DNO Connections to Buildings

Question

Afternoon All 
 Quick 'simple' question hopefully someone has a simple answer to. 
 Why are DNO's (like UKPN, SSE etc.) adverse to providing buildings with more than a single incoming LV supply? 
 Are they concerned about risks/ regs, or is there another reason. 
 Would like to know. 
 Many thanks 
 Ade

mapj1 · Accepted Answer

If both are TNC-s, then as the CPCs of the 2 supplies link to building steels etc, you can get supply neutral currents flowing in the structure of the building between the to intake positions. https://www.tangle-tamers.com/tncs/TNCS-DN-1-RD-V1.6.pdf 
 has a nice illustration that may help. 
 Mike

gkenyon · Answer

mapj1 said: If both are TNC-s, then as the CPCs of the 2 supplies link to building steels etc, you can get supply neutral currents flowing in the structure of the building between the to intake positions. 
 Yes, this is the case. HOWEVER, if subsequent supplies to such a building are TT, they are an extreme case of high prospective earth fault currents in TT systems, as illustrated in IET Guidance Note 6 Protection against overcurrent where the prospective earth fault current (after connection of extraneous-conductive-parts) is similar to the L-N prospective fault current: 
 
 In fact, prospective earth fault currents in TT systems are rarely as low as that predicted for the 'worst case' used for ADS, and therefore: 
 
 RCDs should never be relied upon for protection against earth fault current ... it is the 'overcurrent protective circuit-breaker' element of an RCBO that should be considered. (NOTE: Section 1.5 of GN6 discusses devices that are suitable for protection against overcurrent) ; and 
 Care must be exercised when trying to determine maximum prospective fault currents in TT systems ... even if it's not an extreme case as Figure 6.3 of GN6 (above), earth electrode resistances of the earth electrode itself, and the equivalent for extraneous-conductive-parts, will vary at different times of the year, so may well be much less on some days, than any values you measure on the day of testing.

statter · Answer

Apart from the earthing considerations there were also the old Electrciity Supply Regulations. One of these e.g. 25.1 of the 1988 regs could be read mand was sometimes quoted as requiring only a single point of supply. It was standard practice in some DNOs (probably as a result) at least to insist on supplies having a single point of isolation for safety purposes so that there was no doubt about how to turn off the supply in an emergency. This lead to significant costs for customers (either expensive switchgear or the need to go to an HV supply if more than 400A at LV was required in some cases. Other DNOs would cheerfully provide multiple supplies so there was little consistency.

DNO Connections to Buildings

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