Medium-voltage fuse selection: E-rated, R-rated, C-rated and loadbreak elbow

Start with the protected equipment

The letter in a medium-voltage fuse rating tells you the duty it was designed for. Choosing by current alone is the most common specification error, because two fuses of the same ampere rating in different classes will behave very differently under fault.

• Transformer or potential transformer — E-rated
• Medium-voltage motor or soft starter — R-rated
• Capacitor or capacitor bank — C-rated
• Underground distribution equipment, pad-mount or vault — loadbreak elbow
• Overhead distribution cutout — current-limiting dropout

Expulsion or current-limiting

Medium-voltage fuses divide first into two families, and ANSI C37.40 defines both. The difference is not how fast they are — it is whether the fuse reduces the fault current or only ends it.

Expulsion fuse — A vented fuse in which the gases produced by the internal arc — on their own or assisted by another mechanism — expel the arc and interrupt the current. An expulsion fuse limits how long the overcurrent lasts. It does not reduce how large the fault current gets.

Current-limiting fuse — A fuse whose element, when melted by a current inside the fuse's specified current-limiting range, abruptly introduces a high resistance that cuts both the magnitude and the duration of the current, and then interrupts it. Inside its current-limiting range, this type reduces the peak the downstream equipment sees as well as shortening the event. Outside that range it does not current-limit, which is why the three classes below exist.

This is why swapping an expulsion cutout link for a current-limiting design is a change to the system, not a like-for-like replacement. The let-through energy the rest of the installation experiences is different, and it should be assessed rather than assumed.

Full range, general purpose and back-up

Current-limiting fuses are then sorted into three classes by how far down the current range they can be relied on to interrupt. The class decides whether the fuse can stand alone or needs a second device behind it, and two fuses with the same voltage and ampere rating can sit in different classes.

Full range — Interrupts every current from its maximum rated interrupting current down to the lowest continuous current that will melt the element, at the maximum ambient temperature the manufacturer specifies. If the current is high enough to melt the element, the fuse can clear it. No second device is needed to cover a low-current gap.

General purpose — Interrupts every current from its maximum rated interrupting current down to the current that melts the element in one hour. There is a band below that one-hour current where the fuse can be overloaded but cannot be relied on to clear. Another protective device has to cover it.

Back-up — Interrupts every current from its maximum interrupting rating down to a stated minimum interrupting current — the lowest current it will clear properly. A back-up fuse is deliberately incomplete on its own. A low-current interrupting device, typically the motor overload relay, must sit in series with it.

The practical consequence is at the bottom of the range, not the top. All three interrupt a bolted fault. Only a full-range fuse is defined to interrupt every current that will melt its own element.

What the class letters mean

E, R and the PT designation describe defined melting behaviour, not a manufacturer’s model line. That is what makes them comparable between makers — and it is also their limit: the letter fixes the melting characteristic and nothing else. Voltage, interrupting rating, barrel count, dimensions and mounting all still have to match.

E-rated — defined by ANSI/IEEE C37.46. Used on feeder circuits, power transformers and potential transformers. Ratings 100 A and below must melt within 300 seconds at an RMS current between 200% and 240% of the continuous current rating. Ratings above 100 A must melt within 600 seconds at an RMS current between 240% and 264% of the continuous current rating. The E is a melting characteristic defined by standard, not a manufacturer's designation. That is what makes an E rating comparable between makers — but it fixes only the melting characteristic, not voltage, interrupting rating, dimensions or mounting.

R-rated — defined by ANSI/IEEE C37.46. Used for short-circuit protection of medium-voltage motor circuits, alongside a starter that provides the overload protection. Must melt within 15 to 35 seconds at a current equal to 100 times the R number. R-rated fuses are back-up fuses by class. They are not overload devices, and the R number is not an ampere rating.

PT (potential transformer) — defined by ANSI/IEEE C37.46. Used on the primary winding of a potential or voltage transformer, where the continuous current is a fraction of an ampere. Ratings 100 A and below must melt within 300 seconds at an RMS current between 200% and 240% of the continuous current rating. A PT fuse that conforms to the requirements for an E rating meets the C37.46 performance characteristics. Not every PT fuse is E-rated — fast-acting PT fuses are specified by their own opening-time tests instead.

A fuse that physically fits is therefore not necessarily electrically equivalent, which is the single most common error we see on a replacement inquiry.

The information you need before choosing

Collect these before specifying or requesting a quote. Missing any of them usually means the selection cannot be completed:

• System voltage, and whether the circuit is AC or DC
• Continuous load current, including any cyclic or inrush behaviour
• Available fault current at the point of installation
• Required interrupting rating
• The equipment being protected and its own withstand characteristic
• Physical envelope, mounting style and clip or bolt centres
• Indicator or microswitch requirement
• Ambient conditions — enclosure, outdoor exposure, oil immersion
• The standard or approval the installation must satisfy