Its an interesting one, I can think of a few sites that are TT with Ze values of less than 5 ohms, often a lot lower, due to the amount of steelwork in the ground. Some of these were installed under the 16th edition and fault protection is via an MCCB equiped with an earth fault protection module (so effectivly making it an adjustable RCD) and they are often set at say 1A and about 600ms Delay. Now the submains will comply as you'll get 1s disconnection, but take lighting cirucits for example, they too would ahve required 1s disconnection under the 16th edition, but now they require 0.2sec. Now their might be some lighting circuits say on a B6 that actually meet the MCB Zs, while the ones on a C10 might not.
So your lighting circuit was ok protected by the Vigi incommer unit under 16th edition, under 17th and later its not, but today it actually complies because the MCB allows it to meet the disconnection times, but who knows whether it still will in the summer when the ground dries out. Stranglely enough, the guide doesn't seem to cover this particular situation.....
I also know of sites that are technically TT, but they are effectivly TNCS through someone elses installation - rows of adjoined retail units with common steelwork that have underdone splitting up at some point in the past, original supply was TNCS, DNO would not supply an earth with the extra supplies due to ciruclating currents in the steelwork* so your Zs could be fine for overcurrent protection only, but completely reliant on someone elses bonding and if something happens to that then you are back to TT type loop readings (still good ones due to all the steelwork, but likely to high for at least some of the MCBS)
*They can actually provide multiple TN supplies to such a place under newer rules, they have to run SNE service cables into the building, but these can be off a CNE streetmain as long as the joints are all together in the same place. Often when they do this, in my area at least, they label the head up as SNE(TNS), but really its not TNS, but if its presented as TNS, labelled as TNS, how are you supposed to know whats happening in the street
So your lighting circuit was ok protected by the Vigi incommer unit under 16th edition, under 17th and later its not, but today it actually complies because the MCB allows it to meet the disconnection times, but who knows whether it still will in the summer when the ground dries out. Stranglely enough, the guide doesn't seem to cover this particular situation.....
I also know of sites that are technically TT, but they are effectivly TNCS through someone elses installation - rows of adjoined retail units with common steelwork that have underdone splitting up at some point in the past, original supply was TNCS, DNO would not supply an earth with the extra supplies due to ciruclating currents in the steelwork* so your Zs could be fine for overcurrent protection only, but completely reliant on someone elses bonding and if something happens to that then you are back to TT type loop readings (still good ones due to all the steelwork, but likely to high for at least some of the MCBS)
*They can actually provide multiple TN supplies to such a place under newer rules, they have to run SNE service cables into the building, but these can be off a CNE streetmain as long as the joints are all together in the same place. Often when they do this, in my area at least, they label the head up as SNE(TNS), but really its not TNS, but if its presented as TNS, labelled as TNS, how are you supposed to know whats happening in the street
