Best Fuse Selection Tips for Motor Starters

About this list

These tips apply to medium-voltage motor circuits protected by R-rated current-limiting fuses, with a note on IEC motor-circuit fuse-links. Each one is tied to ANSI/IEEE C37.46, IEC 60644, or a manufacturer's published catalog or technical guide, and figures are attributed to the source that gives them. The list helps you prepare a correct request; it does not size a fuse.

1. Start from the fuse the starter manufacturer specifies

In short: Every new installation and every replacement

A medium-voltage combination starter is designed and tested as a system: contactor, overload relay and fuse. Mersen's catalog states that the motor starter manufacturer generally specifies the R-rated fuse size, and that R-rated fuses must be coordinated with overload relays in combination motor starters.

Record the starter make, model and nameplate, and the full part number printed on the existing fuse, before you order anything. Those two pieces of information answer most replacement questions.

2. Read the R number as a melting point, not an ampere rating

In short: Avoiding wrong-size orders

ANSI/IEEE C37.46 defines an R-rated fuse as one that melts in 15 to 35 seconds at 100 times its R number. A 9R fuse, for example, melts in that window at 900 A. The fuse's continuous current rating is a separate number that the manufacturer publishes for each R rating.

Mersen's catalog adds that the continuous ampere rating reflects the fuse's thermal capability and should not be the sole factor in selection. Order by R number, voltage and part number together.

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3. Collect starting current, starting time and starts per hour

In short: Motors with long acceleration times or frequent starts

A motor fuse has to survive every start without its element being gradually weakened. ABB's medium-voltage fuse technical guide notes that direct-on-line starting current may be up to eight times rated current, and its selection method for IEC motor fuse-links reads the fuse rating from charts for starting times of 6, 15 and 60 seconds and from the number of starts per hour.

Even for R-rated fuses, these three figures are what a supplier needs to confirm that a proposed fuse will not melt or age during normal starts. IEC 60644 formalizes the same concern for IEC motor-circuit fuse-links by setting requirements for withstanding motor starting pulses.

4. Make sure the relay and contactor cover what the fuse cannot

In short: Checking an existing protection scheme

R-rated fuses are back-up fuses. Under ANSI C37.40, a back-up fuse interrupts only down to a stated minimum interrupting current. Below that, it cannot be relied on to clear.

Littelfuse's R-rated page describes the division of duty: the controller's overload relays provide overload and locked-rotor protection, and the controller interrupts low-value short circuits within its own capability, which protects the fuse from sustained overcurrents below its minimum interrupting rating. If a relay setting has been changed or a relay replaced, the fuse selection should be reviewed with it.

5. Tell the supplier when the motor is on a soft starter

In short: Reduced-voltage and soft-start installations

ABB's technical guide notes that soft starting equipment based on power electronics can significantly limit inrush current, and that soft-start motors can then be protected by a fuse-link with a much lower rated current.

That cuts both ways: a fuse chosen for direct-on-line starting may be larger than the soft starter's own protection requires, and a fuse chosen for soft starting may not survive if the motor is later switched to across-the-line starting in a bypass arrangement. Give the supplier the starter model and its bypass arrangement, not just the motor size. DENCO's R-rated family covers motor and soft-starter fuses.

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6. Match voltage, barrel count and mounting

In short: Replacements in existing starters

R-rated fuses come in single, double and triple barrel constructions, in ferrule styles for spring clips and bolt-in styles, some with hookeye attachments. Mersen, for example, publishes ferrule, bolt-on and DIN-dimension R-rated ranges, and Eaton lists R-rated fuses specific to its AMPGARD starters.

A fuse that fits the clips is not necessarily correct. Check the voltage rating, clip centers, barrel diameter and any indicator or striker that the starter uses to signal an open fuse.

7. Replace fuses as a matched set after a fault

In short: Post-fault maintenance

ABB's technical guide, citing IEC 60282-1, suggests replacing all three fuse-links in a three-phase circuit even if only one or two have operated, unless it is certain that no fault current passed through the others. Current that did not open a fuse can still change its characteristics.

The same guide advises against mixing fuse types, classes or manufacturers within one three-phase set, even when the rating plates match.

8. Do not mix IEC motor fuse-links with R-rated fuses

In short: Plants with both North American and IEC-built switchgear

IEC motor-circuit fuse-links are specified to IEC 60644 and IEC 60282-1 and do not carry an R rating. Some manufacturers offer R-rated fuses in DIN dimensions, but a DIN-size fuse is not automatically R-rated, and an R-rated fuse is not automatically suitable for an IEC starter.

Treat a change between the two systems as a new selection, with the starter manufacturer's approval.

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What to send with a quote request

For a medium-voltage motor starter fuse, DENCO needs:

• Existing fuse part number and manufacturer, if replacing
• Starter make, model and nameplate data, and whether it is a soft starter
• System voltage
• Motor full-load current, starting current, starting time and starts per hour
• Barrel count, mounting style and any indicator or striker requirement
• Quantity and required-by date (DENCO quotes a 30-day lead time on all products)

Frequently asked questions