM240-2 Power Module: Selecting Between 3NA gG and 3NE gS Protective Devices

The Siemens SINAMICS PM240-2 power module is the central inverter block behind the G120 and S120 drive families, and the protective device fitted on its line side directly determines how the converter behaves during a short circuit, an input diode bridge failure, or a DC bus fault. Siemens product documentation and the SIZER selection tool reference two distinct fuse families for the same frame size: the 3NA LV HRC gG cable/conductor protection fuse and the 3NE LV HRC gS semiconductor protection fuse. Selecting the correct family is not a matter of preference; it is a coordination decision driven by the converter's let-through energy, the drive's operational criticality, and the applicable installation standard (IEC 60269 versus UL 248).

This reference consolidates the official Siemens protective-device information for the PM240-2 (entry ID 109486009), the relevant Siemens catalog data, and IEC coordination practice into a single decision guide. The article covers fuse-class physics, frame-size current tables, I²t coordination, application-specific trade-offs (general-purpose G120 vs. high-performance S120), UL/IEC variants, and field verification steps.

1. PM240-2 Architecture and Why Line-Side Fusing Matters

The PM240-2 is a three-phase AC/AC converter block. Its input stage is a six-pulse uncontrolled diode bridge feeding a DC link capacitor bank; the IGBT inverter section synthesizes the output. The diode bridge is the most vulnerable sub-assembly under short-circuit conditions because it has no inherent current-limiting capability. When a supply-side fault or a DC-link short occurs, the prospective current is limited only by the upstream transformer impedance and the line-side fuse.

Two design parameters drive the fuse selection:

• I²t let-through of the fuse must be lower than the I²t withstand of the diode bridge and the DC-link capacitor bank at the same clearing time.
• Operating voltage class (400 V, 500 V, or 690 V three-phase) determines the minimum rated voltage of the fuse.

If the fuse is too slow (high let-through), the input rectifier will rupture before the fuse clears. If the fuse is too fast for the drive's normal inrush, nuisance trips occur on every cold start. The two Siemens-recommended families solve the second problem differently.

2. Fuse Class Fundamentals: gG vs gS

Both 3NA and 3NE are LV HRC (low-voltage high-rupture-capacity) blade-type fuses per IEC 60269-1 and -4, but their operating characteristics are different by design.

The gG characteristic protects cables and busbars from sustained overload, with melting at approximately 1.25–1.6 × In within one hour. The gS characteristic is a hybrid defined in IEC 60269-4: it carries normal load continuously (like gG) but clears very quickly at high fault currents (like the older gR). The gS curve was introduced specifically for solid-state power electronics.

3. PM240-2 Frame Size and Converter Current Ratings

The PM240-2 ships in six frame sizes, FSA through FSF, with several sub-variants for 200 V, 400 V, and 690 V three-phase supplies. The frame size determines the recommended fuse rating. The values below are consolidated from the Siemens G120 catalog (D31) and the protective-device product information (entry 109486009).

3.1 400 V class, 3 AC (most common)

3.2 200 V class, 1 AC / 3 AC

For single-phase 200 V supplies, the input current is roughly 1.5–1.7× the 3-phase value due to full-wave rectification into a single-phase line. The fuse rating scales accordingly. Siemens lists gG sizes from 3NA6810 (10 A) at FSA up to 3NA6840 (125 A) at FSC for the 200 V 3 AC family, and gS selections 3NE1814-0 / 3NE1802-0 for the lower-power range.

3.3 690 V class, 3 AC

At 690 V the line current is reduced by a factor of √3 compared with 400 V for the same kW. The fuse family and frame mapping are similar, but the rated fuse voltage must be ≥ 700 V AC; 3NE fuses in the 3NE3 series (e.g., 3NE3227, 3NE3231) are commonly used. A gG at 690 V requires an NH00 or NH1 form factor with a 690 V AC rating (e.g., 3NA6470, 3NA6472).

4. UL 248 / North American Cross-Reference

Siemens PM240-2 hardware carries both CE (IEC) and UL/cUL listings. The UL-listed versions (UL 508C as Power Conversion Equipment) accept Class J or Class CC fuses as the line-side protective device. The Siemens product information PDF lists the UL alternative as Bussmann LPJ (or AJT) series time-delay fuses with the equivalent current rating, and notes: "Protective devices of the same type with lower current ratings in accordance with existing applicable local electrical code" are acceptable.

For UL installations, use a Class J fuse holder and verify the AIC rating of the fuse (typically 200 kA at 480 V for LPJ/AJT). The drive's own SCCR (Short Circuit Current Rating) is stamped on the nameplate; the line-side fuse must support at least that prospective current.

5. Why the Same PM240-2 Sometimes Shows 3NA and Sometimes 3NE

The SIZER configuration tool and individual product catalogs both list 3NA gG and 3NE gS because the choice is not a function of the drive alone. It is a function of the system. The decision path is:

1. Is the converter in a G120 general-purpose application or an S120 high-performance / servo application? A G120 controlling a fan, pump, conveyor, or HVAC AHU typically tolerates the longer clearing time of a gG fuse. An S120 driving a synchronized servo or a high-inertia paper-machine section demands the lowest possible I²t let-through.
2. What is the prospective short-circuit current at the line terminals (Ipk at the drive bus)? Above approximately 10 kA at the PM240-2 input terminals, the gG fuse's pre-arcing I²t approaches the diode-bridge rating. A gS (or a fused disconnect with current-limiting fuses) becomes mandatory.
3. Is the supply a stiff transformer (< 5% Z) or a weak rural feeder? A weak feeder limits Ip naturally and may allow a gG selection. A stiff substation transformer can deliver 50 kA or more.
4. Is personnel safety a controlling factor? A gS fuse clears faster, reducing arc-flash incident energy at the line compartment (relevant per NFPA 70E).
5. Cost and availability of spare fuses in the country of installation? 3NA gG fuses are stocked in every industrial electrical distributor; 3NE gS fuses are not.

6. Application-Specific Guidance

6.1 G120 General-Purpose Drives (Pumps, Fans, Compressors, Conveyors)

For a G120 with a PM240-2 in a non-critical process, the typical recommendation is the 3NA gG series sized to the frame (see Section 3). The justification is:

• Lower capital cost (gG fuses are typically 40–60% the cost of equivalent gS fuses).
• Wider distributor base, faster MTTR after a fault.
• For a fan or pump, a momentary line-side interruption is operationally acceptable.

Select the gG fuse at approximately 1.0 × the drive's rated input current. Do not oversize to reduce nuisance trips; this defeats coordination.

6.2 G120 Critical Process (Food, Pharma, Continuous Web)

If the line-side fault would stop a continuous process whose restart cost is high (clean-in-place, material in the line, FDA batch), upgrade to the 3NE gS selection. The lower I²t let-through reduces the probability of rectifier rupture, which converts a fuse-only repair into a drive replacement.

6.3 S120 High-Performance Drives

For an S120 using the PM240-2 as the Active Line Module (ALM) or Basic Line Module (BLM) input, the recommendation is uniformly 3NE gS. The S120 system has tighter DC-bus monitoring, the converter is typically part of a tightly coordinated multi-axis system, and the cost of an uncommanded line-side interruption is high. Choose the gS fuse at 0.8–1.0 × the input current rating (verify against the gS datasheet for that frame, since the gS continuous derating differs from gG).

7. Sizing Procedure and Verification

Use the following checklist when specifying fuses for a PM240-2 installation.

1. Identify the PM240-2 frame (FSA–FSF) and the supply class (200/400/690 V, 1- or 3-phase). Record the drive's rated input current from the rating plate or the SIZER output.
2. Compute the prospective short-circuit current at the drive input terminals using the upstream transformer kVA, impedance Z_k%, and the cable impedance. For a 3-phase bolted fault, I_sc ≈ I_n,trafo × 100 / Z_k%.
3. Read the PM240-2 diode-bridge I²t withstand from the protective-device PDF or the G120 hardware installation manual. Compare to the fuse pre-arcing I²t + post-arcing I²t at the computed I_sc.
4. Choose the fuse class. Rule of thumb: gG is acceptable when I_sc ≤ 10 kA and the drive is in a general-purpose application. gS is mandatory for I_sc > 10 kA or for S120 / critical process.
5. Select the specific catalog number from Section 3 (400 V) or the equivalent from the protective-device PDF.
6. Verify the fuse's continuous amp rating at the ambient temperature of the cabinet (typically 40 °C). If the cabinet runs at 50 °C, derate the fuse by 5–8% (consult the manufacturer's derating curve).
7. Check the AIC (interrupting rating) of the fuse against I_sc. All Siemens 3NA and 3NE industrial LV HRC fuses are rated 120 kA at 500 V / 100 kA at 690 V, well above the typical I_sc at the drive.
8. Confirm the drive's SCCR on the rating plate is met or exceeded by the fuse's interrupting rating.
9. For UL installations, substitute the equivalent Class J (LPJ or AJT) from Section 4 and verify the same coordination parameters.

8. Common Pitfalls

Oversizing the gG fuse. A common field error is to step up from a 32 A gG to a 50 A gG to "stop nuisance trips on cold start." The PM240-2 charging current typically peaks at 2–3× the input current for < 100 ms, well within the gG's safe region. Oversizing the fuse sacrifices the diode-bridge coordination that Siemens published.

Mixing gG and gS in the same panel. Do not place a gG fuse on the PM240-2 input and a gS fuse on the next drive in the same panel. Under a bus fault, the coordination table is no longer valid; the slower fuse may rupture before the faster one clears.

Ignoring 690 V voltage class. A 400 V class 3NA fuse will operate on a 690 V system only as long as the line-to-line voltage does not exceed the fuse's rated voltage. The 3NE family in the 3NE1xxx-0 range is rated 690 V; the 3NE1xxx-1 (with 1000 V rating) is for 690 V plus 30% tolerance.

Using DC-rated fuses on AC input. Some semiconductor fuses are DC-only or have lower AC ratings. Verify the AC voltage rating on the fuse datasheet, especially when substituting an alternate brand.

Skipping the cabinet temperature derating. Inside a sealed cabinet at 50 °C, a 100 A gG fuse derates to approximately 92 A continuous. If the drive draws 95 A continuously, the fuse will eventually fail. Use the manufacturer's published derating curve.

9. Field Verification After Fuse Replacement

1. With the drive de-energized and locked out, isolate the line-side disconnect upstream of the fuse holder.
2. Verify zero voltage at the fuse clips with a properly rated voltage tester (per NFPA 70E or local equivalent).
3. Inspect the fuse holder and the wiring for signs of arcing, discoloration, or carbon tracking before installing the new fuse. A ruptured fuse on a small fault can deposit vaporized metal on the holder.
4. Install the new fuse of the same catalog number and class. Do not substitute a different class without re-validating the coordination.
5. Re-energize in sequence: upstream breaker, then line disconnect, then the drive's 24 V supply, then the DC bus pre-charge, then the run command.
6. Monitor the drive's input current for at least 15 minutes under normal load and confirm it is below the new fuse's derated rating.
7. Log the fuse replacement in the panel's equipment record, including the part number, frame size, supply class, and the original fault cause (rectifier failure, DC bus short, downstream short, etc.).

10. Quick-Reference Selection Table

11. Reference Documentation

Official Siemens documents to retain on the project file:

• Siemens Support Entry 109486009 – Protective Devices for SINAMICS G120 Power Module PM240-2 (product information PDF, current revision).
• SINAMICS G120 Operating Instructions (entry 109486000 or successor) – hardware installation manual with frame-size tables.
• Siemens Catalog D31 (SINAMICS G120) – frame and power ratings.
• IEC 60269-1 and IEC 60269-4 – low-voltage fuse standards.
• NFPA 70E – Standard for Electrical Safety in the Workplace (for verification step procedure).