Two DNO 11Kv/400/230v transformers on different HV lines originally supplied two separate buildings on an industrial site. The two separate buildings are now one large steel framed one. So we have two DNO intakes and no clear demarcation within the installation. I remember reading somewhere about issues arising in such situations but cannot find the article. I am keen to establish if any issues are serious enough to seek alternatives. One of the intakes has a Biomass plant feed-in with g59 relay. Both intakes are circa 1MVA.
Not wishing to second guess the IDNOs, but a building with two TNC-S LV supplies is one of those things that needs to be done carefully ?
Because in effect you have two NE bonds, and two lots of bonding that put the neutral onto the structural steel, plumbing etc, when those neutrals join up round the back via a common low impedance street network ground / DNO neutral / HV ground there is potential to have a constant load dependent voltage gradient along the building. This is similar to the metal water main serving as a second PEN in a street of houses, but with less impedance to limit the current. Even though the voltages may be fractions of a volt the currents may be tens or in an unlucky case, hundreds of amps, with all the associated magnetic fields and this is 'a complication' , and is is either avoided by tying both together very solidly - easy if side by side, less so at opposite ends of a barn, or more easily using the electrode resistance in a TT system is to reduce the potential for large circulating currents.
In the end the folk who know the network details should be the DNO, but things like building change of use of a building that was separate modules into one big one, or the addition of factory piping running between previously isolated buildings can introduce problems where there originally were none.
Why is bonding them together hard if the supplies are not adjacent (but still local to the same building)? Is this not just the case of a large section bonding conductor between them, and making sure they stay bonded? Maintaining bonding throughout is already an important safety aspect, so I don't see why we should assume this would not be done.
What cross-sectional area do you put on that "large section bonding conductor"? See Reg 542.1.3.3 . Don't forget, though, the current the conductor has to carry may not simply be fault current (whether or not originating from the LV or HV network), but may have to include Neutral currents - and perhaps not just Neutral currents from the installation(s) in question.
Having two seperate HV supplies in the building requires that you have a full understanding of the LV distribution through the building. It also requires that you have the installation set up in such a way that the LV is completely identified and can not be mistaken or otherwise altered or interchanged. If not properly segregated you could in theory end up accidentally backfeeding the other electrical system.
The neutral earth connections are also of concern as you can have circulation currents in there that could suprise the unwary. In an emergency would you be able to identify quickly where the power source was sourced from and how to isolate ? you can have currents in the neutral or earth cables that you would not expect to be there due to the effects of parallel paths.
