Best Ways to Test High Voltage Fuses

About this list

This list covers the checks a qualified maintenance or testing crew can make on a medium or high voltage fuse. It draws on OSHA's electrical work practice rules, S&C Electric's instruction sheet for current-limiting fuse holders, ABB's Medium-voltage fuses technical guide and MV fuses catalog, and the fuse steps in the ANSI/NETA acceptance testing specification.

Nothing here is a live-test procedure. Medium voltage fuses are tested with the equipment de-energized and the fuse removed. If you are not trained and authorized to work on medium voltage equipment, the right test is to call someone who is.

1. Establish an electrically safe work condition before anything else

In short: Every fuse check, every time

Work under your site's NFPA 70E program: a risk assessment, isolation of every source, lockout/tagout, PPE for the task, and verification of absence of voltage. OSHA 29 CFR 1910.333(b)(2)(iv)(B) requires that a qualified person use test equipment to verify that the circuit elements and equipment parts are de-energized, including checking for induced voltage or backfeed. For circuits over 600 volts, nominal, the tester must be checked for proper operation immediately before and immediately after the test.

S&C Electric's instruction sheet for its pad-mounted gear states the same principle from the equipment side: always consider all parts live until de-energized, tested and grounded, and install suitable grounding equipment before touching any device in the high-voltage compartments.

• General industry lockout/tagout is 29 CFR 1910.147; utility generation, transmission and distribution work falls under 29 CFR 1910.269 instead.
• Only qualified persons should open medium voltage compartments or handle the fuses.

2. Read the blown-fuse indicator or striker

In short: A first answer on which fuse operated

Many current-limiting fuses carry a striker or visual indicator that moves when the fuse operates. ABB's guide describes "light" strikers used for visual indication, and heavier strikers that trip a switch-disconnector. DENCO's potential transformer fuses and C-rated capacitor fuses are available with a blown-fuse indicator, and its E-rated fuses with or without a trip indicator or microswitch.

Treat an operated indicator as confirmation, not as the whole story. A fuse whose indicator has not moved can still have been stressed by fault current, which is why the set is normally replaced after a fault (see the last item).

See the DENCO range

3. Check continuity on the removed fuse

In short: A quick go/no-go on a fuse that is out of the circuit

With the fuse out of its clips, an ohmmeter or continuity tester across the end caps shows whether the element is open. A reading from the fuse in its holder can be misleading because it may include parallel paths through the circuit, so test the fuse on its own.

Continuity proves only that the current path is intact. It doesn't show whether the element has been partly damaged, which is what the resistance measurement is for.

4. Measure resistance with a low-resistance ohmmeter

In short: Checking a new or in-service fuse against the manufacturer's data

ABB's installation checklist asks that the measured fuse-link resistance conform to ABB's catalog data or the documentation supplied with the fuse, measured with a milliohmmeter at an ambient temperature of about 20 °C. For its instrument transformer fuse-links, ABB's catalog specifies a bridge method or an instrument with an accuracy class of 0.5 percent or better, at 20 °C plus or minus 2 °C, which shows the level of care a meaningful reading needs.

Temperature matters because element resistance changes with temperature, so a fuse just pulled from a loaded circuit will not read the same as a cold one. Let it reach room temperature, record the ambient, and compare like with like.

5. Compare resistance across the set

In short: Spotting one fuse that differs from its neighbors

Where manufacturer data is not at hand, the fuses in a set can be compared with each other. The 2009 edition of the ANSI/NETA acceptance testing specification lists "Measure fuse resistance" among the electrical tests for switches and says to investigate fuse-resistance values that deviate from each other by more than 15 percent; its medium voltage motor starter section states that resistance values shall not deviate by more than 15 percent between identical fuses.

Current editions are ANSI/NETA ATS-2025 and MTS-2023. Use the acceptance criteria in the edition your contract names.

6. Check clips, holders and connections

In short: Finding heat sources outside the fuse itself

The same NETA switch sections call for verifying that each fuse has adequate mechanical support and contact integrity, measuring contact resistance across each switchblade and fuseholder, and inspecting bolted connections for high resistance with a low-resistance ohmmeter, a calibrated torque wrench or a thermographic survey.

ABB's maintenance guidance for voltage-free inspections adds cleaning the fuse-link body and base insulators, cleaning contacts and connections, applying acid-free vaseline and tightening bolts.

7. Infrared thermography during normal operation

In short: Finding hot fuses or connections without opening the circuit

A thermographic survey is the one fuse check made while equipment is in service, because it doesn't need contact with the fuse. NETA lists it as one of the accepted methods for finding high-resistance connections, and a fuse or clip that runs hotter than its neighbors under the same load is a reason to plan a de-energized inspection.

Opening covers on energized medium voltage equipment is itself energized work. Surveys must be done by qualified personnel under the site's NFPA 70E risk assessment and PPE requirements.

8. Striker trip test on switch-fuse combinations

In short: Fused switches where the fuse striker opens all three poles

In a switch-fuse combination the striker is part of the protection: when one fuse operates, it trips the switch. ABB suggests a switch-disconnector tripping test using a test fuse-link before a fuse-link is installed or replaced, and checking after installation that the fuse is seated correctly in the base contacts with the striker facing the tripping or signaling device.

IEC 62271-105 is the standard for these switch-fuse combinations. Follow the switchgear maker's instructions for the test device and procedure.

9. Know what field tests can't show

In short: Deciding between retesting and replacing

Interrupting performance and time-current characteristics are proven by design tests in a high-power laboratory, as set out in IEEE C37.41. No field meter can confirm them. Continuity and resistance tests catch open or damaged elements; they can't prove a fuse will still clear a fault the way its curve says.

That is the reasoning behind replacing the whole set after a fault. ABB's catalog, citing IEC 60282-1, says all three fuse-links should be replaced even if only one melts, unless it can be verified that the others experienced no overcurrent. When in doubt, replace.

What not to do

A few habits that are fine on a control circuit are dangerous on medium voltage equipment:

• Don't use a handheld multimeter across a fuse in energized medium voltage equipment. Voltage checks for absence of voltage use high-voltage test equipment rated for the system, in the hands of a qualified person.
• Don't trust a reading taken with the fuse still in its holder; remove it.
• Don't put a fuse back in service because it passed continuity after a fault on its circuit.
• Don't mix fuse classes or manufacturers in one three-phase set, which ABB recommends against even when rating plates match.

When the fuse needs replacing

Replace with a fuse of the same class, voltage rating, current rating and dimensions, confirmed against the drawings and the coordination study. If you have the part number of the installed fuse, DENCO's fuse cross-reference page explains how to send it; DENCO engineering will confirm an equivalent. DENCO quotes a 30-day lead time on all products, and tests fuses in-house at its factory lab, with third-party testing at IPH Berlin available on client request.

Frequently asked questions