Multiple 3 phase supplies

Question

I&rsquo;ve got some works to do at a hotel/restaurant. The hotel is on a 3 phase supply and the restaurant on a separate 3 phase supply. They&rsquo;ve asked me to wire in a large induction hob in the restaurant kitchen but unfortunately the load is already at capacity in the restaurants main board so was going to come off the hotel board to a new 3 phase board in the restaurant kitchen and to the induction hob. 
 I can&rsquo;t see this being an issue if everything&rsquo;s labelled correctly etc but would I have to connect the two METS together? 
 Thanks Dan

AJJewsbury · Accepted Answer

the DNO may not like the two installations bonding together 
 
 Agreed, but keep in mind that if the two supplies feed the same building (or a pair of closely associated buildings) - the existing supplies are likely to be already solidly bonded together via bonding to common metallic pipes etc. Provided there's no evidence of overheating, that might give you an arguing point that things would be no worse than they are already, regardless of the DNO's latest rules. Of course the DNO might want to take the opportunity to improve things - but if that's their choice, there's an argument that it should be at their cost. 
 - Andy.

mapj1 · Answer

If there is equipment from two supplies that is simultaneously accessible - i.e. a person could be between the two CPCs via the metal casings of bits of kit, then they need to be solidly bonded. 
 From you description this will almost certainly be the case, if it is not already - if both supplies are already attached to the same water mains and heating systems, structural steels etc.they are probably already informally linked. 
 If both supplies are TNC-s you will need to think a bit about the potential for diverted neutral currents that may be quite large. Can you tell if both supplies come from the same DNO transformer originally ? Mike.

tattyinengland · Answer

I have a note for this same scenario in my regs book for "buildings with multiple supplies" - 542.1.3.3 Earthing and 544.1.1 for Bonding.

Multiple 3 phase supplies

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