Matti Peltola Research on the Mechanical Quality of an Underground Cable

A reliable electricity transmission network has a vital role in modern society. The long service life and the capital intensity emphasize the importance of distribution network design and selecting good quality components. The thesis addresses quality related factors in underground cable connectors. The connections in underground cable connectors are widely established by screwing. The performance and reliability of a connector are important aspects of quality. The reliability of underground cable connectors is determined by standardized tests. All in all, quality consists of every characteristic that makes the product fulfil the set requirements or the expectations of clients. This thesis determines the structural elements that affect the quality of the screw coupling in an underground cable connector. Improving the feel of the screw coupling would enhance the quality perceived by the customer. In particular, the significance of thread tolerance of the screw is explored. A method is developed for measuring the dependency between the torque needed for tightening the screw and the force generated at the tip of the screw. Based on the results of the torque-force tests and the ISO 965-1 standard, it is worth looking into utilizing 4h screws in conjunction with a 6H thread instead of 4g screws. In the tests 4h screws produced slightly more force at any given tightening torque. A screw coupling having a 4h screw felt firmer than a 4g screw. This is a corollary of the smaller clearance. Testing is needed with an actual connector body that utilises screws manufactured to meet the 4h tolerance. Based on benchmarking, the feel of the screw couplings commonly have some free play. Small deformations or chips in the edges of the shearing point and tightening tool grooves affect the feel when turning the screws by hand. Disposing of these imperfections would enhance the quality perceived by a customer. Some phase of the tin coating process can apparently cause imperfections to the finished screw threads. The whole work process needs to be analysed to figure out and eliminate the exact cause or causes of the damages in the thread. The scrutiny of the split connectors under a microscope revealed, inter alia, that the threads in many connector bodies deform under tightening of the screws. Some manufacturers use an internal thread crest shape that yields under loading. The microscope revealed a thread on which the thread bore and tapping may not be entirely concentric. Additionally, a crack noticed under the microscope on a thread of a connector body should be investigated. The internal threads call for further attention.