Design and Testing for Overhead Line Clamps as per BS 3288
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 50% of the specified minimum failing load shall be applied at the 90° of the conductor axis for 30 seconds. And throughout the test, the clamp should not be damaged.
This test is specifically applicable to Tension joints and Tension Terminations if the design is such that the conductor is not continuous through the clamp as shown in the below image.
The electrical test includes various different types of to test the clamp’s capabilities. Let’s discuss them one by one.
Ambient test temperature: 15°C and 30°C
The clamp fitting should be assembled as per the recommendations given by the manufacturer. With the use of a reference conductor, the resistance/mm shall be calculated. And using which, the resistance of the 50mm conductor can be calculated and subtracted from the potential resistance measured points. This result should not exceed 75% of the measured resistance of the conductor.
For measurement, the assembly shall contain a length of the unjointed conductor which shall be used as a reference for resistance & temperature measurements. If the two sizes of conductors are being connected into the assembly, then the smaller one should be taken as reference. The length of the reference conductor should also be no less than 100 times the diameter (maximum up to 4m)
The DC supply shall be used in conducting the test. For the effective contact with the strands of the conductor, the current connection shall be taken at the minimum of the 50 times the diameter of the conductor from the fitting.
The test shall be repeated once again with polarity reversed and the average of the two results should be taken as the measured value. There is one more alternative method to conduct the same test. For which, the BS 3288 can be referred.
Electrical heating cycle test
If the conductor is not continuous through the clamp, this test is being performed.
The clamp fitting should be assembled as per the recommendations given by the manufacturer. The test shall be conducted indoors in the draught free condition. And the care should be taken in the erection of the case assembly to minimize the effects of thermal interference.
1. Before heat cycling, the resistance across each shall be measured as per the method specified in the resistance test.
2. The test current shall then be passed through the whole assembly. Monitor the duration of test current such that the temperature of the conductor rises to 70 °C (±5) above ambient and maintain the same temperature for at least 30 min.
3. After the heating period, the current shall be interrupted and the conductor shall be allowed to cool to within 5°C above ambient.
4. The same heating & cooling cycle shall be repeated for 100 cycles (±20 cycles).
5. During the last 5 cycles of the 100 cycles (±20 cycles), the temperature of the conductor & each fitting shall be measured during the last 15 min of the 30 min period.
6. Then the whole assembly is allowed to cool. And again the resistance of each fitting is being measured.
7. The whole process of temperature & resistance measurement will be repeated for every 100 cycles (±20 cycles) until the 500 cycles have been completed.
8. Then the further 500 cycles shall be conducted with the resistance measurements at every 50 cycles (±10 cycles) and the temperature measurement at every 100 cycles (± 20 cycles).
9. And throughout the test, the fittings shall not be tightened or adjusted.
1. The surface temperature measured at every 100 cycles (±20 cycles) should not exceed the conductor temperature.
2. The electrical resistance of the fitting, measured at the end of every 100 cycles (±20 cycles) at ambient temperature shall not exceed 75% of the resistance of an equivalent length of the conductor.
3. The initial resistance of the fitting shall not differ by more than 50% from the mean of the overall resistance of each of the fittings, or half fittings in the case of tension joints assembled for the test.
4. The average resistance measured over the last 500 cycles shall not exceed the value of initial resistance of the fitting by more than 50%.
5. A graph of resistance against the number of cycles should be plotted. And it should demonstrate that the rise in resistance over the last 500 cycles is not more than 15% of the average resistance over the same period.