What has got the grid managers worried is the large scale toppling of transmission towers, all of which are technically supposed to be designed to withstand much higher gusts of wind than the 115 kmph peak winds that were experienced on May 30.
As per the Bureau of India Standards’ Code of Practice for Design of Wind Loads for Buildings and Structures (IS-875 Part III), the basic design of structures such as buildings in areas around the National Capital Region and most of the northern region should factor in wind speed of 47 metres per second, which works to around 169 kmph.
Additionally, for installations in open areas and high structures such as transmission towers, a margin of 10-15 per cent is to be further built in. For key structures such as high-tension 765kV towers, an extra safety margin of 7 per cent is supposed to have been factored in.
Despite this, an estimated 19 towers of 765kV capacity and another 11 of 400kV and 220kV across three northern states collapsed on account of the dust storm that occurred on Friday evening, with winds estimated to have been blowing at a peak of 92-114.8 kmph from the North-West direction into the northern plains.
In addition, there were at least half a dozen tripping of lines and substations due to faults such as insulators and jumper wires coming loose as the weather system travelled from Punjab across Haryana, and then to Delhi, according to the Northern Region Load Dispatch Centre’s latest outage report.
While the power ministry did coordinate a rescue act across the northern states in the immediate wake of the incident to ensure that power supplies were restored in the affected areas, grid managers are alluding to the larger concern that the transmission tower infrastructure may be “under designed”, especially 765kV transmission towers that are supposed to be designed with additional safety margin.
This could explain their having collapsed like “ninepins” despite being subjected to wind speeds much below the expected design threshold, officials said.