Process Details for the Selection and Installation of 10kV Pole-Mounted Drop-Out Fuses

Fuses are connected in series in a circuit. When the current through the fuse element is less than or equal to its rated current, the element will not melt. Only when the current exceeds the rated value and reaches the fusing current will the element melt. When a short circuit (or overload) current occurs in the line, the current through the fuse element exceeds the specified value, causing the element to overheat and melt, thereby automatically interrupting the circuit. This prevents damage to the power grid or electrical equipment and prevents accidents, thus protecting electrical devices in the circuit. In 3kV–35kV small-capacity installations, fuses can be used to protect lines, transformers, motors, and voltage transformers.

Below, we will discuss the structural features, selection, and some technical details of installation for 10kV pole-mounted expulsion-type fuses.

1. Structure and Features of Common 10kV Pole-Mounted Expulsion-Type Fuses

The RW10–10F and RW11–10 models are two commonly used types of general-purpose expulsion-type fuses, as shown in Figures 1 and 2. Each model has its own characteristics. The former primarily uses the spring force of a coil spring to press the contacts tightly, with an arc extinguishing chamber and arcing contacts installed at the upper end, enabling live-line operation for opening and closing. The latter mainly relies on spring force to press the contacts but cannot be operated under load. The fuse tubes and upper/lower contact conductive systems of these two models have slightly different structural dimensions. To ensure interchangeability of fuse tubes and fuse wires during fault handling and reduce the number of spare parts required, it is advisable to use only one model of expulsion-type fuse within a maintenance area.

Under normal operation, the fuse wire is reliably tightened by a tensioning device, securing the movable joint of the fuse tube and keeping the tube in the closed position. When an overcurrent causes the fuse wire to melt, an arc is generated at the break inside the fuse tube. The arc extinguishing tube lining produces a large amount of high-pressure gas under the action of the arc, promoting rapid arc extinction. Subsequently, the spring bracket quickly ejects the fuse wire from the tube, while the fuse tube rapidly drops open under the combined force of the upper and lower elastic contacts and its own weight, creating a clear isolation gap and completing the circuit interruption.

At the upper end of the fuse tube, there is a pressure-release cap containing a low-melting-point fuse plate. When interrupting high currents, the thin fuse plate in the upper cap melts, creating dual-end gas exhaust. When interrupting low currents, the thin fuse plate remains intact, resulting in single-end gas exhaust.

2. Selection Principles for Expulsion-Type Fuses

1) Selection of Fuse Specifications:

• Rated Voltage: Select a voltage equal to or higher than the grid's rated voltage. For a 10kV distribution network, select a 10kV expulsion-type fuse, such as the RW10–10F or RW11–10.
• Rated Current: The fuse's rated current should be greater than or equal to the rated current of the fuse element.

2) Selection of Fuse Element Rated Current:

• For distribution transformers above 100kVA, the rated current of the high-voltage side fuse wire is selected at 1.5 to 2 times the transformer's high-voltage side rated current.
• For distribution transformers of 100kVA and below, the rated current of the high-voltage side fuse wire is selected at 2 to 3 times the transformer's high-voltage side rated current.
• The rated current of the low-voltage side fuse wire for distribution transformers is selected at 1 to 1.2 times the transformer's low-voltage side rated current.

3. Hazard Control and Safety Precautions During Installation

1) Hazard Control:

• Risk of falling from height or being struck by falling objects.
• Before climbing the pole, check that the pole base, climbing tools, and foot spikes are secure.
• Workers must wear safety harnesses and safety helmets. The safety harness should be attached to the pole or a sturdy component, avoiding sharp objects that could cause cuts.
• Materials, tool bags, and tools should be passed using ropes. Workers on the pole must prevent dropping items, and a barrier should be set up on the ground.
• Prevent slipping when using foot grips to climb the pole.
• Use an appropriate wrench to prevent slippage and injury.
• Before work, emphasize the names of adjacent energized equipment and the specific line, starting and ending pole numbers.
• Clearly communicate information about adjacent, crossing, overpassing, or parallel energized lines and assign a dedicated supervisor.
• Pole climbing inspections must be performed by two people: one working and one supervising. Before climbing, confirm the de-energized line name and pole number. The supervisor may only participate in work when the worker is safe, but the worker must remain within the supervisor's line of sight.
• For pole climbing inspections, all low-voltage lines and street light lines that are crossed must be verified as de-energized and equipped with temporary grounding wires.

2) Safety Precautions:

• Power-off installation work should be conducted in good weather. Do not work during thunderstorms, rain, snow, or high winds.
• After installation, perform open/close tests on the fuse tube to ensure good contact.
• Copper-aluminum connections should use copper-aluminum transition measures.
• Check that the selected fuse wire matches the capacity of the protected equipment.
• It is strictly prohibited to use copper or aluminum wire as a substitute for high-voltage fuse wire.

4. Preparations Before Installation

1) Personnel Organization:

• 1 work supervisor, 1–2 line workers.

2) Required Tools, Equipment, and Materials:

• Hoisting rope.
• Expulsion-type fuse.
• Crossarm for the expulsion-type fuse.
• Conductors.
• Copper-aluminum terminal connectors.
• Copper stranded wire (or aluminum stranded wire).

3) Pre-Installation Checks:

• Verify that the fuse specifications and model are appropriate, with a manufacturer's name and factory certificate of conformity.
• Check that all fuse components are complete and undamaged, with no cracks or damage on the porcelain parts.
• Ensure the shaft is smooth and flexible, with no cracks, sand holes, or rust on cast parts.
• The fuse tube should show no signs of moisture absorption, swelling, or bending.
• Check that the static and dynamic contacts have good contact and that the contact spring elasticity is appropriate.

5. Installation Procedure

• Verify that the specifications and model of the expulsion-type fuse match the design, and that documentation is complete.
• Assemble and adjust the expulsion-type fuse, fuse tube, and upper/lower leads. Use equipment clamps to connect the leads to the fuse.
• Install the crossarm and other fittings; install the crossarm in the designated position according to design requirements.
• Install the expulsion-type fuse:
  • During installation, tighten the fuse element to prevent overheating at the contacts.
  • The fuse must be securely and reliably mounted on the crossarm (structure), with no shaking or wobbling.
  • The angle between the axis of the fuse tube and the vertical ground should be 15°–30° to allow the tube to drop quickly under its own weight when the element melts.
  • The fuse should be installed on a crossarm (structure) at a vertical height of no less than 4.7m from the ground. If installed above a distribution transformer, maintain a horizontal distance of more than 0.5m from the outermost boundary of the transformer to prevent secondary accidents if the tube falls.
  • The length of the fuse tube should be properly adjusted. After closing, the duckbill tongue should engage more than 2/3 of the contact length to prevent unintended dropping during operation. The tube should not be jammed in the duckbill to ensure it can drop promptly after the element melts.
  • 10kV expulsion-type fuses are installed outdoors, requiring a phase-to-phase distance greater than 0.5m.
• (5) Connect the upper and lower leads of the expulsion-type fuse; the connections to the line conductors must be tight and reliable.

6. Installation Craftsmanship Requirements

• The upper and lower leads of the expulsion-type fuse must be reliably connected with good contact.
• When connecting copper to aluminum, use copper-aluminum transition clamps.
• After installation, the RW-type fuse tube should form an angle of approximately 30° with the pole.
• The fuse tube should be clean, crack-free, undeformed, and free of welding marks. The indicator should be intact and pointing downward. The resistance value of the fuse tube should meet the manufacturer's standards, or the difference between the three-phase resistance values should be less than 20%.
• The contact seat should be clean, free of rust or burn marks. If uneven, use a fine file to level it and sandpaper to polish it. Replace if the required condition cannot be achieved after treatment.
• Clean the base of the connection plate, polish with sandpaper, wipe clean, apply electrical grease or neutral petroleum jelly, and tighten the bolts.