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Dual LV Supplies With Downstream UPS - Neutral Continuity

RichHaze

Hello,

I have been looking for the "right" way to derive the following supply architecture at one of our sites:

  • Supply A - LV
  • Supply B - LV
  • LV supplied feeds a LV main switchgear assembly in the main switchroom
  • LV main switchgear has dual sections and a normally open bus coupler (N+1 sizing / run whole switchgear from one TX if needed)
  • Downstream of the LV switchgear we connect sizeable UPS systems
  • The UPS OEM has asked us NOT to switch the neutral (No 4 pole breakers) because their system is not designed for loss of N-E reference

NOTE:  We own and operate our own HV and LV networks on our industrial sites (private supplies). 

I have been trying to get my head round the requirements of the wiring regulations and have come up with the attached schematic:...

*EDIT* Picture replaced (hopefully more readable now)

I would appreciate peoples thoughts on:

  1. Have I missed anything that would be considered a no-no in BS 7671
  2. I am considering this a TN-S installation from the LV switchgears point of view
  3. Do people have any improvements or suggestions for the pole arrangements of the incomers and bus couplers?
  4. I am not switching the neutral over (ref 463.1.4 compliance) but as we own Supply A and Supply B I am assuming this is permitted as per 551.6.2
    1. Have I interpreted this correctly
    2. Is the exemption for not switching the neutral discussed more explicitly elsewhere in BS 7671?
  5. Figure 9A note (b) "The function of this conductor is similar to a PEN" - Why so vague? Is this moot point?
    1. The TX's LV side are part of the electrical installation by BS 7671 definition
    2. This conductor is carrying neutral current and also acting as the connection to earth from the TX's "neutral" point of view

With the wiring regulations I sometimes struggle to see the wood for the all the trees!

Any feedback is gladly received

Thanks

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  • 0

    The UPS OEM has asked us NOT to switch the neutral (No 4 pole breakers) because their system is not designed for loss of N-E reference

    So if you needed to carry out any maintenance on the system upstream of the UPS (e.g. replacing or re-routing the cable) which would necessitate breaking the N, you'd be obliged to shutdown the UPS and everything it supplies? To my mind that's quite a limitation...

       - Andy.

  • 0 in reply to AJJewsbury

    Or it could be read that it is safest to provide a temporary N-E bond on the UPS side of the neutral that is about to open.
    Mike

  • 0 in reply to AJJewsbury

    Hello AJJewsbury,

    I think we would only re-route or replace the cables if something went wrong or a major modification to our facility was required and in that instance we would look to consider either a temporary shutdown or look to implement a short duration parallel of neutrals whilst we switched the UPS over to an alternative supply.

      has planted the seed that paralleling two separate neutrals with independent N/E links could possibly be okay especially if only for a short duration?

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  • 0 in reply to AJJewsbury

    Hello AJJewsbury,

    I think we would only re-route or replace the cables if something went wrong or a major modification to our facility was required and in that instance we would look to consider either a temporary shutdown or look to implement a short duration parallel of neutrals whilst we switched the UPS over to an alternative supply.

      has planted the seed that paralleling two separate neutrals with independent N/E links could possibly be okay especially if only for a short duration?

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