Do 2-phase (split phase) lines ALWAYS need to be switched together?

[Mark42]

Here’s the scenario:

240/480V split phase, rural overhead, ‘Farm’ supply.

From the main house Hager DB, two ways are used to supply 50A, both phases, on 16mm2, to a sub main DB in the workshop.

Is it OK to use two separate 50A Type C MCBs in the DB (cheap and available) rather than a 2-pole 50A MCB? (Expensive and very difficult to source).

No (old, exotic) 480V two-phase equipment will ever be used in any part of the installation, so there’s no question of failing to disconnect the phases substantially at the same time as is required for polyphase equipment.

The only reason in having both phases in the workshop is for load balancing.

The biggest breaker in the workshop is a 32A Type B, so the only chance of the 50A C’s ever tripping because of a fault would be cable damage, which is extremely unlikely.

The twin breakers would be clearly labelled as both supplying the single workshop DB, which itself would be labelled as having a twin source of supply.

I feels a bit wrong but is it OK electrically, or regulation-wise?

This is effectively two, separate, 240V supplies in a single box. This is so rare that the regulations appear silent on the matter.

Opinions?

Cheers, Mark

 

[lewlec]

hi

could you use a tripple pole mcb? breaking the neutral and two of the phases?

regards
gary

 

[Risteard]

Two phase and split phase are not the same thing.

 

[Mark42]

hi
could you use a tripple pole mcb? breaking the neutral and two of the phases?
regards
gary

Hi Gary
Thanks for the comment, but as Risteard implies, split-phase is a different animal. Without going into tedious detail it arrives from the pole transformer as 2 x 240V phases at a 180 degree phase angle, therefore 480V phase-to-phase.

I’ve used a standard Hager TPN board, commoning L2 & L3 to save ways.

Therefore the panel ways go L1-L2-L2 … L1-L2-L2 … L1-L2-L2 … and so on.

Using a standard 3-pole breaker would ‘waste’ the second L2, be untidy, and potentially confusing. I may just order a 2-pole breaker, but since I have plenty of single pole MCBs which are very cheap, I’m still tempted to use two of them instead. I really want to know what the regulatory issues are.

Cheers

Mark

 

[lewlec]

hi

done many split phase installations but never on a tp&n db and can see what you are saying regarding the 3rd pole, what i normally do is put a tp mcb in a seperate enclosure at origin and then feed out from there switching l1,l2 & neutral. i think in your case maybe a dp mcb would be best.

regards
gary

 

[Sintra]

You need to be careful with split phase on a 3 phase db as the maximum rated voltage of the db may be exceeded ie. 480V + permitted 10% = 528V.

 

[Mark42]

You need to be careful with split phase on a 3 phase db as the maximum rated voltage of the db may be exceeded ie. 480V + permitted 10% = 528V.

Hi
Yes! Exactly, I'd thought about that, and it's why I joined this forum looking for opinions and advice four weeks ago:

http://www.electriciansforums.net/c...-switchgear-used-480v-split-phase-ok-not.html

Is it generally done or not?
Note that the DNO's equipment, including a TPN meter and radio-teleswitch (both with only two phases connected) is all marked up as 230/400 - 240/415 but is of course running at 240/480 in this case.
Nothing's burned down yet But is it 'legal'?

Cheers, Mark.

 

[Sintra]

Hi Mark

Had a quick read of your thread.

Personally i would install 2 metal clad consumer units stacked together.

 

[davelerave]

hey

double pole c50s shouldn't be hard to get -i used a load of DP mcbs c63 etc on transformer work lately


a DP isolator may do if you have switch-fuse on the supply side of sub-main

be careful if you have to switch neutral as well (all lives) -it must break last on the split-phase supply
a 3-pole mcb can't be used for this imo

practically all tn-c-s here(including outbuildings/sub-main boards)so it's less complicated

 

[lewlec]

hey

double pole c50s shouldn't be hard to get -i used a load of DP mcbs c63 etc on transformer work lately


a DP isolator may do if you have switch-fuse on the supply side of sub-main

be careful if you have to switch neutral as well (all lives) -it must break last on the split-phase supply
a 3-pole mcb can't be used for this imo

practically all tn-c-s here(including outbuildings/sub-main boards)so it's less complicated

hi there

what is the reason for breaking the neutral last on a split phase system? sorry if im missing the obvious..

i got annual nic inspection next week and one of the jobs have this set up.... i wonder if he will pick up on it?

regards
gary

 

[davelerave]

didn't mean to be definitive-i'm assuming the standard 3 pole switch/mcb are not allowed (for all lives)

in case the center tap is lost momentarily

we don't switch them here

 

[Mark42]

...Personally i would install 2 metal clad consumer units stacked together.

So might I had I known the planned upgrade to ‘normal’ three phase was going to be priced by the DNO at £20,000! (and hence the plan be swiftly abandoned).

I’d already bought and fitted three Hager TPN boards.

Plus I need 40+ SP ways in the main house board, with many RCBOs, so I fancied a bigger commercial panel anyway for ease of wiring.

But I’m also not wholly comfortable with the 480V issue. But overvoltage is rare here – it’s normally the other way - especially when my goose farmer neighbour switches on all his incubators.

Although I’m hoping that there is such tolerance built into modern designs and materials it should be fine. I mean, modern plastic insulation can withstand thousands of volts for decades before breaking down. (Can’t it?)

double pole c50s shouldn't be hard to get -i used a load of DP mcbs c63 etc on transformer work lately
a DP isolator may do if you have switch-fuse on the supply side of sub-main
be careful if you have to switch neutral as well (all lives) -it must break last on the split-phase supply
a 3-pole mcb can't be used for this imo
practically all tn-c-s here(including outbuildings/sub-main boards)so it's less complicated

This installation is all TN-C-S as well.

Yes, I’ve fitted a new TP switchfuse upstream, with solid neutral, fed from the meter tails. It’s just under three meters from there to the main DB (space is tight otherwise) so I felt better protecting (and switching) the DB supply cable (4 x 25mm SWA) that way. It also enables the barn sub-main to come directly from this switchfuse rather than from DB1. Also it eases the changeover from old to new installation, with much less working live.

But since the switchfuse is 100A, I’d rather not use only a DP isolator downstream for the workshop supply, even though it’s on 16mm. It’s a good idea, and I have plenty of old isolators in my junk box, but I like the idea of having 50A protection downstream. Plus, strictly, it’s no doubt a requirement due to the CSA reduction.

By hard-to-get I meant not listed anywhere online (I usually use TLC), so I may - heaven forefend - have to pick up the phone and actually talk to a real person! Maybe next week.

Gary – I’m not sure I understand you. Do you really mean a 3P over-current MCB, or an isolator (ie just a switch)? Surely you can’t run two phases plus neutral through a 3P MCB? This may be reaching the outer edges of my understanding, but would the third phase not be trying to monitor and protect the ‘reverse’ neutral current? It feels odd to me.

Thanks for all the advice gentlemen; it’s much appreciated.

 

[davelerave]

ya for tn-c-s you only need DP mcbs/switching on supply/distribution for split-phase

and SP for 230v final circuits




i may be overly cautious about switching star points/centre taps

3-pole mcb may not be an issue if all poles make/break substantially together-maybe someone can confirm?

(if the centre tap is lost on the supply/distribution side the 480v may divide unevenly across the loads)

prob not a great idea feeding the 2 boards from the 'main' TPN switchfuse imo -i would fit separate switchfuse for the sub-main and an isolator at the sub-board for tn-c-s

assuming you can export the earth in your case(we don't convert sub-mains to TT here at all)

 

[lewlec]

So might I had I known the planned upgrade to ‘normal’ three phase was going to be priced by the DNO at £20,000! (and hence the plan be swiftly abandoned).

I’d already bought and fitted three Hager TPN boards.

Plus I need 40+ SP ways in the main house board, with many RCBOs, so I fancied a bigger commercial panel anyway for ease of wiring.

But I’m also not wholly comfortable with the 480V issue. But overvoltage is rare here – it’s normally the other way - especially when my goose farmer neighbour switches on all his incubators.

Although I’m hoping that there is such tolerance built into modern designs and materials it should be fine. I mean, modern plastic insulation can withstand thousands of volts for decades before breaking down. (Can’t it?)


This installation is all TN-C-S as well.

Yes, I’ve fitted a new TP switchfuse upstream, with solid neutral, fed from the meter tails. It’s just under three meters from there to the main DB (space is tight otherwise) so I felt better protecting (and switching) the DB supply cable (4 x 25mm SWA) that way. It also enables the barn sub-main to come directly from this switchfuse rather than from DB1. Also it eases the changeover from old to new installation, with much less working live.

But since the switchfuse is 100A, I’d rather not use only a DP isolator downstream for the workshop supply, even though it’s on 16mm. It’s a good idea, and I have plenty of old isolators in my junk box, but I like the idea of having 50A protection downstream. Plus, strictly, it’s no doubt a requirement due to the CSA reduction.

By hard-to-get I meant not listed anywhere online (I usually use TLC), so I may - heaven forefend - have to pick up the phone and actually talk to a real person! Maybe next week.

Gary – I’m not sure I understand you. Do you really mean a 3P over-current MCB, or an isolator (ie just a switch)? Surely you can’t run two phases plus neutral through a 3P MCB? This may be reaching the outer edges of my understanding, but would the third phase not be trying to monitor and protect the ‘reverse’ neutral current? It feels odd to me.

Thanks for all the advice gentlemen; it’s much appreciated.

yes a tp mcb as overcurrent protection. ive done this on two jobs and had no problem but not saying that its a normal way of doing it. normally put L1 - N - L2 in the breaker and the neutral will never exceed the line current being that it is split phase. hope i am not confusing matters.

regards
gary

 

[davelerave]

yes a tp mcb as overcurrent protection. ive done this on two jobs and had no problem but not saying that its a normal way of doing it. normally put L1 - N - L2 in the breaker and the neutral will never exceed the line current being that it is split phase. hope i am not confusing matters.

regards
gary

i could well be wrong as i'm not overly familiar with TT
it may indeed be ok for all llives to switch simultaneously for TT- i know that for isolators they may make first /break last

a lost neutral on the split-phase could cause a lot of problems