Resistance and continuity
Fuse resistance testing measures the very small resistance across a fuse with a low-resistance ohmmeter. It finds damaged or partly melted elements and poor end-cap or clip connections in switches, motor starters, drives and network protectors, and it is judged by comparing phases, manufacturer data and earlier readings.
A fuse resistance test measures the small electrical resistance from one end of a fuse to the other, using a low-resistance ohmmeter that injects a controlled direct current and reads the voltage drop. The fuse is a deliberately weak link in the circuit, and its element is designed to carry normal current without heating much and to melt promptly under a fault. A healthy fuse of a given type and rating has a characteristic low resistance that is consistent from one sample to the next. The test is applied to medium-voltage power fuses in air, oil, vacuum and SF6 switches and metal-enclosed switchgear, to cutout fuses on distribution lines, to the fuses protecting adjustable speed drives and medium-voltage motor starters, and to the fuses in network protectors. It is quick, and it is carried out with the fuse out of service, either removed from its holder or isolated at both ends. The result tells the technician whether the element and the end connections are sound, which a visual inspection cannot confirm because the element is sealed inside the body.
A fuse does its job only if it is intact and correct. An element that has been stressed by repeated inrush currents, overloads or a fault elsewhere in the system can be partly damaged, with a section thinned, a joint weakened or a strand broken in a multi-element fuse. Such a fuse may still carry load today, but its resistance is higher, it heats more, and it may open at the wrong time or fail to clear a fault as designed. A poor connection between the fuse end and its clip or terminal adds heat as well, and the heat accelerates the same degradation. Consequences include nuisance outages when a healthy circuit loses a phase, single-phasing of a motor that is then damaged by unbalanced current, and in the worst case a fuse that fails to protect the equipment behind it. A resistance check is a low-cost way to catch a suspect fuse before it is installed, or during a maintenance outage, instead of finding out after a failure.
Every conductor has a resistance determined by its material, length and cross-section, and a fuse element is a deliberately narrow conductor shaped for a specific melting behavior. When a direct current flows through a fuse, the voltage drop across it is the current times its resistance, and the low-resistance ohmmeter calculates and displays that resistance. The method uses separate current and voltage connections, often called a four-wire or Kelvin arrangement, so the resistance of the leads and clips does not appear in the reading, which matters because the fuse resistance is tiny. If the element has been thinned or partly broken, the cross-section carrying the current is smaller and the reading rises. If the contact between the end cap and the instrument probe or between the fuse and its holder is poor, the reading also rises, which is why a high result calls for a second look at connections before blaming the fuse itself. Because the quantity of interest is small and sensitive to temperature, the measurement is made when the fuse has had time to settle at ambient temperature, and a comparison between like fuses is more telling than any single number.
The circuit is de-energized, isolated, verified dead and locked out, and the fuse is either removed from its mounting or isolated so that no other circuit path runs in parallel with it. A parallel path, such as a bypass, a connected load or a resistor across the fuse, would let the instrument measure that path instead. The fuse ends are cleaned where the probes will touch, because oxide and dirt produce misleading high readings. The low-resistance ohmmeter is connected with its current leads placed outside its voltage leads, so that the measured section is the fuse alone, and the clips are applied firmly on bare metal at both ends. The instrument injects its test current, and the reading is allowed to settle, then recorded. Where the instrument offers a choice of direction, a reading in both directions can be taken and averaged to cancel thermal electromotive force. Readings are taken for each fuse in the set, so the phases can be compared like with like, and the fuse type, rating and manufacturer are noted from the label. The ambient temperature is noted because resistance varies with temperature. If a fuse reads differently from its siblings, the measurement is repeated after re-cleaning and re-seating the connections before any conclusion is drawn.
The record holds the resistance reading for each fuse in the set, the fuse type, rating and manufacturer, the circuit or position it belongs to, the instrument and its calibration status, and the ambient temperature. It also notes the as-found condition of the fuse and its holder, for example clean, discolored, corroded or showing signs of heating, and whether the fuse was measured in place or removed. These details let a later reader compare like with like, and a fuse that was replaced after a questionable reading is documented along with the reason.
The shape of the criteria is comparison, not a single absolute limit, and this page gives no numbers. The first comparison is among the fuses of the same set, such as the three phases of one circuit, where fuses of the same type and rating should read closely alike and an outlier is suspect. The second is comparison with the manufacturer's published resistance data or a reference value for the same fuse type, since each design has its own expected range. The third is comparison with earlier results on the same installation, where an upward trend matters. When a reading is elevated, connections are re-cleaned and the test repeated, because poor contact is a common cause, and a fuse that is still out of family is replaced. The tests and the equipment on which they are called for appear in ANSI/NETA ATS-2025 for acceptance work and in ANSI/NETA MTS-2023 for maintenance work, covering switches, drives, motor starters and network protectors. Refer to your own licensed copy of these standards for the governing values and wording.
At acceptance the test checks that the fuses supplied and installed are sound and consistent, and it creates the baseline for later work. In maintenance it is repeated at outages and trended, and it is most valuable on fuses that have seen heavy duty or on circuits where a fuse has recently operated elsewhere. Power fuses in medium-voltage switches are large, easily measured and often hand-replaceable, while cutout fuses on lines are measured on the ground or in the shop. Fuses protecting drives and electronic equipment are often fast-acting current-limiting types, where a very small resistance and a tight match between phases matter, and the leads must be placed carefully on the end caps. Fuses in network protectors are handled with the unit isolated according to the manufacturer's procedure. In every case the measurement sits beside other low-resistance checks, such as contact resistance and bolted connection resistance, which look at the connections that carry the same current.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Yes. The element is sealed inside the fuse body, so a thinned, partly melted or broken element cannot be seen from outside. A low-resistance reading that is higher than that of identical fuses in the same set, or higher than the manufacturer's data, is one way to find such damage without destroying the fuse.
A fuse has a very small resistance, and a multimeter lacks the test current and the separate voltage leads needed to read it reliably. The lead and contact resistance would swamp the result. A low-resistance ohmmeter uses a four-wire connection so only the fuse is measured.
First clean the contact points, reseat the clips and measure again, because a poor connection is a common cause. If the fuse still reads out of family compared with its set, the manufacturer's data and earlier results, treat it as suspect and replace it before returning the circuit to service.
Only if the circuit is de-energized and no parallel path, such as a bypass, load or resistor, can carry the test current around the fuse. Otherwise the instrument reads the parallel network. When in doubt, remove the fuse or isolate it at both ends before measuring.
UltraDb references ASTM, ANSI, IEEE, NETA, NFPA, NEMA, FAA, TIA, and IEC standards descriptively, to identify the published methods its forms are built from. UltraDb and Verlecta are not affiliated with, endorsed by, certified by, or licensed by NETA, ASTM, ANSI, IEEE, NFPA, NEMA, FAA, TIA, IEC, or Megger. Standard names and section numbers are the property of their respective organizations. Always test to the edition your contract specifies. Where a standard is sold through the ANSI webstore, Verlecta may earn a commission on a purchase made through our link, at no extra cost to you. A commission never decides which standards we cite or how a form is built.