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.
TT and the other TN if they are neither separated nor tightly tied together such as second supplies for certain type of fire system)
Then the HV side - are these underground lines bringing in an HV earth on the armour with them and if not how are the HV earths arranged. Presumably they are supplied by the same DNO, but are they private transformers, or if they are DNO property do they have an opinion as to how they are wired and how they should be interconnected or not . I presume they are not side by side on a common ground plane.
Hi.
Interesting that you mention TN + a TT supply, this has been suggested to me by IDNOs before, however, without any technical justification. Is this approach due to the idea that the two earths (star points) will become disconnected somehow, so it's better to design it that way in the first place? Just TT means an RCD and that is rather counter to the purpose of many secondary supplies.
Guessing in this case the supplies are at LV. The issue for DNO/IDNO is circulating currents, if they make two N-E bonds (as mapj1 pointed out). Also from their perspective, of course they need to make it so one supply cable can be worked on if the other is live.
From the installation perspective, don't forget, if you have two supplies serving the same location(s), you need to be careful to comply with Regulation 411.3.1.1 - especially second para:
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
So if you have two supplies, you must either segregate them physically at all points (even if exposed-conductive-parts are shared), or bond them together ... but see Reg 542.1.3.3.
TT and the other TN if they are neither separated nor tightly tied together such as second supplies for certain type of fire system)
Then the HV side - are these underground lines bringing in an HV earth on the armour with them and if not how are the HV earths arranged. Presumably they are supplied by the same DNO, but are they private transformers, or if they are DNO property do they have an opinion as to how they are wired and how they should be interconnected or not . I presume they are not side by side on a common ground plane.
Hi.
Interesting that you mention TN + a TT supply, this has been suggested to me by IDNOs before, however, without any technical justification. Is this approach due to the idea that the two earths (star points) will become disconnected somehow, so it's better to design it that way in the first place? Just TT means an RCD and that is rather counter to the purpose of many secondary supplies.
Guessing in this case the supplies are at LV. The issue for DNO/IDNO is circulating currents, if they make two N-E bonds (as mapj1 pointed out). Also from their perspective, of course they need to make it so one supply cable can be worked on if the other is live.
From the installation perspective, don't forget, if you have two supplies serving the same location(s), you need to be careful to comply with Regulation 411.3.1.1 - especially second para:
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
So if you have two supplies, you must either segregate them physically at all points (even if exposed-conductive-parts are shared), or bond them together ... but see Reg 542.1.3.3.
