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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

    In partial answer  to

    The function of this conductor is similar to a PEN" - Why so vague? Is this moot point? -

    they mean that both in normal operation, and during phase to CPC earth fault conditions this  conductor carries current - at this point in the circuit, fault currents and load currents cannot be distinguished and RCDs do not work. That's all it means.

    I'm afraid I cannot read your schematic at any level of zooming in or out. I assume the ratio of thick to thin lines  means that the fine information has lost in the quantization. Additionally the contrast on the coloured fonts is poor, but even down loaded and converted to soot and white-wash, as I quite often have to do,  the text is not clear.

    However, in general, there should be no problem to configure 2 supplies from 2 private transformers you control as TNS or TNS-PNB and on a TN 3 phase system, not switching neutral is common, and  switching a PEN is absolutely forbidden !!

    Note that the bit about only having one NE link must be one of the more commonly ignored bits of the regs for sites with multiple transformers if they are not side by side. There are very few countries that worry about this as much as the UK ;-)

    Mike

Reply
  • 0

    In partial answer  to

    The function of this conductor is similar to a PEN" - Why so vague? Is this moot point? -

    they mean that both in normal operation, and during phase to CPC earth fault conditions this  conductor carries current - at this point in the circuit, fault currents and load currents cannot be distinguished and RCDs do not work. That's all it means.

    I'm afraid I cannot read your schematic at any level of zooming in or out. I assume the ratio of thick to thin lines  means that the fine information has lost in the quantization. Additionally the contrast on the coloured fonts is poor, but even down loaded and converted to soot and white-wash, as I quite often have to do,  the text is not clear.

    However, in general, there should be no problem to configure 2 supplies from 2 private transformers you control as TNS or TNS-PNB and on a TN 3 phase system, not switching neutral is common, and  switching a PEN is absolutely forbidden !!

    Note that the bit about only having one NE link must be one of the more commonly ignored bits of the regs for sites with multiple transformers if they are not side by side. There are very few countries that worry about this as much as the UK ;-)

    Mike

Children
  • 0 in reply to mapj1
    mapj1 said:
    I'm afraid I cannot read your schematic at any level of zooming in or out.

    Hello mapj1 - I have replaced the jpeg with a smaller .dpi version...  Is this any better?  Fingers crossed!

    mapj1 said:
    Note that the bit about only having one NE link must be one of the more commonly ignored bits of the regs for sites with multiple transformers if they are not side by side.

    For the application I am considering here the two supplies do come from two separate buildings which are quite some distance away from one another (diverse supplies and routes).

    Is the implication that if each supply neutral is connected to earth at each source this would be "okay" - I read this as a mandatory No No?

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