In overhead power lines or substations, hardware used to suspend conductors and lightning arresters from insulators, or to suspend lightning arresters from towers, is referred to as suspension hardware. It is also known as support Fiber Optic Cable Hardware or suspension clamps (for consistency, hereinafter collectively referred to as "suspension clamps"). Modern installations increasingly utilize specialized fiber suspension clamps and fiber optic suspension clamps alongside traditional conductor suspension systems.
Basic Structure of Suspension Clamps
A suspension clamp typically consists of a clamp body, a pressure plate (including fastening bolts), and an R-pin. Figure 2-1 shows a standard suspension clamp, also known as a parallel hanger-type suspension clamp. Its operating principle involves using two U-bolts to clamp the pressure plate, securing the clamp within the clamp body.
Figure 2-1: Actual Photo of Standard Overhead Cable Clamp
Figure 2-2 shows two additional configurations of suspension clamps. Their basic structures resemble those of standard suspension clamps. The bowl-head hanger plate suspension clamp incorporates a bowl-head hanger plate onto the hanger plate suspension clamp. The U-shaped hanger plate suspension clamp adds a U-shaped hanger plate to the parallel hanger plate suspension clamp.
Figure 2-2 Actual Photos of Overhead Clamps
(a) CGU-XXX Overhead Clamp (with Bowl-Head Hanger Plate); (b) CGU-XXX Overhead Clamp (with U-Shaped Hanger Plate)
Cable Clamp Hull
Wire clamps and pressure plates are typically manufactured from malleable cast iron, aluminum alloy, or formed by stamping steel plates. Conductors and ground wires are placed within the clamp's channel, whose curvature radius should exceed eight times the diameter of the suspended conductor.
(1) Wire clamps made from forged cast iron. Conductors made of forged cast iron form a closed magnetic circuit around the conductor. Even with minimal magnetic resistance, alternating magnetic fields from AC current induce hysteresis losses and generate heat, raising conductor temperature. Conductor heating reduces mechanical strength, thereby limiting transmission capacity. Thus, while iron (or steel) conductor clamps offer low initial investment, they increase line losses and annual operating costs.
(2) Steel plate stamped suspension clamps feature a hull formed by stamping steel plates, eliminating the need for a hanger plate. The suspension point is positioned above the conductor axis, with U-bolts installed upward, facilitating construction. This type offers advantages such as simple production processes, short cycles, high yield rates, lightweight design, and fewer components. It is suitable for installing steel-core aluminum stranded conductors and aluminum stranded conductors of small to medium cross-sections.
(3) Clamps made from aluminum alloy. Aluminum alloy suspension clamps are currently being widely promoted for their energy-saving properties, anti-halation capabilities, and lightweight design.
Clamping Plate
The clamping plate utilizes the clamping force generated by tightening U-bolts to grip the conductor and ground wire securely.
Closed-End Pin (R-Pin)
The closed-end pin, also known as an R-pin, is a component designed to prevent bolts from loosening.
Types and Examples of Overhead Clamps
Types of Overhead Clamps
Figure 2-3 illustrates the classification of overhead clamps. The correspondence between the designation numbers and the nominal breaking loads of overhead clamps is shown in Table 2-1.
Figure 2-3 Model Designation of Suspension Clamps
In the new standard, suspension clamp models begin with "X." The old standard prohibited 'X' and instead used "C." Manufacturers currently employ both designations.
Table 2-1 Correspondence Between Designation Digits and Nominal Breaking Loads
Example: CGU-3 suspension clamp: "C" denotes suspension clamp; "G" denotes fixed type; 'U' denotes with U-bolt; "3" denotes combination number. Referring to Table 1-4 or hardware product catalogs, it is suitable for aluminum stranded wire diameters of 16.0–18.0 mm and steel-core aluminum stranded wire diameters of 13.0–14.5 mm.
CGU-5A suspension clamp denotes a U-bolt fixed suspension clamp. The additional letter "A" following the combination number "5" indicates it features a bowl-head hanger plate.
XTS-2A suspension clamp: "X" denotes suspension clamp, "T" denotes jumper wire use, the number '2' indicates wire combination number (wire diameter), and "A" indicates it features a bowl-head hanger plate.
Suspension Clamp Model Examples
According to DL/T 683-2010, suspension clamp model examples are shown in Table 2-2.
Table 2-2 Suspension Clamp Model Examples
Specialized Fiber Optic Cable Suspension Systems
Modern telecommunications infrastructure integrated with power transmission systems requires specialized suspension clamps for fiber optic cables. These include:
(1) ADSS Suspension Clamps
ADSS suspension clamps (All-Dielectric Self-Supporting) are specifically designed for self-supporting fiber optic cables that contain no metallic components. These aerial cable suspension clamps provide secure support while accommodating the unique mechanical properties of dielectric cables. The design ensures minimal stress concentration on the cable jacket and maintains proper clearance from energized conductors.
(2) OPGW Suspension Clamps
OPGW suspension clamps (Optical Ground Wire) are engineered for cables that combine fiber optic elements with ground wire functionality. These Tension clamps must provide both electrical grounding and fiber protection, requiring specialized designs that prevent fiber strain while maintaining electrical continuity.
(3) Helical and Preformed Suspension Clamps
Helical suspension clamps utilize preformed helical rods that grip the cable through elastic tension distribution along the cable length. Preformed suspension clamps offer significant advantages including:
- No mechanical damage to cable surface
- Uniform stress distribution
- Quick installation without special tools
- Enhanced vibration damping characteristics
- Self-adjusting grip under varying load conditions
The preformed suspension clamp design is particularly effective for fiber optic suspension clamp applications where cable integrity is critical. These clamps feature helical armor rods that wrap around the cable, distributing mechanical loads evenly and protecting against aeolian vibration damage.
Requirements for Suspension Clamps on Operating Lines and Failure Analysis
(A) Technical Requirements
Technical requirements for suspension clamps must comply with the provisions of DL/T 756-2009 "Suspension Clamps."
(1) General technical conditions for suspension clamps shall conform to GB/T 2314-2008 and be manufactured according to approved drawings following prescribed procedures.
(2) Design shall minimize the impact of micro-vibrations on conductors and ground wires. Clamps shall exhibit favorable dynamic characteristics, with freely rotating bodies and minimal rotational inertia relative to the pivot axis.
(3) Electrical energy losses due to hysteresis and eddy currents shall be minimized.
(4) The curvature radius of the conductor channel in the clamp body shall not be less than 8 times the diameter of the conductor or ground wire.
(5) Sufficient contact area shall exist between the clamp and the conductor/ground wire to minimize conductor damage caused by short-circuit currents.
(6) Surfaces in contact with conductors or ground wires, such as conductor channels and clamping strips, shall be smooth and flat, free of burrs, protrusions, or defects that could cause conductor abrasion.
(7) The structure shall facilitate live working operations, minimize the number of components, and permit installation or removal using live working tools.
(8) Both ends of the hull shall feature a smooth flared shape, and both ends of the clamping strips shall have smooth curved profiles.
(9) The design shall accommodate various installation conditions, including bare conductors, ground wires, or conductors wrapped with armor tape or protective strips.
(10) The groove radius value shall be selected according to Table 2-3.
Table 2-3 Groove Radius Values for Conductor Clamp Hulls (mm)
(11) Primary dimensions include total length, overall height, total mass, maximum exit angle per hull side (generally not less than 25°), minimum exit angle (generally not greater than 3°, except for Drop Wire Clamps for large spans), and allowable rotation angle of the hanger.
(12) Nominal breaking load, hanger (ear) bolt diameter, and bolt spacing shall be selected according to Table 2-4. For conductors with cross-sectional areas of 300–400 mm², the breaking load shall not be less than 60 kN; for 630–720 mm², not less than 80 kN.
Table 2-4 Nominal Breaking Load of Suspension Clamps
(B) Failure Analysis of Suspension Clamps in Operating Lines
1. Wear of Suspension Clamps
The pins of suspension clamps form moving pairs with components such as hanger plates and rings, creating point and line contact at the interface. This relative motion generates excessive localized pressure, leading to wear, fracture, and subsequent operational failure.
Primary Causes:
(1) Under wind forces, relative motion occurs between the clamp body and hanger plate. The hanger plate pivots at a small angle around the body's pivot pin. Due to the hanger plate's thinness, it acts like a blade cutting into the body, gradually reducing the cross-sectional area of the body's pivot pin. When wear reaches a certain level, the conductor or ground wire may detach from the suspension clamp under its own weight, causing an accident.
(2) Oversized clamps or insufficiently tightened conductors/ground wires generate relative movement under wind or lightning current forces, causing the conductors/ground wires to be cut or burned through. This results in the conductor/ground wire detaching from the clamp.
2. Impact of Product Manufacturing Quality and Environmental Conditions
(1) When steel components are used, substandard hot-dip galvanizing applied for rust prevention may fail due to environmental contamination and corona corrosion during operation.
(2) Cracks inherent in the structure itself, or internal stresses resulting from improper manufacturing processes or heat treatment, can lead to fatigue failure after prolonged operation.
(C) Special Considerations for Fiber Optic Cable Suspension Systems
When installing aerial cable suspension clamps for fiber optic applications, additional failure modes must be considered:
(1) Cable Jacket Damage: Improper clamp installation or excessive clamping force can damage the cable's outer jacket, exposing fiber elements to environmental degradation and moisture ingress.
(2) Fiber Strain: Excessive mechanical stress transmitted through the suspension clamp for fiber optic cable systems can cause micro-bending losses or fiber breakage, degrading optical performance.
(3) Vibration-Induced Fatigue: Fiber suspension clamps must effectively dampen aeolian vibration to prevent fatigue damage to both the cable structure and internal fiber elements. Helical suspension clamps and preformed suspension clamp designs excel in this application due to their superior vibration damping characteristics.
