Engineers were replacing the gas supply to a property in Bromsgrove Road, Hunnington, when the leak occurred on 20 August 2010.
The existing supply was connected to a low pressure main on the opposite side of the road so they took the decision to connect it to a medium pressure iron main on the same side of the road as the house.
They then left the street, not realising they had inadvertently drilled through a plastic main contained within the iron main which they did not know was there. Gas leaked out and found its way into another house approximately 150 metres away from the first.
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It's common for plastic mains to be inserted into an existing iron main, when iron mains are replaced. The gas is then contained within the plastic main.
Later that afternoon a member of the public reported a smell of gas and a second team of engineers came out to investigate.
They visited a number of properties and found gas had entered a property some distance away from where the main had been drilled but were unable to gain access to the house as the owners were away on holiday. They inserted a probe through the letter box which confirmed high levels of gas in the house and were in the process of trying to gain access when a neighbour heard knocking and let them in with a spare key.
The escaping gas was finally made safe in the early hours of the following morning.
The Health and Safety executive (HSE) told Dudley Magistrates' Court that the leak would not have happened had National Grid Gas PLC updated its records to show the existence of the plastic main, which they had known about since 2006.
The company, of The Strand, London, pleaded guilty to breaching Section 3(1) of the Health and Safety at Work etc Act 1974 and was fined £17,500 with £8,192 costs.
(CD)
