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Case Study When Single cable was found hotter than 3 core cable
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Being in the service, we always used to get various calls regarding problem in their plants. However, they were lucky that this problem didn’t occur on Sunday or Holiday (Majority of the breakdown occurred on those days only). Jokes apart, we got these call that, their power cable is getting heated up. We took few details on call and asked for photos.
The client had 3R X 3C X 185 SQMM, XPLE Armoured Cable with the earthing braid connected to the earth bus-bar at both ends of the cable. Cable termination was as per drawing shows.
In above figure consider the nomenclature as C1R is R-Ph of the first Cable run, C2Y is Y-Ph of the Second Cable run and so-on.
After some maintenance activity and some consideration, they have reconnected the cables from both end as follows:
In above connection, client has terminated all cores of the First run to R-Ph, Second run to Y-Ph and Third run to B-Ph. This was done considering that there are three cable of the same size and final cross-section area provided by reach cable to each phase remains same.
After observing the installation of the cables, it was concluded that all the cables armours were connected to the gland plate with Earthing Braids at both the ends.
After putting the above connected cables in service, and with the time lapsed, client has found that the Gland Plate is getting carbonized at Glands due to heating up.
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The difference between first installation of the cable and the later installation of the cable is that, in first installation due to the three-phase core are in one cable, resultant flux linkage to the armour is null.
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Whereas in the later case, each three-phase cable serves as a single core cable and flux linkage to the armour generates EMF. This EMF in turn generated circulating/eddy currents in armour once the close circuit is formed. In Later case this closed circuit is formed through Armour – Earthing Braid at one end – Earthing – Earthing Braid at other end – Armour. Due to this continuously flowing Circulating/eddy current in Plates, it is heated up and carbonizing is observed.
Such kinds of problem are reported more in single core cable of 11 / 22 / 33 kV, where, by mistake or unknowingly, engineers connect earthing braid with earthing at both ends. Thus, it is recommended to earth only one end of single core armoured cable. We hope, this case study will help and if you have any suggestions do let us know.
Solution to the problem is that, anyhow we have to open this closed circuit. We can do that by disconnecting One earthing braid with earth at any end of the Cable.
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www.systemprotection.in
+91-265-2225137