Fuses: fundamentals of fuses

A utility fuse protects equipment from being damaged. When an overcurrent event happens, equipment can be at risk for serious impairment or destruction. In this situation, a fuse will stop the flow of electricity providing a designated level of protection. When a fuse detects an overcurrent (fault) that is higher than its designated rating, it will operate and open.

Utility fuses are a necessity for applications such as the protecting equipment against violent failures, enhancing power quality, extending equipment life and limiting outages on the system. Fuses save users considerable amounts of time from having to replace expensive, long lead time equipment by operating when an overcurrent happens.

Fuses are widely used and are present in overhead and underground applications. The range of fuses vary between very low current protection (protection on a street light) all the way to very high current protection (solar farm transformer). Environmental initiatives have put the spotlight on the importance of protecting distribution equipment against failures without the emittance generated from operation. Innovation and expansion within the industry is broadening the spectrum of coverage. This is leading toward larger fuses, environment and fire-safe fuses, and dual protection fuses.

Current-limiting fuses

Current-limiting fuses are classified in three ways:

• Back-up fuse
• General purpose fuse
• Full-range fuse

All classifications can limit system fault currents to a fraction of available system fault level and are defined by IEEE standard C37.40.

Operating advantages

• Fast clearing
• Reducing burning at the point of fault
• Minimal line damage
• Reduction of damage to other equipment in the faulted circuit

Energy let through is the amount energy that is passed through the circuit before the fuse can operate. This energy can be harmful to equipment on the circuit. Traditional fusing requires energy to travel a full cycle after a fault occurs before the fuse will operate. This graph is a demonstration of how quickly a current-limiting fuse will operate vs a K-Link expulsion fuse. The reduced amount of energy that is passed into the circuit after a fault occurs can reduce damage to equipment within the circuit – saving the customer from experiencing an outage and avoiding the replacement of expensive equipment. Current-limiting fuses can be thought of as “zero-forcing devices” because of their ability to force the cycle back to zero – preventing excess energy let through.

Back-up current limiting fuses

Back-up current-limiting fuses are designed to clear high fault current. High fault currents occur when there is an internal equipment failure. Back-up current-limiting fuses minimize the effects of the high fault current stresses on other equipment, along with the distribution system. Back-up fuses require a low current interrupting device to be run in series for full current protection (high and low). Typical applications are on overhead distribution transformers, distribution single-phase deadfront padmount transformers and distribution three-phase deadfront padmount transformers. 

General purpose fuses

A general purpose fuse interrupts currents from the maximum interrupting rating, down to the current that causes the fuse element to melt in one hour. This is approximately 170% – 240% of the fuse rating. General purpose fuses do not include all the possible currents that the fuse may be required to clear. For example, the fuse could be required to interrupt a current much lower than the value stated by the one-hour criterion.

Full-range current-limiting fuse

A full range current-limiting fuse can interrupt all currents from the maximum rated interrupting to the minimum continuous current that causes melting of the fusible element. Full-range fuses provide protection by successfully interrupting any normal 60-cycle current that will melt its element (within the designated rating). This fuse does not have a one-hour limitation. Full range fuses provide comprehensive coverage and fit within an interchangeable cutout mount. They operate differently than an expulsion fuse link and provide environmentally safe operation. Typical applications are on overhead cutouts, riser poles, 3-phase deadfront padmount transformers and capacitors. 

Expulsion fuses

Expulsion fuse operation is based on two criteria:

• Melting open an element while building up adequate dielectric strength to clear a fault
• Waiting for current to pass through reference point zero to clear the fault. 

Common applications for expulsion fuses:

• Overhead systems such as fuse links mounted in open cutouts
• Underground systems known as Bay-O-Net links

Other types of expulsion fuses are vacuum, SF6 and boric acid fuses.

Expulsion fuses do not absorb appreciable energy, which means they do not modify the circuit during the fault interruption process – giving them the reference “zero-awaiting devices.” The faulted current will travel through the zero reference before the fuse will be able to clear the circuit. In this figure, the expulsion link has a greater energy let through as the faulted circuit travels through the zero reference.

Overhead system protection/cutouts

Cutouts and fuse links are the most common expulsion fuses used on all electrical systems. Classification for distribution fuses is defined by IEEE standard C37.47. Distribution fuses are farther from the substation, lower in voltage (2.8kV – 38kV) and have a lower X/R Rating. Cutouts and expulsion fuse links are an economical way to protect electrical distribution equipment. Different types of expulsion links can be inserted for use into a fuse holder. Types of links include Kearney and Edison and types of fuse series include: E, K, T, D, H, N, QA, S, X along with Bay-O-Net links cartridge fuses. Fuses are interchangeable within the fuseholder if they have the same Time Current Characteristics (TCC) and the same long-time continuous current rating. 

Transformer protection

Bay-O-Net fuses provide economical, field replaceable and readily available transformer fuse protection that come in a variety of links:

• Current sensing
• Dual Sensing
• Dual Element
• High amp overload

Bay-O-Net Fuses must be used in series with an isolation link or current-limiting fuse to prevent the possibility of closing in on a fault. Bay-O-Net fuses are an industry standard for transformer protection and reliability.