Dead end clamps and suspension clamps are the most crucial parts of our distribution system. They perform one of the key roles in forming the electrical power distribution lines. In this article, we will discuss the design and testing for dead-end clamps & suspension clamps as per BS 3288. Let's start the discussion with the design & function dead-end clamps & suspension clamps.
Design & function of Dead End clamps
Dead-end clamps are often used at two spots in the distribution system. They are used to anchor the line conductor from the pole and when the line's direction is being changed (at extreme angles). Thus, Dead-end clamps are installed at the start & end of the line and also at the places where the line is taking the extreme turns.
There are various dead-end clamp designs available. Generally, these products are designed so that the whole assembly has no loose parts and the entire construction consists of aluminum alloy. The clamp consists of a clamp assembly through which the messenger wire will be passed. Generally polymeric or porcelain insulators are provided to separate lines and supporting structures. The bracket will be mounted on the pole using either the metallic strap or bolted directly. The bolt, nuts & washers are made out of galvanized steel.
Design & Function of suspension clamps
The main function of Suspension Clamps is to support & safely carry the cable at each intermediate poles. Except for the start & end of the pole and at extreme turns, the suspension clamps are installed at each pole.
The design of these clamps is fairly simple. The whole body is made out of aluminum alloy. The bolt, nuts & washers are made out of galvanized steel. The clamp body will be mounted on the pole using either a metallic strap or bolted. The messenger wire will be passed through the clamp. Polymeric or porcelain insulators are provided to separate lines and supporting structures.
Testing of the clamps
There are mainly two types of tests performed on the clamps i.e. Mechanical & Electrical tests. The dead-end clamps include both types of tests. Whereas, suspension clamps only include mechanical tests. As per the BS 3288, all the major tests are mentioned below.
To perform the tensile test, the clamp should be held in the tensile testing machine. The arrangement should be as close to the actual working condition of the clamp as possible. A rigid bar should be placed in the place of cable. The one-half of the minimum failing load should be applied at the 90° of the conductor axis. And it should be increased at a steady rate. And the failure should not occur at the load less than the specified minimum failing load.
Conductor slip test
The conductor should be fitted into the clamp. The conductor should be tightened to the recommended installation torque. And either on the conductor or the clamp, the axial tensile load shall be applied.
And the conductor shall not slip through the clamp at the load of 7KN or below the 5% of the rated tensile strength. Whichever is the greater value, the clamp should bear the load.
And the conductor shall slip at the load not greater than the 30% of the rated tensile strength. And apart from some marks of surface flattening of the strands, there should not be any physical damage to the conductor.
Clamp bolt tightening test
To perform this test, the situation of actual working should be created around the clamp. The nuts & bolts should be tightened with the specified torque value given by the supplier.
And the torque should be increased to 1.1 times. The connection shall remain serviceable for any number of subsequent installations & removals. And no unacceptable damage shall occur for either the conductor or the clamp components.
Now, the torque shall be doubled to the specified installation value or the maximum torque value given by the bolt supplier (whichever is lower). And once again, no unacceptable damage shall occur to either the conductor or clamp components.
The sample tests include four different types of tests.
1. Dimension verification
This one can be simply verified by the manual or the product specification plate or guide given by the suppliers. They shall match the ones given by the supplier.
2. Mechanical test
The mechanical tests are already discussed above.
3. Galvanizing test
The clamps will have many galvanized parts. They should match the standards and requirements of BS EN ISO 1461.
4. Routine test
This test is only applicable to the clamps that have castings as the principal load-carrying members or those fabricated with the welding. The clamp should be mounted like its actual working conditions. And the conductor shall be replaced by a rigid bar. And a tensile load equal to at least