Am I Missing Something, Or Was Everyone Else Missing It?

[kme]

Scenario - commercial property, out in the wilds. Pub / hotel.

Supply is 2 phase. 

Pub have an online generator, powers up within 3 seconds on failure of mains.

The incomer is 250A per phase fuses L1 and L2, Into a main isolator ( not looked inside yet - interlocked.) - from there into a metering cabinet with Cts ( one red conductor, 2 black  -70mm BS7889 in free space), and also into the generator housing ( BS5467 70mm singles, two red, one black.)

Back out of the same hole in the generator, and over to a Hager TPN DB ( there are 4 TPN boards on the premises - all different makes!, but NO polyphase loads.)

The Main Hager TPN DB has a 250A, TP switch disconnector, with L1 linked out to L3. The three TP submain DBs are fed from TP MCBs

Here`s the "big problem"

Because we`re only having 2 phase ( "split phase"), where the phases are 180 degrees from each other ( i.e. centre-tapped for N), we`ve got a phase-phase voltage of 480V

Hager main switch rated 415V, TP MCBs rated 400V

According to Hager & Merlin; their boards and breakers / disconnectors are only safe to a maximum of 460V. Obviously, if we allow the DNO`s 10% - we could be looking at 506V (460V +10%/-6%).

I`m thinking they`d be better with 2 DBs; each barred out to SP - so each board is only running 240V.

But the initial distribution of that, from this 3 wire 2 phase - unless I can get an enclosure, rated at say 600V, with a pair of single pole MCCBs? 

I know Hager do NOT produce 600V rated switchgear for their TP DBs.

Anyone else ever had to deal with this issue? Its been installed for 10 years............

n.b. Install is shocking. Photos to follow in gallery - silicon around every cable entry top surface, BS951 used to terminate a SWA, 25mm tails damaged in enclosure by edges of front cover.

What a blinking mess!

 

[steptoe]

sorry, ive never had much dealings with split phase,

in fact, never even seen it until I came to England, only ever saw, or worked with 'normal' 120degree  2 phase before.

 

[Andy™]

never done a split phase install myself, but worked on many with a standard TP board with only L1 & L2 used. but as you say, it doesnt comply because of the higher phase-phase voltage.

never seen any boards designed for split phase, but then i havent really been looking for them

 

[ProDave]

Interesting scenario.

Clearly all the previous installers failed to realise the issue.

Taken in isolation, each phase has the same peak voltage as a 3 phase system. But it's the phase to phase voltage seen by two adjacent mcb's that's the issue here.

Your solution of rewiring it as two single phase db's will solve the "problem" so that's surely the way to go.

Either that, or say it's worked fine for the last 10 years, so there is no problem? :coat

 

[Evans Electric]

Not come across this before Martyn.

I've known lots of large houses ,divided into flats , using 2 phases of the 3 Ph supply but didvided into single phase at source.

 

[Marvo]

I thought you UK guys worked with the new harmonised voltages.... ie single phase is nominal 230v, 3-phase nominal 400v so split phase would be nominal 460v. Therefore your 460 volt rated switchgear and enclosures etc scrape through. We always spec voltage ratings according to nominal, not actual.

 

[steptoe]

I thought you UK guys worked with the new harmonised voltages.... ie single phase is nominal 230v, 3-phase nominal 400v so split phase would be nominal 460v. Therefore your 460 volt rated switchgear and enclosures etc scrape through. We always spec voltage ratings according to nominal, not actual.

ah,

therein lies a very good point.

 

[wozz]

im sure you have but, have you actually measured the voltage? plenty of supplies round here have two phases coming in but both are the same phase, if not measured it could be assumed to be split phase.  

Also does the generator mind having on two phases being pulled on?  surely this means having the massively oversizeing the genny?  Not that it would bother you much just interested as never thought about it before.  

good point by marvo we dont worry about 230V mcbs on 240v supplies.  

 

[Marvo]

From what I understood the genny would be a 2-phase configuration at 180degrees (460v phase to phase), center tapped neutral, not a 3-phase genny (400v phase to phase) with one phase redundant. Maybe the OP can clarify?

 

[ProDave]

I thought you UK guys worked with the new harmonised voltages.... ie single phase is nominal 230v, 3-phase nominal 400v so split phase would be nominal 460v. Therefore your 460 volt rated switchgear and enclosures etc scrape through. We always spec voltage ratings according to nominal, not actual.

Good point.

THEN when it all goes  bad day explode It will be the DNO in court for supplying too much voltage.

 

[SLIPSHOD & SLAPDASH]

I also  have never worked on split phase. Why is it supplied in this case ?

Reading up on this, should split phase be used in one property, or is it meant for , say two farmhouses, each with one phase ?

 

[Marvo]

Not sure about the EU and the UK but split phase is occasionally seen in the rural African areas and it's distributed in both ways ie 1phase and neutral to 2 separate houses or 2 phases and neutral to the same house. With the latter they just split and balance the single phase circuits between the 2 phases same as you would on a residential 3 phase supply. Obviously they don't run loads between the 2 phases although in theory you could if the load was of the appropriate voltage. They stopped installing it locally to me about 10 years ago but they've been in no rush to alter the systems already installed. Most of them are so far out in the sticks that cost saving was probably the motivation for them.  

 

[poni]

first of all where would you buy a 2 phase generator ?

second of all why would you buy a 2 phase generator?, 

and why would you install one? 

 

[SLIPSHOD & SLAPDASH]

This has got me thinking about some of the two phase jobs i have worked on. Many parts of South London, especially Croydon & Thornton Heath have only a two phase distribution as it has been converted from various old DC systems, or is it actually split phase ??

Back to the OP. From your description there are only single phase loads, so the switchgear is only switching 230V , no problem as its rated to 460V.

Better not say much more or Steptoe will have a go at me for not being compedant  :coat

Edit, just re-read the 1st post and see you have 3 phase MCB's.

 

[kme]

Marvo. 

Accept your point regarding nominal - but nominal allowing for supply characteristics is (230x2)+10% =506V, so it doesn`t "scrape through".

If Hager & Merlin are dead set against >460VAC between their phases - then the whole of the DB is operating beyond manufacturers spec.

I`m thinking SP boards - but what about the sub-mains.............

Unless I can take all the DBs as single phase - but 2 DBs are being fed from 63A TP mcbs - so I`m going to need MCCBs to deliver the necessary load!

:shakehead

 

[Marvo]

Marvo. 

Accept your point regarding nominal - but nominal allowing for supply characteristics is (230x2)+10% =506V, so it doesn`t "scrape through".

I'd beg to differ, allowing for supply characteristics would no longer be nominal. Many products are rated at 240v but wouldn't be incorrectly rated because of allowable supply fluctuations. Example, starter cable plug and light fittings etc are al rated at 240v but would all be out of spec on a UK domestic supply if that were the case. Designers and manufacturers build in capability to withstand supply anomalies which would create real life conditions beyond the nominal specified ratings of their products. 

 

[wozz]

agreed on the data sheet for the MK sentry MCBs they are rated to 230v, nothing in the small print about voltage tolerances.

 

[kme]

OK - again, if we proceed on that proviso:

The 400V rated MCBs have 460V nominal (480V actual), and the manufacturers say they would class >460V as "dangerous" (Hager technical`s words).

I don`t see "just ignoring it" to be a viable solution - but before I can convince the clients to spend a substantial amount of money re-jigging the distribution to eliminate the problem - I have to be 100% certain that this IS a problem. I think it is, but the installer, and muppet who previously PIR`d it didn`t ( The PIR gives nominal as 400/230 - so someone didn`t actually measure it).

We can`t USE 400 as nominal, as that is for 120 degree phases.....

Read this - under "system voltage" 

If they spec 460 for this supply..........

 

[Phoenix]

For main distribution you could go for an MEM glasgow feeding into an exel HRC fuseboard. MEM literature indicates that both are rated to 660VAC. Then install SPN MCB final boards as required

 

[Steve3948]

If its a Pub/Hotel in the sticks, could you not ask DNO if they can tweak the voltage.