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Review of power poles? design a must: Study

india Updated: Oct 16, 2006 01:11 IST
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FREQUENT TUMBLING of electricity poles in the city as well as in the state are due to stress corrosion crackings and defective design selection of the poles themselves.

A study by an expert of Motilal Nehru National Institute of Technology (MNNIT) has established that there is an urgent need to review the designs of the power poles as well as their installation and maintenance procedures to prevent such incidents.

"We studied a number of power pole collapses in Allahabad and found corrosion to be the main culprit. For example, on October 6, two electric poles came down on their own in a busy locality at the Matiara Road in Allahpur. The sudden collapse of poles leading to live wires falling right down to the ground height could have resulted in human deaths as well but sheer luck saved the residents," said Dr HS Goyal, assistant professor in the Mechanical Engineering Department of MNNIT.

Dr Goyal said that following scrutiny, he found the welded steel tubular poles fractured from the root due to stress corrosion cracking.

Citing the main problems, Dr Goyal said that transmission cables are being attached to the tubular poles in a cantilever pattern. "If the pole is considered as a central line, all the transmission wires are being placed on one side that is towards the centre of the road and this leads to a typical load pattern resulting into unequal stress distribution at the base where the pole comes into contact with the ground," he said.

Dr Goyal said that if equal numbers of transmission wires get placed on both sides of the poles, the load pattern becomes more uniform at the base leading to reduced stressing in a particular direction. A congested street like Matiara Road in Allahpur demands that the power transmission cables be placed in one direction and accordingly extra care in the design, selection and installation of poles needs to be taken for safety against corrosion failure during thunderstorm  and truck collision etc," he added.

Dr Goyal explained that the critical zone in an electric pole is between ground level and to a depth of about four feet where the effects of soil moisture and its contents are most damaging.

"The continued excess loading on one side of the plate along with corrosion of steel due to soil moisture is resulting into stress corrosion cracking at the most severely loaded section and leading to the ultimate brittle fracture of the tube poles without showing any ductile deformation," he said.

Citing the Matiara Road incident, Dr Goyal said that the two poles fractured from the neck of the cement collar and the cement collar on the steel tube with cantilever loads provided a wedge action and further helped in crack propagation.

"Safety of other Tubular Electric Poles loaded in similar manner and of life equivalent to the poles involved in accident—all purchased or produced in the same lot installed all across city and the state— need to be checked to ensure that corrective technological steps are implemented," he said.

Dr Goyal said that wherever tubular poles are being used, before the installation coating of black bituminous paint both internally and externally needs to be ensured on the portion that has to remain submerged in the ground.

"For welded steel tubular poles bottom section of the poles should be coated with black bituminous paint both internally and externally up to 1.6 metres for 9 metre poles and 2.1 metres for 12 metre poles. The working portion of the exterior should also be painted with one or more coat of Red-oxide primer," he said.

Dr Goyal said that once a pole of inferior material or uncoated with bituminous paint gets installed, the metallurgical methods to detect the corrosion rate for ensuring safe working life of poles is quite difficult.

"The requirement of Electric Poles in Magh Mela (a temporary structure) ground are quite different as compared to the need within the city and if the same temporary structure material gets used within city a man made accident can not be averted," he warned.

The suitability of welded steel tubular poles with high possibility of corrosion over the solid I section of High Strength Steel poles needs to be re-evaluated by the transmission system designers/ material selectors and the incidents taking place must act as eye opener.

High number of pole fall related incidences –within city limits after every thunder storm also can be checked by re-evaluating the pole selection / quality assurance methodologies. Our engineering departments should become more concerned against engineering design linked failures and must take corrective measures to reduce man made disasters, Dr Goyal added.

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