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Topic Title: Short-time parallel in-feed exceeding fault rating of switchgear
Topic Summary: Electricity at Work Regulations
Created On: 04 November 2017 01:53 PM
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 04 November 2017 01:53 PM
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timothyboler

Posts: 239
Joined: 25 July 2008

Interesting one... I've be told that it's a criminal offence to have a switchboard installed if it's fault capacity is exceeded when connecting two in-feeds in parallel for a no-break transfer - even if this occurs very infrequently and for a short-time.

The EaW regulations state that:
"5. No electrical equipment shall be put into use where its strength and capability may be exceeded in such a way as may give rise to danger."

I suppose if I can demonstrate there's no risk of personal injury then I can worm my way out of the requirement but as a designer with no input to future operations procedures this may be difficult.

However, G59 7.3.4c seems to allow overstressing of switchboards in this case the short-term if risks are "controlled"?

Does anyone have any experience with this scenario? I was thinking that an automatic switching arrangement so that operators would not need to be stood in proximity of the switchboard would suffice.

Thanks.

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Everyone loves a fireman - but hates the fire inspector.
 04 November 2017 02:49 PM
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sparkingchip

Posts: 10094
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My experience is domestic, power goes off then the generator kicks in.

With what you are describing surely the generator if connected to the DNO network temporarily feeds into the grid, so there must be DNO engineering documents stating design requirements?

Andy B.
 04 November 2017 03:51 PM
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broadgage

Posts: 2432
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The installation MIGHT not involve a generator at all.

I have seen installations with 2 LV supplies each one from a dedicated transformer.
The design intent is to provide at least some redundancy, for example one end of a busbar chamber is connected to transformer A and the other end to transformer B.
In the middle is a bus-coupler, normally open.
Each transformer then supplies a part of the load and the switchgear is then selected to withstand the fault current from a single transformer.
At a time of reduced load, the bus-coupler may be closed, and one transformer then switch opened in order that the transformer may be serviced, inspected or replaced.
Therefore, briefly, BOTH transformers are paralleled and the combined fault current may exceed the capacity of the switchgear.

This arrangement or some variation thereof is quite common and seems to be accepted, though there is some danger if a bad fault occurs when both transformers are connected.

My personnel view is that it is acceptable if and ONLY IF the switchgear is remotely operated. And if procedures are in place to ensure that paralleling is only done with no person present, and is "un done" before persons are admitted.
There is thus a small chance of a very expensive blow up, but no direct life risk.
If however the equipment is hand operated, then I would not consider it an acceptable design for a new installation.
Either the switchgear should be rated for the TOTAL fault current from both transformers, OR back up protection via HRC fuses should be considered.
 04 November 2017 05:39 PM
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timothyboler

Posts: 239
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Yes, it's as Broadgage discribes (site with 2 private transformers as infeeds). Only reason I refernced G59 is that's the only place that seems to mention this type of setup in terms of safety although maybe not fully relevant.

I agree with the hand operated comments - seems like a sensible solution to me - but other than sticking a warning label on the switchboard its otherwise difficult to prevent manual operation. Also Reg.5 of EaW is an absolute requirement. There's no "as is reasonably practicable" wording you normally see.

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Everyone loves a fireman - but hates the fire inspector.
 04 November 2017 05:52 PM
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Martynduerden

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Is it not possible/impractical to have automatic operation of both functions simultaneously?

In the example given second supply isolation at the point of bus coupler connection.

Genuine question - no idea.

Edited sphellin

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Regards

Martyn.

Only a mediocre person is always at their best
 04 November 2017 06:05 PM
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broadgage

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Originally posted by: Martynduerden

Is it not possible/impractical to have automatic operation of both functions simultaneously?
In the example given second supply isolation at the point of bus coupler connection.
Genuine question - no idea.
Edited sphellin


Whilst it could be automated, the brief paralleling of the two supplies is unavoidable if any loss of supply is to be avoided.
 04 November 2017 06:48 PM
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Martynduerden

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Well there is always going to be some measure of timeI guess.

It could however be limited to mechanical switching time and if necessary linked to an Amperage measurement to prevent changeover occuring at periods of high load?

Just exploring...

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Regards

Martyn.

Only a mediocre person is always at their best
 04 November 2017 06:59 PM
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ToniSM

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Originally posted by: timothyboler

Does anyone have any experience with this scenario? I was thinking that an automatic switching arrangement so that operators would not need to be stood in proximity of the switchboard would suffice.

Thanks.


On a number of occasions:

Bus section close.
Incoming No1 or No2 open.

It took longer to write that than it takes to do the switching operation. You'd be dambed unlucky if it went wrong during the swap over.

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Could there be a better way?

In theory yes, but in practice?
 04 November 2017 08:50 PM
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Nedryerson

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Hi Timothybowler,

Good one this. I would say that so long as the paralleling is momentary then the switchboard could be fault rated for only one transformer.

Presumably we are talking about two identical transformers here ?

Ned
 04 November 2017 09:54 PM
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sparkingchip

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If both transformers share a distribution network connection doesn't the network limit the fault current?

Do two transformers in parallel double the fault current if the network connection only allows the maximum demand of one transformer to be supplied?

Andy B
 04 November 2017 11:55 PM
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alancapon

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Yes, because the impedance of the transformer limits the fault current. Two transformers in parallel will give you half the impedance, therefore twice the fault current, as long as the network can provide it.

Regards,

Alan.
 05 November 2017 02:01 AM
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Martynduerden

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Thats intresting Alan, what would be a usual breaking capacity on the HV side of a transformer, say 1000kva?

I supposethe HV amps being significantly lower might not assist in thos case.

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Regards

Martyn.

Only a mediocre person is always at their best
 05 November 2017 06:12 AM
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Nedryerson

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Martyn,

The fault levels at HV can be difficult to obtain from the Network Operator.

This being the case it is safe to base the calculation on the rating of the HV breaker which in my day was usually 250MVA. The thinking was the designer of the network must have based his selection on something.

Just add in the impedance of the HV cable and you have it.

Ned
 05 November 2017 09:08 AM
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Zoomup

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The running of two identical transformers in parallel looks mighty tricky to me. This linked article discusses the matter. http://electrical-engineering-...parallel-connection-1

Z.
 05 November 2017 09:23 AM
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Zoomup

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This company knows a thing or two about transformers. Perhaps their technical team can advise you on matters.

http://www.siemens.com/content...-product-brochure.pdf

Z.
 05 November 2017 09:41 AM
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alancapon

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Originally posted by: Nedryerson
. . . The fault levels at HV can be difficult to obtain from the Network Operator. . .

They can be, as they need to be calculated depending on the network configuration, which will change as open-points are moved and lines & cables are constructed / removed. The DNO should be able to calculate the maximum value at any point on their network.

Regards,

Alan.
 05 November 2017 09:57 AM
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timothyboler

Posts: 239
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No issue with paralleling the transformers assuming normal operation (i.e. no fault/short-circuit). They're the same size and vector group etc. And the fault levels have been calculated including the HV contribution. Actually the majority of the fault loop impedance comes from the transformers themselves so the HV fault level doesn't affect it that much.

I think a big label on the front (procedural interlock) saying not to manually parallel while in proximity of the board with a timeout coil to close the last incomer if paralleled for over say 5 seconds is reasonable... just that Reg. 5 above gives no credit of 'reasonableness' it seems.

I suppose if I'm not doing the switching I'm not the one "putting into use".

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Everyone loves a fireman - but hates the fire inspector.
 05 November 2017 10:16 AM
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AJJewsbury

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I supposethe HV amps being significantly lower might not assist in thos case.

That sounds like a good point to me. If my maths is right 25kA at 230V only equates to about 523A at 11kV - which feels more like an overload than a fault.

The EaW regulations state that:
"5. No electrical equipment shall be put into use where its strength and capability may be exceeded in such a way as may give rise to danger."

At the risk of making the question bigger rather than smaller - what would be the opinion about other events causing large currents to flow or excessive voltages - e.g. nearby lightning strikes (or even direct onto the building in some circumstances) - we don't install lightning protection systems on every building that could possibly be struck by lightning and most ordinary electrical equipment would go "pop" in a pretty dramatic (I guess dangerous) way if struck. Does that imply that there is a degree of not-quite-so-absolute under every possible circumstance about the regulation?

- Andy.
 05 November 2017 02:06 PM
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mapj1

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It took longer to write that than it takes to do the switching operation. You'd be dambed unlucky if it went wrong during the swap over.


However it depends why the swap-over is happening. All very well if it is switching around for standard maintenance, but if it is because one of the transformers has raised an alarm - (boiling oil anyone?) , then very strange things are possible during that brief moment of parallel operation. In that case energy limiting with very high value fuses may be worthwhile to minimise the extent of the installation to which the excessive PSSC is available.
In cases with gensets it would normally be done rather differently - in a modern system if the mains contactor is still on, then the genset contactor will not close unless the lock-out permits it - and that lock-out requires the difference voltages between genset phases and supply phases to be controlled within low levels - typically within 10-15 degrees phase jitter and less than about 20v or so RMS, so the fault correction current is about the same that associated with a 10% voltage drop- far more manageable than a hard switching and praying.
The exact limits are set up in close co-operation with the genset makers.
Even if there is HV involved the impedance contribution from that is normally treated as "very smal"l compared to the impedances on the LV side.
After all, in terms of dissipation, every ohm of load at 400V is worth about 600 ohms at 11kV (square of the transformation ratio - as volts go up and also current goes down.) After all that is why the HV lines are long, and LV lines are short..

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regards Mike
 05 November 2017 02:56 PM
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timothyboler

Posts: 239
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Originally posted by: AJJewsbury

At the risk of making the question bigger rather than smaller - what would be the opinion about other events causing large currents to flow or excessive voltages - e.g. nearby lightning strikes (or even direct onto the building in some circumstances) - we don't install lightning protection systems on every building that could possibly be struck by lightning and most ordinary electrical equipment would go "pop" in a pretty dramatic (I guess dangerous) way if struck. Does that imply that there is a degree of not-quite-so-absolute under every possible circumstance about the regulation?



I was thinking the same myself. There will always an element of risk to personnel in any installation and nearly all electrical equipment "may" have it's capability exceeded in a way that "may" give rise to danger... in the wrong circumstances, so dare I say it, I think it's poorly drafted. Nearly every other section has the phrase "as so far as is reasonably practicable" inserted.

-------------------------
Everyone loves a fireman - but hates the fire inspector.
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