Mersen OT-45 Class K5 Fast-Act...
IN STOCK (leaves warehouse same day if ordered before 3pm ET)
$14.09
Mersen OT-45 Class K5 Fast-Acting Amp-Trap® Fuse, 400A/250V

Mersen OT-45 Class K5 Fast-Acting Amp-Trap® Fuse, 400A/250V

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Model: OT45

In Stock (leaves warehouse same day if ordered before 3pm ET)

$14.09
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Mersen OT-45 Class K5 Fast-Acting Amp-Trap® Fuse, 400A/250V

The Mersen OT-45 Class K5 Fast-Acting Amp-Trap® Fuse is a One-Time general purpose fuse engineered to protect feeder and branch-circuits serving lighting, heating, and other non-motor loads. Rated 400A at 250VAC/250VDC, this Class K5 fast-acting fuse safely interrupts available short-circuit currents up to 50,000 amperes (50 kA), delivering dependable overcurrent protection where it is needed most. Designed for use on both AC and DC systems at 50/60 Hz, this fuse is available with either Blade or Ferrule style connections, letting installers match the fuse to the physical configuration of their fuse holders and blocks for straightforward, secure installation.

Key Features

  • Amperage & Voltage Rating: 400A rated for 250VAC/250VDC circuits, providing reliable overcurrent protection for feeder and branch-circuit, non-motor loads.
  • Fuse Class: Class K5 fast-acting construction for consistent, predictable interruption performance.
  • High Interrupting Capacity: 50 kA AC maximum interrupting rating/breaking capacity safely clears available short-circuit currents up to 50,000 amperes.
  • AC/DC Rated: Suitable for both AC and DC applications, rated at 50/60 Hz frequency for versatile installations.
  • Connection Options: Offered in Blade and Ferrule connection styles to fit a range of compatible fuse clips and holders.
  • RoHS Compliant: Manufactured to meet RoHS compliance requirements.
  • Non-Rejection Fuse: As a non-rejection style fuse, replacement fuses should never carry a lower interrupting rating than the original to maintain system protection.

Certifications & Compliance

  • RoHS Compliant

Technical Specifications

  • ROHS Compliant: Yes
  • AC Max I.R./Breaking Capacity (I1): 50 kA
  • Frequency: 50/60 Hz
  • Rated Voltage AC UL: 250 V
  • Rated Voltage DC UL: 250 V
  • Speed/Characteristic: Fast-Acting
  • AC or DC: AC/DC
  • Fuse Class: Class K5
  • Connection Type: Blade or Ferrule

Typical Applications

  • Feeder and Branch-Circuit Protection
  • Lighting and Heating Circuit Loads
  • Non-Motor Load Overcurrent Protection
  • Commercial and Industrial Electrical Systems
  • General Purpose Circuit Protection Requiring Replacement Fuses

Why Mersen?

Mersen's Amp-Trap® OT-45 Class K5 fuse combines fast-acting protection with a 50 kA interrupting rating to safely clear available short-circuit currents in feeder and branch-circuit applications. Available in Blade or Ferrule connection styles and rated for both AC and DC use, it delivers dependable, code-compliant overcurrent protection for lighting, heating, and other non-motor loads. RoHS compliance further supports its use in demanding commercial and industrial electrical installations where consistent performance and safe replacement practices matter most.

Manufacturer Resources

Mersen Product Catalog

Product Specs

Mounting Method:
Clip
AC Max I.R./Breaking Capacity: I1:
50 kA
ROHS Compliant:
Yes
Frequency (Hz):
50/60 Hz
Rated voltage AC UL:
250 V
Speed/Characteristic:
Fast-Acting
AC or DC:
AC/DC
Rated voltage DC UL:
250 V
Fuse Class:
Class K5
list_batch:
MERSEN-20260825
Department:
Electrical
Product Group:
Circuit Protection
Product Type:
Fast-Acting Fuse
Voltage Rating:
250V
Amperage Rating:
400A
Standards:
RoHS
Product Subgroup:
Fuses

Mersen OT45 – Frequently Asked Questions

1) Do I order the blade or the ferrule version of the Mersen OT-45 400A fuse?
That depends entirely on how your fuse block or holder grabs the fuse. The 400A Mersen one-time fuse comes two ways: blade (knife-blade) connection, ordered as OT400, and ferrule connection, ordered as OT45. Both are clip mounted. Pull the old fuse and look at the ends: flat metal tabs sticking out means blade, plain round caps means ferrule. They are not interchangeable in the same holder.
2) Can I use this Mersen Class K5 fuse to protect a motor circuit?
No. The OT-45 is a fast-acting one-time general purpose fuse built for feeder and branch circuits serving non-motor loads such as lighting and heating. A motor's inrush on start would open a fast-acting fuse. For motor branch circuits you want a time-delay fuse instead, for example the Littelfuse CCMR Series Class CC time-delay fuse in the amperage your motor requires.
3) Will a 250V fuse work on my 480V system?
No. The OT-45 is rated 250 VAC and 250 VDC UL, so it can only be applied on circuits at or below 250 volts. Installing it on a 480V system is a misapplication and it will not safely interrupt a fault at that voltage. You need a 600V class fuse for that panel.
4) Is a 50 kA interrupting rating enough for my service, and can I substitute a fuse with a lower rating?
The OT-45 clears available short-circuit current up to 50,000 amperes at 250 VAC. Compare that to the available fault current at the point of installation; if your short circuit study shows more than 50 kA at that location, this fuse is not the right choice. Because it is a non-rejection style fuse, nothing physically stops someone from dropping in a lower interrupting rating later, so any replacement must match or exceed 50 kA to keep the system protection intact.
5) What does Class K5 mean, and will it drop into my existing Class H or Class K fuse clips?
Class K5 is a one-time, non-rejection fuse class that shares the dimensional envelope of standard Class H one-time fuses at the same amperage and voltage, which is why it mounts in ordinary fuse clips rather than a rejection holder. The practical difference from Class H is interrupting capacity: this one clears 50 kA where a Class H is only 10 kA. Confirm your clip style, blade for OT400 or ferrule for OT45, and the 400A 250V rating of the block.
6) Can a Littelfuse CCMR040 Class CC fuse be used in place of the Mersen OT-45?
No. The Mersen OT-45 Amp-Trap is a Class K5 fast-acting fuse rated 400A at 250VAC/250VDC with a 50 kA AC interrupting rating, mounted in clips. The Littelfuse CCMR040 is a Class CC time-delay fuse rated 40A, a different class, a different amperage, and a time-delay rather than fast-acting characteristic. Class CC fuses use a rejection-style ferrule and will not seat in a 400A Class K5 fuse block, and a 40A element cannot carry a 400A feeder. Use the CCMR040 only where a 40A Class CC time-delay fuse is called out, such as motor branch-circuit or control transformer protection.
7) Is the Mersen OT-45 rated for DC circuits as well as AC?
Both. It carries a 250 V DC UL rating alongside the 250 V AC rating, and it is listed for 50/60 Hz AC service, so the same fuse covers DC buses and AC feeders up to 250 volts. That makes it usable in battery and DC distribution gear as well as standard 120/240V AC panels.
8) Is this a one-time fuse or can it be reset or refilled?
One-time only. Once the OT-45 opens on an overcurrent it is replaced, not reset and not renewable. For a 400A feeder that means stocking spares, and matching the replacement to the same 400A, 250V, 50 kA ratings and the same blade or ferrule end style.
9) I need a small fast-acting fuse for a control circuit, not a 400A feeder fuse. What should I be looking at instead?
The OT-45 is a 400A feeder and branch circuit fuse, far too large for control or instrument work. For low-amperage fast-acting protection look at the Littelfuse BLF Series midget fast-acting fuse rated 250V, or for metering, the Square D PowerLogic energy meter fuse pack, a set of three rated 0.5A 600V. Small glass tube options include the Littelfuse AGA and AGU Series.
10) What is the difference between the Mersen OT-45 and a Littelfuse BLF midget fuse at 250V?
Both are fast-acting and both are used on 250V circuits, but they are not interchangeable. The Mersen OT-45 is a Class K5 one-time fuse rated 400A at 250VAC and 250VDC with a 50 kA AC interrupting rating, supplied in blade or ferrule connection styles for clip mounting in feeder and branch-circuit blocks. The Littelfuse BLF is a midget fast-acting fuse rated 250V in a laminated tube body, a physically much smaller midget-dimension fuse used in small control and supplementary protection holders. Choose the OT-45 when the drawing calls for a 400A Class K5 feeder fuse and the BLF when a midget fuse holder is in the panel.
11) Is the Square D PowerLogic meter fuse pack an alternative to the Mersen OT-45?
No, they serve entirely different roles. The Square D PowerLogic energy meter fuse pack is a set of three 0.5A 600V fuses used to protect the voltage-sensing leads of a power meter. The Mersen OT-45 Amp-Trap is a 400A, 250VAC/250VDC Class K5 fast-acting fuse with a 50 kA AC interrupting rating that protects feeder and branch circuits serving lighting, heating, and other non-motor loads. If you are fusing meter potential taps, the PowerLogic pack is the correct item; if you are protecting the feeder itself, the OT-45 is.
12) Does the Mersen OT-45 meet RoHS requirements for our facility specs?
Yes, the OT-45 is RoHS compliant, which is usually what an institutional or government purchasing spec is asking for on electrical consumables. That applies to both the blade and ferrule versions.
How fast can I get the Mersen OT45 delivered?
Most orders ship the same day from our distribution partners, with quick delivery throughout the continental U.S. Need it urgently? Contact Us for expedited shipping options.
Can I return it if it doesn’t work for me?
Yes—ZenSupply offers a hassle-free return policy. If the product isn’t the right fit, reach out to our support team for a return authorization or setup a return by following our return instructions.
Does the Mersen OT45 come with a warranty?
Absolutely. The Mersen OT45 is backed by a manufacturer’s warranty. For full warranty terms, check the product documentation or Contact Us.
How do I get technical support or installation help for the Mersen OT45?
You can call, text, or email our expert support team for fast answers. We’re here to help with product selection, installation tips, and troubleshooting. Contact Us
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