Current-Limiting Fuses

Description

The Type FM fuse is a totally-enclosed current-limiting fuse designed for motor-starting applications. It is available in basic voltage ranges of 2.4 and 4.8 kV, with current interrupting ratings of 80,000 amperes rms asymmetrical.

The fuse consists of silver elements surrounded by inorganic sand, all enclosed in a strong polyester base insulating tube. Circuit connection is made by silver-plated copper ferrules on each end of the tube. The silver elements are internally connected to the ferrules. When mounted, low resistance is maintained in its supports by the completely silver-plated ferrules.

A mechanical-type indicator projects from the lower end of the fuse when the fuse elements melt. This indicator is operated by a mechanism separate from the fusible elements, releasing only after the fuse elements have melted.

Current designations 2R through 36R are used to distinguish one size of fuse from another within the same voltage rating. Ampere ratings are not directly associated with the fuses; selection is determined by application requirements, involving motor-starting current and the time-current characteristics of the thermal overload relay.

Figure 1 illustrates a Siemens-Allis Type FM fuse alongside its corresponding fuse clips.

Fuse Application

For short-circuit protection, high voltage motor starters typically employ either fuses or circuit breakers. Standard fuses are classified as current-limiting or non-current limiting.

Current-limiting fuses are preferred for motor-starting circuits due to their fast operating time and current-limiting ability. They limit extreme dynamic and electrical stresses associated with short-circuit current and reduce the short-circuit current magnitude significantly below the available short-circuit current.

In selecting fuses, all factors affecting a particular application must be considered, including the motor full-load current, motor rotor current, and overload relay characteristics to ensure proper coordination. Other important factors include voltage rating, current rating, frequency rating, interruption rating, coordination, location, mounting, and motor data.

Most fuses are self-protecting, capable of extinguishing the arc for any current value within their interrupting capacity rating. Current-limiting fuses for motor-starting service are designed to carry low overload currents for extended periods, allowing for repetitive motor starting or jogging. These fuses must be used with a control contactor or similar device that can interrupt currents requiring over 100 seconds to melt the fuse, to prevent overheating and potential failure to interrupt.

Figure 2 depicts the operating characteristics of a current-limiting fuse, showing a graph of current versus time that illustrates fuse melting, arcing, and the peak current of the available symmetrical short-circuit current.

The interrupting ability of FM fuses has been tested in accordance with AIEE and NEMA standards, covering currents from minimum flowing levels up to 80,000 rms amperes at maximum design voltage. Without the fuse, the current would have reached approximately 80 kA rms (100 kA crest value) in the first loop.

Fuse time-current operating characteristics are typically given in terms of melting time versus current. The time-current characteristics of Type FM fuses can be derived from given characteristics. Figure 6 shows total clearing time curves, and Figure 7 shows minimum melting curves. The document refers to Figure 8 for current-limiting characteristics, illustrating the maximum instantaneous current peaks permitted by each fuse current designation for various available currents, as demonstrated by the time-current curves.

Type Designations

An explanation of type designations is provided. For example, a designation like '48-FM4R-4' breaks down as follows:

• Nominal voltage kV: 48 (for 4.8 kV)
• Fuse Type: FM
• Current Designation: 4R
• Number of elements: Indicated by the designation (e.g., Size 4 are single barrel fuses, Size 5 are double barrel fuses, Size 6 are triple barrel fuses).
• Fuse tube size: Indicated by the designation.

Application Diagrams

Figure 3 presents a schematic diagram of a typical motor-starting fuse application, showing a transformer connected to Type FM motor starting fuses supplying power to motor starters and the load.

Figure 4 shows a SpaceMaker II motor controller equipped with Size 5 double barrel Type FM fuses.

Characteristic Curves

Figure 5 is an oscillogram detailing the let-through current, melting time, and clearing time during a typical fuse interruption.

Figure 6 is a 'Clearing Time' graph plotting time in seconds against current in amperes for various Type FM fuse designations (e.g., 2R, 4R, 6R). The data is based on tests at 5500 volts AC at low power factor, starting at 25°C with no initial load, with curves plotted to minimum test points.

Figure 7 is a 'Minimum Melting' graph, similar to Figure 6, plotting time in seconds against current in amperes for different Type FM fuse designations, using the same data basis.