Best Fuse Types for Transformer Protection

About this list

This list groups transformer fuses by how they interrupt current, using the ANSI C37.40 and ANSI/IEEE C37.46 definitions the rest of this site uses, and by where they are installed. It is not a ranking of manufacturers. Figures quoted come from a named standard or a manufacturer's published application guide.

1. E-rated current-limiting fuses for switchgear and unit substations

In short: Primary protection of power transformers in indoor switchgear, load-interrupter switches and unit substations

An E rating is defined by ANSI/IEEE C37.46: at 100 A or less the fuse melts in 300 seconds at 200% to 240% of its rating, and above 100 A it melts in 600 seconds at 220% to 264%. Mersen's catalog describes E-rated fuses as general purpose fuses; ANSI C37.40 defines a general purpose current-limiting fuse as one that interrupts everything from its maximum rating down to the current that melts it in one hour.

Because they are current-limiting, they also cut the peak fault current the transformer and switchgear see, not only the duration. That is the main reason they are specified where available fault current is high.

• Defined melting point makes the rating comparable between makers
• Formats include ferrule, bolt-on and clip-lock mountings
• Available with indicators or microswitches for remote signaling

See the DENCO range

2. Full-range current-limiting fuses

In short: Distribution transformers where one fuse must clear everything from a low overload to a bolted fault

ANSI C37.40 defines a full-range fuse as one that interrupts every current from its maximum interrupting rating down to the lowest continuous current that will melt its element. There is no low-current gap for a second device to cover.

Manufacturers publish full-range designs for pad-mount and overhead distribution equipment; ABB's Hi-Tech line, for example, lists several full-range series. ABB's own technical guide notes that full-range performance is often achieved by combining a back-up element and an expulsion element in one device.

• Single device covers the whole melting range
• Published for distribution equipment

3. Back-up current-limiting fuses in series with a low-current device

In short: Oil-filled pad-mount transformers and switchgear where an expulsion link or switch handles low currents

A back-up fuse interrupts only from its maximum rating down to a stated minimum interrupting current. It is deliberately incomplete: another device, such as a Bay-O-Net expulsion link, a weak link or a switch tripped by a striker, must clear currents below that minimum.

ABB's technical guide gives a typical figure: back-up fuse-links have a minimum breaking current of approximately four times rated current. The pairing gives current limitation for high faults while the series device handles overloads and low-level faults.

• Must be coordinated with the series low-current device
• Oil-submersible designs sit in wet-well holders

See the DENCO range

4. Bay-O-Net expulsion fuse links (oil-immersed)

In short: Expulsion protection mounted in oil-filled pad-mount transformers

A Bay-O-Net holder carries an expulsion fuse link in an oil-filled transformer. Under ANSI C37.40, an expulsion fuse limits how long a fault lasts but not how large it gets, so on higher fault-current systems it is normally paired with an oil-submersible current-limiting back-up fuse.

DENCO's oil-immersed family supplies both parts: Bay-O-Net holders and links, and oil-submersible current-limiting back-up fuses.

See the DENCO range

5. Current-limiting dropout fuses for overhead transformers

In short: Pole-top transformers where a cutout expulsion link would allow too much fault energy

Overhead distribution transformers have traditionally been protected by expulsion fuse links in cutouts. A current-limiting dropout fuse fits a cutout mounting but limits the fault current as well as its duration, which matters where available fault current is high.

Swapping an expulsion link for a current-limiting design changes the let-through energy the rest of the circuit sees, so treat it as a system change and check coordination rather than assuming a like-for-like swap. DENCO's DHS/DHHS family is designed as a replacement for cutout expulsion fuse-links.

See the DENCO range

6. PT fuses for potential and voltage transformers

In short: The primary winding of metering and relaying PTs

A potential transformer draws a fraction of an ampere, so its fuse is sized very small and its job is to disconnect a failed PT without disturbing the system. Mersen's application information notes that PT primaries require current-limiting fuses or equivalent, applied in the 1/2 to 5 ampere range.

Some PT fuses meet the E-rating criteria; others are fast-acting designs specified by their own opening-time tests. ABB's guide adds that the fuse must not operate while the PT is energized, yet must still react quickly to a fault, so the rating is always a compromise.

See the DENCO range

7. Solid-material power fuses

In short: Substation and large-transformer protection where the design calls for power fuses with E-rated units

Solid-material power fuses interrupt current inside a chamber lined with solid arc-extinguishing material. S&C Electric, for example, publishes SM and SMD power fuses with E-rated fuse units for transformers and cables through 34.5 kV, in several time-current speeds.

S&C does not describe them as current-limiting, so do not count on them to reduce peak fault current. S&C's case for them is precise, stable time-current characteristics that keep a system coordination plan valid over time, with a choice of speeds.

8. Type HH (DIN-style) HV HRC fuse-links

In short: IEC-built switchgear using switch-disconnector and fuse combinations

ABB's guide calls transformer protection the main application for high-voltage fuse-links of this kind in IEC switchgear. They are often back-up fuses fitted with a striker that trips a switch-disconnector, so the switch handles overloads and the fuse handles faults. ABB's guide states that IEC/TR 62655 gives the selection method for transformer protection with these fuses.

They are specified to IEC 60282-1 rather than by an E rating, so do not mix them with E-rated fuses without checking curves and dimensions.

See the DENCO range

Four checks every transformer fuse must pass

ABB's medium-voltage fuse technical guide sets out the selection criteria that apply whatever fuse type you choose:

• It must not operate at the transformer's rated current.
• It must not operate at the transformer's permissible overload.
• It must ride through the magnetizing inrush when the transformer is energized. Mersen's application note, for example, checks its transformer fuses against 12 times full-load current for 0.1 second and 25 times for 0.01 second.
• It should clear a fault at the secondary terminals as quickly as possible, and its total clearing curve should stay below the transformer's damage curve.

Where the code and the equipment maker come in

In the United States, NEC Article 450.3 sets maximum overcurrent device ratings for transformer primaries. Mersen's application note cautions that sizing a fuse at the maximum the code allows may not provide adequate protection, and that the transformer manufacturer should be consulted for overload and short-circuit withstand capability. Treat published sizing charts as a cross-check, not as the selection.

Frequently asked questions