Automatic Transfer Switching - Dual Supplies - Neutral Earthing

Hi All,

Thanks everyone for the replies - very informative. 

Please see below for responses (collated from everyone's responses in one place)

Terry osifo said:

A 3-pole ATS with an unswitched neutral effectively parallels the neutral conductors of Supply A and Supply B, even when only one source is supplying the load.

The ATS specification originally asked for a 4 pole automatic transfer switch with overlapping neutral i.e. Supply B neutral conductor make, supply A line conductors break, supply B line conductors make, supply A neutral conductor break.  This doesn't appear to have been carried into the design.

Terry osifo said:

If the supplies originate from separate transformers with independent N-E links, a 4-pole ATS with switched neutral would normally be the preferred arrangement to keep the systems separated during transfer.

Agree - per "444.4.7 Transfer of supply"

Terry osifo said:

I would also want to establish whether the two substations are part of the same LV network or genuinely independent sources, and whether the DNO or designer intended the neutrals to be interconnected.

They are from different LV substations on an large industrial site. Thinking more deeply it is likely that a global earthing system is in effect across site due to density of buildings, earth electrode nests, substations, etc

AJJewsbury said:

especially those with a number of single-phase loads - as any imperfection in timing that results in N open when two or more lines are connected, even momentarily, can lead to severe overvoltages (e.g. 400V being presented to 230V equipment)

This is an interesting point of clarification - If the 4 Pole ATS with "guaranteed" momentary overlapping of the neutral were installed (per the original specification) then this risk seems to be mitigated.

AJJewsbury said:

Maybe your ATS does that, maybe it doesn't

As above.

AJJewsbury said:

so critical loads probably want a UPS downstream of the ATS as well

The design does include for local UPS backup within the server room - the ATS is there to ensure that the UPS autonomy time is not exceeded and that delay from switching over from supply A to Supply B are prevented.  I believe the ATS is operating "in line" but could transfer to internal bypass so I am not accounting for any isolation from input to output side.

AJJewsbury said:

no longer a recognised approach for safety-critical systems

Agree - The ATS and second supply from supply B and UPS are intended for mission critical equipment (not safety critical)

Gyaneshwar Kumar said:

The ATS should ideally be switching neutral. Although there is a risk of neutral breaking first or making last causing overvoltage across single phase load but that risk is negligible for correctly selected ATS from reputed manufacturer which generally have EMLB(Early make late break neutral)

Agree - please see above

Gyaneshwar Kumar said:

The LV earthing maybe combined provided both subs are cold sites and the corresponding earthing is designed for similar fault level, disconnection times etc

Are you considering the earth potential rise on the MV side here i.e. ensuring the means of earthing of supply A does not overlap with an EPR zone (>470 V?) of the means of earthing of supply B?

Gyaneshwar Kumar said:

I understand they are private transformers

Correct

mapj1 said:

The actual problems it causes in practice are minimal. The biggest issue is that earth fault relays and RCDs see the other NE link as a fault, and that the CPC and neutral share any fault current, and both need to be sized accordingly for the maximum current, or there is scope for overload

mapj1 said:

parallel neutrals is the dodgy part, and switching with an offset contact so neutral breaks later than phases and makes before is the common solution.

 but as noted, the real risk is low.

Agree - as above the original intent / specification was 4 pole ATS with momentary overlapping neutral which seems to be optimal but for some reason not implemented.  I will investigate implementing an improvement on this topic.

Thanks everyone for their insightful contributions!

Richard

transfer switch with overlapping neutral i.e. Supply B neutral conductor make, supply A line conductors break, supply B line conductors make, supply A neutral conductor break. 

Even that sounds less than ideal .. I can see upstream RCDs for one not being happy during the 'both N connected' period.

  - Andy.

RichHaze said:

EPR

Yes, exactly. It’s 430V though for standard protection. 

RichHaze said:

EPR

Yes, exactly. It’s 430V though for standard protection.