Having two supplies is a totally different situation as they are outwith your control so the DNO could change one, even to put it on a totally different sub-station, etc.I think I’m with you here. If we had two adjacent houses, the first TNCS then a looped supply to the 2nd which is TT, we wouldn’t dream of saying that the gas pipe entering the 2nd house is only an exposed conductive part because it’s connected to the MET next door.
Really this thread highlights that there should be greater consideration when multi building installations sharing a common supply have their earthing characteristics changed by the DNO.
By a strange coincidence this month's IET magazine has this article covering the rather real dangers of TN-C-S bonding:
Industry must acknowledge dangerous network fault, warn experts
While networks and the government are well aware of the dangers of neutral current diversion, they have failed to publicly acknowledge the extent of the fault. This is delaying potentially lifesaving work, E&T has learned.
tinyurl.com
Mackenzie, the former SPEN employee, first became aware of a potential public safety issue in 2014 when he was alerted to an incident in a property in Galashiels, Scotland. A resident’s coat had fallen onto the property’s gas meter and had caught fire. Readings taken at the scene detected 35 amps of current flowing through the metallic gas service pipe entering the property. Mackenzie said it was fortunate the resident was at home at the time.
I always though the bonding to the customer's side of the gas meter was bonkers from an electrical point of view, but that is what the regs say to do, presumably due to the legal ownership change (supply pipe & meter are Transco's property, etc, but pipe after the meter belongs to the installation).