What Is a Multimeter Fuse For

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What is a multimeter fuse for? It guards the current input against one predictable error: a short created when the test leads are still in the current jacks and the meter is connected across a voltage. Ratings differ between models, so each meter’s own manual is the source for its numbers. This page covers why that fuse exists, what it protects, how two manuals print its rating, and what a blown fuse indicates.

Quick answer

The fuse sits in the current path so that a short through the current jacks opens the fuse first. Fluke describes it as protection against a simple but serious mistake, and LibreTexts describes it as protection against excessive current.
Ratings come from the manual of the model in hand. Klein’s MM400 manual pairs a 500 mA fuse at 600 V with a 10 A fuse at 600 V, and Keysight’s 34410A manual gives 3 AT, 250V, fast-acting for its current input.
A blown fuse, per Klein’s manual, follows an input beyond the rating. The breaking figure is absent from both manuals we fetched.

Electrical safety

Work on mains circuits can kill. Before you take any reading, confirm that the category rating printed for the meter covers the circuit in front of you. The CAT figure should come from the listing or the manufacturer’s manual, not from memory. Panel and service work belongs to a licensed electrician.

Several sources point the same way. The Klein MM400 manual states that ignoring its warnings can lead to serious injury or death, and it treats voltages above 25V AC RMS or 60V DC as a shock hazard. Its specifications list a CAT III 600V safety rating, and its test-lead note requires CAT III or CAT IV leads. Under the manual’s definition, CAT III covers measuring circuits on the distribution side of a building’s low-voltage supply. The Fluke guide ties voltage and current measurements made without a tester’s designed fuse protection to burns and, possibly, death. OSHA frames its electrical rules around shock, electrocution, fire and explosion risks.

A fuse is one protection feature, and the CAT rating is a separate limit. The Klein manual states that limit plainly: never apply the meter to a circuit whose voltage is above the category rating it carries. This page does not describe any measurement on live mains, panels, outlets, or service wiring. Those tasks fall outside its scope.

Why current ranges have a fuse

A voltmeter is connected across a source, while an ammeter has to sit in the current path. The LibreTexts lesson on ammeter use explains the risk: an ammeter will act as a short circuit when it is placed in parallel across a substantial source of voltage. Ammeters are generally protected by a small fuse located inside the meter housing. A surge from a substantial voltage source blows that fuse and leaves the meter unable to measure current until the fuse is replaced.

The fuse exists because of a common mistake. The Fluke guide describes a user who makes a current measurement with the leads in the current jacks, then tries to read voltage and forgets the leads are still in the amps jacks. “This effectively places a short across the voltage source.” To guard against that error, meter manufacturers started putting a fuse in series with the meter’s test lead jacks.

Voltage inputs work differently. The Fluke guide notes that voltage measuring inputs have a high input impedance, which makes an overcurrent unlikely, so they are generally built with overvoltage protection instead of a fuse. The Klein MM400 shows that this is a generalization rather than a rule for every meter. Its manual limits the V (A/mA) jack to a 500 mA fuse. Check the manual for each jack instead of assuming that a voltage jack is unfused.

What the fuse protects against

The main hazard is a short through the current jacks. It happens when the leads are still in those jacks and the meter is placed across a voltage. The current is set by the voltage and by the very small resistance of the current path. The Fluke guide works through an example: a CAT III 1000 V meter with the leads in the amps jacks has a series resistance of approximately 0.1 ohms. Across a 1,000 V source, Ohm’s law gives a current of 10,000 A (1,000/0.1 = 10,000). That is the guide’s worked example, not a rating for any meter, and it shows why a fuse has to break that current quickly.

The guide also explains that specially designed high-energy fuses are built to keep the energy of such a short inside the fuse enclosure, which protects the user from electric shock and burns. It adds that not all fuses with the same amperage and voltage rating are the same. The sources do not say how this protection compares across other meters.

What the manuals state: fuse rating and breaking capacity

A fuse rating and a breaking capacity answer different questions. The rating gives the amperage and voltage a fuse is specified for in its position. The breaking capacity is the high current the fuse must be able to open at safely. The Fluke guide notes that fuses can be designed to open at a high instantaneous current as well as at a steady rated current, and it calls that figure the minimum interrupt current. The Fluke fuse list we opened uses an abbreviation, IR, next to many of its fuse parts without defining it.

The table below lists only what the two manuals we opened state. Where a manual is silent, the cell says so.

Manual (copy opened)	Fuse position	Fuse rating as stated	Breaking capacity as stated
Klein Tools MM400 instruction manual, English pages (PDF on a UBC course site)	V (A/mA) jack: 500 mA/600 V fast-blow, Klein Cat. No. 69033. 10A jack: 10 A/600 V fast-blow, Klein Cat. No. 69032.	“Overload Protection: 500mA / 600V and 10A / 600V Fuses”	Not stated in the copy we opened
Agilent (now Keysight) 34410A/11A User’s Guide, Fifth Edition, March 2012 (PDF on a Boston University course site)	Current input path, in a screw-in holder on the left side of the rear panel. The manual gives the supplied fuse as 3 AT, 250V, fast-acting, 5x20mm.	DC current specification: “Externally accessible 3 A, 250 V fuse”	Not stated in the copy we opened

Two points follow from the table. Each manual prints its fuse rating, and the wording differs, so read the manual for your own model. Neither copy states a breaking capacity, so this page does not give one. The Fluke fuse list shows IR values for fuse part numbers, running from 100 A to 100 kA. Because those values belong to fuse parts rather than to a meter named on this page, they are not used as ratings here.

The manuals also cover replacement, and this page leaves those steps out. The Fluke guide notes that ratings appear in the manual and often on the meter itself. A Fluke list warns, “Do not substitute other brands or types of fuses”. Keysight limits replacement to the specified type and rating, and Klein’s manual asks the user to take the leads off any voltage source before reaching the fuses.

What a blown fuse tells you

A blown fuse is a symptom, and the Klein manual links it to input above the rating. The manual says a fuse “may blow if more than 500mA is applied to the V jack”, and it gives the same rule for the 10A jack at 10 A. The LibreTexts lesson describes what the user sees: the meter cannot measure current until the fuse is replaced. The sources do not say what happens to voltage or resistance readings in that state.

A blown fuse therefore suggests the meter was connected to something beyond its rating, such as a voltage placed on the current jacks. The Fluke guide describes this as a common mistake. That reading is an inference from the manuals, not a statement any source makes about every blown fuse. The sources do not say whether a fuse can blow without an overload, so check how the meter was connected before you replace the fuse with one of the same type and rating. If you need the test side of that task, our step-by-step guide to checking a fuse with a multimeter covers it.

Common mistakes

Leaving the leads in the current jacks. The Fluke guide describes the user who forgets the leads are still in the amps jacks before a voltage measurement.
Assuming every jack has the same fuse. The Klein MM400 fuses its V (A/mA) jack at 500 mA and its 10A jack at 10 A, so the jack sets the limit. The Keysight 34410A fuses its current input path separately from its power-line fuse.
Replacing a fuse with one of a different type or rating. The Fluke guide says that a fuse without the right ratings, or a wire wrapped around the fuse connections, is a serious hazard, and it calls the result a “thermal hand grenade.” The Fluke fuse list warns against substitutes.
Using test leads that do not match the meter. The Klein manual requires that test leads be CAT III or CAT IV rated.
Operating the meter with the back housing off. The Klein manual says not to operate the meter while the back housing is removed.

How we chose

We chose sources by four rules. Fuse ratings had to come from the named meter’s manual or from a manufacturer page that lists that meter’s fuses. Each safety claim needed three opened sources, and the electrical safety section draws on the Klein manual, the Fluke guide and OSHA for that. A Klein catalog page was used only for listing facts, such as which meters a fuse part fits, and never for procedure or safety. Forum posts and search snippets were not used. Every quotation in this draft was checked against text fetched with curl on 2026-10-22.

Seventeen fetches were attempted. Seven are cited, and three were opened but not needed. The rest returned 403 responses, a denial page, an HTML page instead of a manual, or no response. Among the failures were a Klein MM400 datasheet copy on a distributor host, Keysight 34461A support and product pages, and a Fluke product page for the 87V. Third-party manual sites were not used for the 87V, because the copies found were older or belonged to other models.

What we could not verify

None of the points listed here is settled by the material we fetched, so this draft leaves each one open.

Breaking capacity for the MM400 and the 34410A. No interrupt or breaking figure appears for the fuses in the two copies we fetched.
Any fuse rating for a Fluke 87V. The 87V manual could not be fetched. The Fluke fuse list we fetched has no 87V row and says “Check with Fluke” for the base 83, 85, and 87 models.
Any fuse rating for a Keysight 34461A. Its user guide could not be fetched, because the Keysight pages returned 403.
Whether the two university-hosted manual copies match each manufacturer’s current revision. The manufacturers’ own manual pages for these meters could not be reached, so the revision is unconfirmed.
Whether every meter with a current range has a fuse, and whether every current jack is fused. The sources here cover two meter manuals and one fuse list.
Whether a fuse can blow without an overload, and how long a fuse lasts in service.
Whether any local rule requires a licensed electrician for panel or service work. The sources do not state a licensing rule for any area.

FAQ

Why does a multimeter have a fuse?

It protects the current input from a short through the current jacks, such as leads left in the amps jacks while the meter is across a voltage. The Fluke guide and the LibreTexts lesson both describe that purpose.

Does the voltage jack have a fuse?

It depends on the meter. The Fluke guide says voltage inputs are generally built with overvoltage protection instead of fuses. The Klein MM400 manual, however, places a 500 mA fuse on its V (A/mA) jack, so check the manual for your meter.

Which fuse should I use?

Use the type and rating stated in your meter’s manual. The Fluke fuse list warns against other brands or types. The sources we opened do not give a general rating that applies to every meter.

Does a blown fuse mean the meter is broken?

The sources do not say that. The LibreTexts lesson says the meter cannot measure current until the fuse is replaced, and the Klein manual says a fuse may blow when its rating is exceeded. Neither source says a blown fuse always means other damage.

Is the fuse rating the same as the breaking capacity?

No. The rating is the amperage and voltage a fuse is specified for in its position. The Fluke guide’s term for the high-current figure is minimum interrupt current. Neither the MM400 copy nor the 34410A copy we opened states that figure.

Does a fuse make a meter safe for mains work?

This page does not treat a fuse as a safety rating for mains work. The CAT rating and the test-lead requirement are separate limits, and the Klein manual states them separately. Work on mains circuits can kill, and panel and service work belongs to a licensed electrician.

Bottom line

Treat the current-jack fuse as protection against one specific mistake, not as a general safety rating. Before any measurement, check the jack you are using, the fuse rating in that meter’s manual, and the CAT rating for the circuit. If you are choosing a meter, compare the options in our multimeter comparison and read the manual of the model you pick. If your task is DC current, our guide to DC current meters covers that choice.

Sources

Fluke learning blog article on picking a replacement fuse: https://www.fluke.com/en-us/learn/blog/digital-multimeters/choosing-the-correct-fuse-for-your-tester (opened 2026-10-22)
Fluke knowledge base, KA-02781, fuse part list for Fluke meters: https://my.fluke.com/en-us/article/KA-02781 (opened 2026-10-22)
Klein Tools, MM400 Instruction Manual, English pages, PDF on a UBC course site: https://www.eoas.ubc.ca/courses/atsc303/Instruments/manuals/0_Klein-Tools-MultiMeter-MM400-UsersManual.pdf (opened 2026-10-22)
Klein Tools, 69033 fuse listing (5×20 mm, 500 mA, 600 V): https://www.kleintools.com/catalog/test-measurement-accessories/fuse-5×20-500ma-600v-mm400 (opened 2026-10-22)
Agilent (now Keysight), 34410A/11A Multimeter User’s Guide, Fifth Edition, March 2012, PDF on a Boston University course site: https://sites.bu.edu/engcourses/files/2016/08/agilent34410.pdf (opened 2026-10-22)
Kuphaldt, “How to Use an Ammeter to Measure Current,” Electric Circuits VI – Experiments, LibreTexts: https://workforce.libretexts.org/Bookshelves/Electronics_Technology/Electric_Circuits_VI_-_Experiments_(Kuphaldt)/02%3A_Basic_Concepts_and_Test_Equipment/2.04%3A_How_to_Use_an_Ammeter_to_Measure_Current (opened 2026-10-22)
OSHA, Electrical safety overview: https://www.osha.gov/SLTC/electrical/ (opened 2026-10-22)