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Secondary Source of Supply fed from other building

Eddie Currents
We have a scenario whereby we have 2 public buildings located within 20 meters of each other but separated by a small public highway. Both buildings are owned by the same authority. Both buildings have independent DNO / tx transformers located in their own sub and are both fed off the same network running in the public highway. One of the buildings has a Generator set and UPS Back-Up facility covering the whole building and the other does not have any Generator set or UPS Back-up. In the building with no Generator set or UPS Back-Up facility it is proposed they plan to install 2 lifts primarily used for moving goods in/out and of which they will also double up the lifts to be used for assisted disability usage. In the unlikely event of an emergency ie if there was both a fire and a power outtage then the lifts will need to continue to operate. The required combined power to run the lifts is around 63A. 

 

The PM is presenting the client with 3 back-up options for the lifts primarily to overcome any emergency situation:


Option 1 - is to have a 30min UPS and changeover switch installed in a purpose made enclosure to operate the lifts during a power outtage. Downside is it is costly and large quantities of batteries and cooling are needed. 


Option 2 - (is my preferred option - belt & braces) which is to have a purpose made enclosure constructed comprising of both a UPS & Gen Set via a Auto-Changeover Switch which in the event of power outtage will keep the 2 lifts operating. 30 mins UPS was mentioned but with a gen set installed ultimately this time can be reduced to a less critical time and in turn bringing battery sizing/cooling sizing down. Downside is that this idea is very expensive and i feel against the wishes of all parties including space and location for aesthetics.


Option 3 - the designer has suggested feeding the lifts from the Back-up Generator located in the building next door. Suggesting the cable can be installed across the public highway running in ducting between both buildings and all notices can be served with the local authority for road crossing and wayleaves - i think section 50 was mentioned.


I have suggested with Option 3 that 2 different supplies into the same building is no longer acceptable but the designer is adamant that this is ok and has seen this done many times before. I feel both the Electricity & Work Regs and Fire Authority would not like this scenario but i have however seen something similar myself with high rise buildings where feeding life safety circuits. They also feel that the earth potential will not be an issue also as both buildings are fed from the same Network located in the public highway. I have asked for more clarity. 


It would be great to receive some sort of feedback on this topic as to it`s feasibility, it is something i have certainly never seen Option 3 and to me does not appear acceptable but seems the way they are heading.
  • 0
    If it can be fed from the other building it rather begs the question why it has its own supply for so many years. There may be issues with earthing that require the second supply to present as TT in the other building.  Sharing a substation is not enough if either is TNC-s then when you cmbine the earths you are also introducing a bypass on a section of the neutral of the street cable. This may be asked to carry many hundred amps even when both buildings are switched off. The DNO will need to agree this is OK.

    I'd also be worried about credible faults where a fault in one building cuts off something critical in the second, and what happens to the earth if the fault that removes the mains supply removes the mains earth as well it may if the incoming cable is cut by a digger or something.

    I'm not saying it cant be done, but there is a lot to think about.

    Step 1 is to get the DNO to comment, as they know the street cable arrangements.
  • 0
    Thanks Mike. Lots to consider. They have talked about the earth issue. I wouldn't see the earths of both buildings being linked in anyway unless DNO consent. If they do then not so much of a problem. If the SWA from the building with the generator was connected via an adaptable box prior to the changeover switch serving the lifts then the earth can possibly be segregated but this would mean maybe switching both the neutral and the earth over if mains fails so the circuit feeding the lifts have all 3 phases and the neutral and the earth all coming from the building with the generator back up. Not sure if switching the earth in circuit is an acceptable way to do this. Also the lifts I would imagine would in part be earthed whilst touching steelwork etc within the building they sit so impossible to split the earth off. To me the ultimate decision would be with IDNO.
  • 0
    Not sure if switching the earth in circuit is an acceptable way to do this.

    Generally frowned upon as any kind of switching tends to be less reliable than a solid joint, but is permitted under certain conditions (see reg 543.3.3.101). Switching an Earthing conductor can pose extra challenges though as it's likely to be acting as as main bonding conductor as well as a c.p.c. - thus any switch rating would have to consider possible currents flowing to extraneous-conductive-parts (e.g. diverted N currents as well as fault currents from elsewhere) which may significantly exceed the rating of the circuit you're switching.


       - Andy.

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