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Mersen US221I UltraSafe US22 Modular Fuse Holder, DIN Rail Mount, Polyamide
The Mersen US221I is part of the UltraSafe US22 modular fuse-holder line, engineered for DIN rail installation in circuits up to 750 VAC / 690-750 VDC and rated 125A. Built from flame-retardant Polyamide with a glow wire flammability index to 960°C, this finger-safe, IP20-rated holder is designed for motors, transformers, low voltage distribution, and control circuits. Pressure plate terminals accept 1/0 to #14 solid/stranded copper wire, and tin-plated copper contacts help ensure a stable, low-resistance connection over the life of the installation.
Key Features
- DIN Rail Mounting: Installs directly onto DIN rails per EN 60717 for fast, tool-free panel integration.
- Modular Design: Supports a multi-pole assembly kit for building 1, 1+N, 2, 3, or 3+N pole configurations from a single-pole base unit.
- Durable Construction: Molded from UL94V2-rated Polyamide with a glow wire flammability index to 960°C for flame-retardant performance.
- Finger-Safe Design: Enclosed contact areas reduce the risk of accidental contact during operation and fuse replacement.
- Sealable & Lockable: Can be sealed in both the open and closed position and is lockable for controlled access and tamper resistance.
- Connection Terminals: Pressure plate terminals accept 1/0 to #14 solid/stranded copper wire with tin-plated copper contact materials.
- Optional Accessories: Available with an optional visual blown fuse indicator and microswitch for remote fault signaling.
- Degree of Protection: IP20-rated enclosure for standard indoor panel and enclosure applications.
Certifications & Compliance
- UL Recognized Component, UL4248, Guide IZLT2, File E52283
- CSA Certified, C22.2 Class 6225, File 321
- CE Conformity
- RoHS Compliant
Technical Specifications
- Ac Voltage: 750 VAC
- Amps: 125 A
- Application: All circuits up to 750V for protection of motors, transformers, low voltage distribution, control circuits
- Body Material: Polyamide
- Ce Conformity: Yes
- Connection Terminal Type: Pressure Plate
- Dc Voltage Range: 690 - 750 VDC
- Lead Free: Consult Technical Services
- Materials Contact: Tin-plated Copper
- Mounting Method: DIN-rail Installation On To DIN Rails To EN 60717
- Number Of Poles: 1 1 + N 1, 2, 3 2 3 3 + N
- Rohs Compliant: Yes
- Sccr: 100 kA 200 kA
- Size Per Standard: 22x58
- Standards: UL Recognized Component, UL4248, guide IZLT2, file E52283 CSA Certified C22.2, class 6225, file 321
- Wiring Range: 1/0 to #14 Solid/Stranded Cu Wire
Typical Applications
- Motor Circuit Protection
- Transformer Protection
- Low Voltage Distribution Panels
- Control Circuit Protection
- Industrial Panelboards and Enclosures
Why Mersen?
The Mersen US221I combines flame-retardant Polyamide construction, finger-safe operation, and IP20 protection with modular DIN-rail mounting for flexible panel design. Backed by UL Recognized Component and CSA certification, it delivers code-compliant fuse protection for demanding industrial and commercial electrical systems.
Manufacturer Resources
Product Specs
- Ac Voltage:
- 750 VAC
- Amps:
- 125 A
- Body Material:
- Polyamide
- Ce Conformity:
- Yes
- Connection Terminal Type:
- Pressure Plate
- Dc Voltage Range:
- 690 - 750 VDC
- Configuration:
- IP20
- Lead Free:
- Consult Technical Services
- Materials Contact:
- Tin-plated Copper
- Mounting Method:
- DIN-rail Installation On To DIN Rails To EN 60717
- Number Of Poles:
- 1 1 + N 1, 2, 3 2 3 3 + N
- Rohs Compliant:
- Yes
- Sccr:
- 100 kA 200 kA
- Size Per Standard:
- 22x58
- Wiring Range:
- 1/0 to #14 Solid/Stranded Cu Wire
- Department:
- Electrical
- Product Group:
- Wiring Devices
- Product Type:
- Fuse Holder
Mersen US221I – Frequently Asked Questions
1) Will the Mersen US221I fuse holder take a 22x58 fuse?
That's exactly what it's built for. The US221I UltraSafe US22 holder is sized per the 22x58 standard, so it takes 22mm x 58mm cylindrical fuse links and nothing else. Class CC, midget 13/32 x 1-1/2, and 10x38 links will not seat in it. Check the body size stamped on the fuse you're replacing before you order.
2) What wire size can I land in a Mersen US221I fuse holder?
Pressure plate terminals on the US221I accept 1/0 through #14 solid or stranded copper. That is a wide window, so it covers both a 125A feeder conductor at the top end and small control wiring at the bottom. Contacts are tin-plated copper. Aluminum conductors are outside what the terminals are rated for, so land copper or transition to copper ahead of the holder.
3) Can I use the Mersen US221I on a 600V or 690V DC circuit?
Yes on both counts. The US221I is rated 750 VAC and 690 to 750 VDC, so a 600V AC panel or a 690V DC bus is inside its rating. DC ratings on fuse holders are usually the limiting number, and here it is nearly the same as the AC figure, which is why this one turns up on solar and drive DC links as well as AC distribution.
4) How do I get a 3-pole fuse holder out of the Mersen US22 single-pole base?
You gang them. The US221I is a single-pole modular base, and a multi-pole assembly kit joins bases into 1, 1+N, 2, 3, or 3+N configurations. So for a three-phase motor feeder you order three bases plus the assembly kit rather than a factory-built three-pole block, and for 3+N you add a fourth. Order the kit separately; it does not come in the box with the base.
5) What is the SCCR on the Mersen US221I and will it hold up on a 100kA service?
The US221I carries a 100 kA and 200 kA short circuit current rating, so a panel fed from a service with 100 kA available fault current is within it. The actual interrupting performance comes from the 22x58 fuse link you install, so pair the holder with a correctly rated link to hold the assembly rating.
6) Is the Mersen US221I safe to change a fuse on in a live panel?
It is finger-safe with enclosed contact areas and IP20 rated, which means the live parts are guarded against a probing finger during operation and fuse replacement. IP20 offers no water protection at all, so it belongs inside a dry indoor enclosure. It also seals in both the open and closed position and locks, which is what you want when a contractor needs an isolation point signed off.
7) Can I put a Mersen UltraSafe US22 fuse holder in a wet or outdoor location?
No. The US221I is an IP20 DIN rail holder for dry indoor panels and enclosures only. For a wet location or a direct-burial style branch feed, an in-line watertight holder is the right type instead, such as the Littelfuse WPB1 Series in-line watertight fuse holder. That is a different install entirely, spliced into the conductor rather than clipped to a rail.
8) Can a Littelfuse WPB1 in-line fuse holder be used in place of a Mersen US221I?
No, they solve different installation problems. The Mersen US221I is a 22x58 modular holder that snaps onto DIN rail per EN 60717 inside a panel or enclosure, carries 125 A at 750 VAC and 690-750 VDC, and uses pressure plate terminals for 1/0 to #14 solid or stranded copper. The Littelfuse WPB1 is an in-line watertight fuse holder, meaning it is spliced into a wire run rather than mounted to rail, so it does not land in a DIN rail panel assembly. If the fuse block has to sit on rail beside contactors and terminal blocks, the US221I is the form factor; if the fuse sits out on a cable in a wet location, the WPB1 is the form factor.
9) The panel location is damp and outside the enclosure. Is the Mersen US221I the right fuse holder there?
The Mersen US221I is IP20 rated, which is a finger-safe indoor panel and enclosure rating, not a wet or washdown rating. For a fuse that has to live out on a cable in a damp or exposed spot, the Littelfuse WPB1 in-line watertight fuse holder is the style built for that condition. Keep the US221I for the protected interior of the panel, where its DIN rail base and sealable, lockable cover give controlled access to the fuse.
10) Does the Mersen US221I mount on standard 35mm DIN rail?
It clips onto DIN rail to EN 60717, which is the standard symmetric top-hat rail used in industrial panels. No panel drilling and no mounting hardware needed, so it drops into an existing rail run beside your breakers and terminal blocks. It is not a panel-mount or in-line holder, so if you have no rail in the enclosure you will need to add one.
11) How do I know a fuse has blown in a Mersen US22 holder without opening the panel?
Two options are available on the UltraSafe US22 line: a visual blown fuse indicator on the holder itself, and a microswitch for remote fault signaling back to a PLC or annunciator. Both are optional accessories, so if you want the remote contact you have to specify it at order time. The base US221I on its own gives you neither.
12) Is the Mersen US221I the right holder for a 125A motor feeder, or do I need something else?
At 125A and up to 750 VAC it is sized for motor circuit protection, transformer primaries, low voltage distribution, and control circuits. Where it is the wrong choice is residential and light commercial plug-fuse work: for a screw-in plug fuse in a box cover unit at 125V 15A you would be looking at something like the Littelfuse LSRY Series plug fuse box cover unit instead. Different fuse body, different world.
13) What is the difference between a Littelfuse LSRY box cover unit and a Mersen US221I fuse holder?
They are built for opposite ends of the system. The Littelfuse LSRY is a plug fuse box cover unit rated 125V 15A, which is branch-circuit plug fuse territory on a small enclosure cover. The Mersen US221I takes 22x58 class fuses at 125 A, 750 VAC and 690-750 VDC, with a 100 kA to 200 kA short circuit current rating, and mounts on DIN rail for motor, transformer, low voltage distribution and control circuit protection. Anything running at 750 V or feeding a motor branch is outside what a 125V 15A plug fuse unit covers.
14) Will a Polyamide fuse holder body stand up to heat in a packed industrial enclosure?
The US221I body is molded from Polyamide rated UL94V2 with a glow wire flammability index to 960 degrees C. That glow wire figure is the one panel builders look at, since it covers the case where a nearby connection overheats against the plastic. Tin-plated copper contacts keep the resistance, and therefore the heat rise at the terminals, stable over the life of the install.
15) Can the Mersen US221I be locked out for maintenance?
It can be sealed in both the open and the closed position and it is lockable, so a facility can use it as a controlled access point and catch tampering on a circuit that is supposed to stay energized. Seal it closed on a critical feeder, seal it open when a circuit is out of service.
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Does the Mersen US221I come with a warranty?
Absolutely. The Mersen US221I is backed by a manufacturer’s warranty.
For full warranty terms, check the product documentation or Contact Us.
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