http://www.bbc.co.uk/news/uk-england-merseyside-15696187

Why does it cause such a problem, and how did it get into what should be a pressurised gas main.

A few years ago, heard of some plumbers who connected the incomeing water main to the gas pipe, they didnt know until they turned on the gas and meter went backwards,and water shot out of the cooker hob, took the gas board ages to pump the water out.

because it is at a higher pressure than the gas main

Pretty common around the manchester area too at the mo

Actually the gas board (national grid?) Seem quite adept at completely fudging things up just now

seems as if it was the water lot, who where replacing the mains

Causes a problem because it can block the main at ‘dip points’ – the pressure head of water is higher than the IIRC 45mbar of the main, in any event physically reduces the main effective area so you start getting low pressures in the main – lots and lots of complaints and problems with boiler shutting off etc. Plus it’ll **** up your boiler and the like super quick if you start burning gas with too much water in it.

We had that in our pipes last year, froze solid in the cold.. Took ages for the gasman to figure it out never mind sort it!

Water in the main is a pain in the arse!

As the link explains, a burst water main.
Low Pressure gas mains typically run at somewhere between 25 and 50mbar, a little higher if all plastic systems. High pressure water leaks can blow holes in metallic and plastic gas pipes, allowing water to flood the relatively low pressure gas network.
This has to be removed. Once clear & repaired the gas system has to be purged from air to gas. Before this is done all properties served by the affected system should be capped off at the meter, or services physically disconnected where there is no access, to avoid the risk of gas passing into the property. Re-commissioning has to be done progressively to prevent an explosive gas/air mix. This takes time if it is to be done in a safe, controlled manner.

Skinnysteel thanks for that, sems as if a lot of peeps dont know that and are not very happy, no heating or hot water over the weekend, also the Mr Kipling factory is nearby so possibly a cake shortage soon,if theyre affected.

a real pain.. capping off/ turning off each property is only an issue if you cant get in to each individual property . the issue becomes a real pain when restating supply to each home. each home has to be visited and every appliance has to be individually checked/ purged and that leads to problems
for example the little old lady we had on friday only source of heating gas fire in lounge.. only problem no fire place so someone, many years ago had made one out of a pallet and a cardboard box.. the easiest decsion i have ever made..