Fuse cutouts are an iconic form of protection for overhead distribution lines. They can typically be found mounted on utility poles and have a porcelain or polymer insulator and a fuse holder attached via a hinge. If the fuse is blown or the fuse cut-out has tripped after an operation, the holder will hang from the insulator at an angle. Although they appear to be simple devices, fuse cutouts provide an important role in protecting transformers, feeder lines, and branch lines from fault conditions and overloads.
For the utilities, contractors, and electrical purchasers to comprehend a fuse cutout‘s operation is more than just an academic interest. It provides knowledge for selecting a product, maintaining it, analyzing faults, and ensuring continued reliability of distribution systems over time. This article will discuss the workings of a fuse cutout as well as the individual components, how they function during a fault, and what should be considered when choosing one for use in a distribution network.
What Is a Fuse Cutout?
The protective device known as a fuse cutout is commonly referred to as an electrical cutout or cutout fuse, and it is used primarily as a form of protection for medium voltage overhead distribution systems. A fuse cutout disconnects an electric circuit, when a large short circuit occurs along with either a transformer fault, line fault or extreme overloading, by interrupting the flow of excessive current through that fuse cutout.
There are basically two functions available from one device; protection by a fuse, and visually isolating circuits. First, when the fault current exceeds the fuse’s rated value, the fuse element will melt, and as a result, the holder will open to provide a clear indication that an interruption has occurred in the circuit. A visible open point is especially helpful to line crews while inspecting, troubleshooting and performing maintenance.
Fuse cutouts protect pole-mounted distribution transformers, capacitor banks, lateral lines, and small branch circuits. The type of fuse link used in combination with the cutout is dependent on how the application is designed; i.e., if the cutout will be used with an expulsion fuse link, a current limiting fuse, or if the circuit is equipped with loadbreak attachments .
How Does a Fuse Cutout Work?
Combining a fuse cutout with the use of a fuse link allows the fuse cutout to identify and stop abnormal current. The fuse cutout supplies an uninterrupted flow of current under normal operating conditions and its current path is typically from the top contact, through the fuse holder and fuse link, to the bottom contact. The fuse link has been properly constructed so that it can carry current at a normal load level without melting.
When a fault occurs downstream, the current rises rapidly. If the current exceeds the fuse link’s rated operating characteristics, the fuse element heats up and melts. This opens the circuit and stops the flow of current. In an expulsion type fuse cutout, the arc created during melting is extinguished inside the fuse tube. Gases generated by the arc-interrupting material help blow out and de-ionize the arc, allowing the device to interrupt the fault safely.
When the fuse link is activated, after that time, gravity will pull the fuse holder down away from the line (open). The action of dropping will make it easy for the utility crews to see that the fuse has opened and they can easily find the area of the line where the fault is and restore service after repairs have been made.
The Main Components of a Fuse Cutout
Fuse cut-outs have many common parts that work together to provide dependable safety, no matter the manufacturer or voltage class of the design.
Insulator
The insulator is a support element for the energized components, providing an electrically insulating separation from the mounting bracket and pole structure. The insulator can be made from either porcelain or polymeric materials. Historically, porcelain insulators have been the most commonly used due to their high mechanical strength and long service life. Porcelain insulators are generally the heavier traditional style of the two materials. Polymer insulators are advantageous due to their relatively low weight; their hydrophobic properties; and their ability to survive transportation and installation without significant breakage.
Fuse Holder or Fuse Tube
The fuse holder (fuse tube) contains the fuse link within its body and forms part of the current path; one function of the interior of the fuse tube in an expulsion fuse cutout system is to assist with extinguishing the arc when the fuse link has melted and separated. The fuse holder / tube is designed to handle mechanical forces / stresses, the thermal effects associated with arcing and melting of the fuse link, and outdoor weather-related environmental conditions (forces).
Fuse Link
The fuse link provides protection and is a replaceable part of the system. The fuse link is selected based on its current rating, time-current curve, transformer size, and coordination requirements. When the current reaches the fuse link’s maximum rating, it will melt and open the circuit. It is important to select the correct size for the fuse link as selecting the wrong size can create nuisance operations or provide too little protection.
Contacts and Terminals
Terminal connections, upper and lower, establish an electrical connection from line to fuse holder. Quality contacts help to minimize resistance, reduce temperature increases and minimize power loss. The terminals are designed so that conductors are securely connected and perform reliably when used outdoors.
Mounting Hardware
Typically, fuse cutouts are installed onto crossarms or pole structures, where they are supported by a mounting bracket that is designed to provide both stable mechanical support and proper alignment. Proper installation processes will help ensure that the cutout will operate smoothly when it drops out of the closed position and that the cutout will have long-term reliability.
Step-by-Step Operation During a Fault
To better understand how this process functions, let’s use an example where a fuse cutout protects a transformer. When in normal operation, the transformer is loaded and current is being drawn from it. The fuse cutout is in the closed position and carries the same current as the transformer without any trouble.
Current rises significantly when there is a fault either within the transformer or on the secondary side of the transformer. The characteristics of how quickly the fuse link will respond to this increase in current are described by the time-current characteristic of the fuse link. The fuse link will react almost instantaneously to a major fault causing a short circuit but could take much longer for a smaller amount of current; the total response time will be determined by the style and rating of the fuse link used.
When the element in a fuse melts, there is an arc inside the fuse holder. The process of expulsion will help to extinguish this arc. After interruption, a mechanical latch releases the fuse holder, causing it to swing downward to an open position. Thus, the circuit is now disconnected, with the faulted device isolated from the energized line.
Protecting upstream systems against localized faults potentially becoming widespread outages. Additionally, allowing maintenance personnel to see where they have operated cutout before performing inspection/replacement activities.
Why the Drop-Open Function Matters
Cutouts are still commonly used in overhead distribution systems because they have a benefit of having an open point after they have performed their function. This allows workers in the field to visually see that the cutout was opened for operational reasons, and thus provides additional safety for the workers, as well as makes it easier and quicker to troubleshoot.
An example would be that if power were to go down to a lateral line, the crews can patrol the feeder and quickly locate where a fuse cutout had dropped. The open fuse cutout would indicate the probable faulted section or protected equipment. Once the cause has been identified and corrected, crews will then be able to replace the fuse link and re-close the cutout using an insulated operating stick.
Common Applications of Fuse Cutouts
Fuse cutouts are typical in numerous overhead supply applications because they provide cost-effective, visible, and diverse protective means. Typical applications include transformer protection, lateral line protection, capacitor bank protection, as well as separation of power lines running into buildings and/or facilities.
The cutout is located on the primary side of the transformer, which is usually mounted on a pole. If there is an internal failure of the transformer, it will automatically disconnect from the primary line supplying power to the transformer. This provides additional protection to the upstream feeder for transformers and prevents continuous service interruption.
An important element in the reliability of a lateral line is having fuse cutouts to allow for the isolation of branches of the main feeder that contain faults. The benefit of isolating such branches is to prevent a problem on one branch from impacting all customers connected to the feeder through other branches. Proper fuse coordination is critical to accomplishing this goal.
Fuse Cutout vs. Disconnect Switch: What Is the Difference?
A standard disconnect switch provides no overcurrent protection or interruption of fault currents. Both types of devices can create a visible open point; however, the two devices function differently from one another. A fuse cutout operates in conjunction with the fuse link to provide overcurrent protection. A standard disconnect switch operates simply as an isolation means for equipment without providing any means of interrupting fault currents.
Not all types of cutouts are designed to be switched on or off with a load, but can have loadbreak hook or loadbreak tool accessories to allow switching under load. All products must have their ratings and application requirements verified by the purchaser prior to mounting, installing, or otherwise utilizing a cutout electrical device for switching reasons.
Key Ratings to Consider When Selecting a Fuse Cutout
To ensure safety and performance, it is essential to evaluate several ratings and operating conditions before choosing the proper distribution Fuse Cutout for the application. You must consider more than just the system voltage.
The distribution systems rated voltage must comply with the expectations of the distribution systems (e.g., 11kV, 15kV, 24kV, 27kV, 36kV). The rated current must be capable of handling the expected loads and be compatible with the fuse link selected. Furthermore, the interrupting capacity must be sufficient to clear any available fault current at the installation location.
The factors of insulation level, creepage distance, pollution performance, mechanical strength, and material selection must be considered when selecting a product, particularly in coastal, industrial, high-altitude, or severely polluted environments. When selecting an outdoor application, consideration should be given to how UV resistance, corrosion resistance, and sealing quality will impact service life.
When purchasing products, one of the most important aspects of the purchase will be whether they conform to certain specifications (for example, IEC, ANSI or whatever regional utility specifies). Additionally, a reputable manufacturer will have available to their customer technical specifications as well as type test reports and dimensional drawings. They should also be able to assist their customers in matching the fuse links to their usages or applications.
How Fuse Link Selection Affects Performance
The fuse link plays a crucial role in providing protection. The cutout may unnecessarily operate when the link rating is too low due to inrush or overloads that occur under normal operating conditions. Conversely, if the link rating is too high, the fuse link will not clear faults quickly enough to protect the equipment from damage.
Transformer kVA (kilovolt-amperes), circuit voltages, expected or actual loads, inrush currents, fault current levels and coordination with upstream and downstream protection devices are used to determine the three types of fuses – primary, secondary and multiuse. Time current characteristic curves are used to verify the appropriate operation of the three types under fault conditions while avoiding unnecessary outages.
Coordination between the protective devices in multiple distribution systems is critical. This will ensure that only the nearest protector (circuit breaker, recloser) operates first to isolate any fault on the circuit. The result of proper coordination is minimizing customer impact and providing improved reliability index for utilities.
What Happens After a Fuse Cutout Operates?
After a fuse cutout has operated, the first action is not only to replace the fuse link, but also to investigate the cause for operation. A blown fuse link could indicate a temporary fault or a failed transformer, damaged arrester, tree contact, animal interference, conductor fault, worn out insulation of equipment, etc.
An inspection of the protection equipment and adjacent line section should be conducted by qualified personnel. Upon completion of an operation to clear an operational fault and ensure that the system is safe to operate, a missing fuse link can then be replaced. The fuse holder is then to be re-installed into the equipment and replaced in accordance with the appropriate approved hot-line tools and procedures.
Never ignore repetitive fuse actions. They can signify that either the fuse is sized wrongly or being coordinated incorrectly, that the equipment has aged, or that there are ongoing environmental issues that may continue impacting the system. Reviewing this carefully will help prevent future outages and assist in increasing the system’s reliability.
Maintenance Tips for Reliable Operation
Fuse cutouts are devices that are considered as being fairly simple, but they still need to be inspected on a regular basis as they are subjected to the elements, pollution, corrosion, loose connections, and mechanical wear over time.
Usually during Utility Crew Inspection, they look for cracked Insulators, burnt Contacts, corroded Hardware, broken Fuse Tubes, loose Terminals and any sign of Tracking or Flashover. The Cutout should also be Installed in proper alignment so that when the fuse holder is removed, it drops down easily. The Contact Surface must be free of debris or defects; therefore, they will be mechanically sound throughout the inspection.
When there is significant pollution or salt fog, or industrially-contaminated areas, more cleaning and higher creep design might be required than other areas. UV exposure in the area will also produce different requirements for the durability of the polymer materials used. Regularly maintaining the electrical equipment can result in less unexpected down-time and increased life expectancy of the equipment.
Safety Considerations
Only trained personnel with authorization are allowed to operate fuse cut-outs. Due to their apparent simplicity, some operators could cause arcing and/or damage to equipment, as well as serious injury when operating these devices incorrectly.
When opening a cutout under a load, Operators should ensure they have adequate protection by using the correct insulator and by following utility standard operating procedures for switching. Furthermore, they should verify that the cutout can conduct a loadbreak operation prior to attempting to open the cutout with a load. A cutout that does not have loadbreak capabilities should not be handled as a standard load switching device unless proper equipment designed for loadbreak use has been installed.
How to Choose a Reliable Fuse Cutout Supplier
Quality of goods and customer service of a vendor are as important to a buyer as the price of the goods. A satisfying vendor will have an understanding of the distribution network’s needs and will provide the buyer with goods that will conform to accepted levels of quality. Clear documentation, consistent manufacturing quality and responsiveness from the vendor’s technical support staff can help shorten the procurement and installation process.
When choosing a vendor, consider the voltage class of the product, its rated current, its interrupting capacity, the type of insulation material used, the creepage distance between the pole and ground, the quality of the hardware, and whether or not the vendor has provided some sort of testing certification for the product being evaluated. It is also helpful to ask if the vendor has the ability to meet any custom requirements such as all types of brackets, types of terminals, materials for the fuse tube, or special designs and manufacturing for extreme conditions.
An electrical cutout having an excellent construction will provide reliable contact pressure, reliable dropout, excellent electrical insulation and long outdoor life; therefore each of these factors plays an important role in determining: safety, outage rate and maintenance expense.
FAQs
What is a fuse cutout?
An overhead distribution protection device, known as a fuse cutout, interrupts excess current flow through the use of a fuse link. When the fuse link melts, the fuse holder opens up and opens to the circuit giving a messenger of operation. The fuse holder also opens the circuit to isolate the faulted circuit.
Where is a fuse cutout used?
Fuse cutouts are often employed on medium voltage overhead lines to provide protection for transformers, lateral branches, capacitor banks and all other types of distribution equipment.
Why does a fuse cutout drop down?
When the fuse links operate, the drop opens. Drop opening, indicating that a circuit has been interrupted, helps line crew locate the defective piece of equipment or line.
Can a fuse cutout be used as a switch?
A few fuse cutouts have the capability of being used as a switch when they have proper loadbreak features or tools. Not all cutouts are able to interrupt load current but will state so on their product rating and/or manufacturer instructions. Before using one of those items as a switch it is always best practice to verify its application and adhere to its manufacturer instructions.
How do I select the correct fuse link?
Factors such as voltage level, transformer rating, circuit load, inrush current, fault current, and coordination requirement will dictate the proper fuse size for a given system. To ensure proper protection, review time-current curves for that fuse type.
What is the difference between an expulsion fuse cutout and a current-limiting fuse?
An expulsion fuse cutout uses the gases produced from the arc inside the fuse tube to put out the arc while current-limiting fuses are built to rapidly lower maximum fault currents within a given time frame. Which device is best suited depends on the fault current level, the level of protection needed, and what procedures & practices your utility follows.
The Fuse Cutout is able to carry regular currents through the fuse link. The Cutout will open automatically when the current flowing through it exceeds the limits of the fuse link based on an internal trip mechanism. This simple design, the visible dropping-open indicator, and the cost-effectiveness of the protection provided by a Fuse Cutout make this item a must-have for all overhead distribution networks. To have reliable performance, the buyer and engineer need to ensure they select the correct voltage rating, interrupting capacity, type of fuse link, material type, and supplier support for the actual operating environment.