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Topic Title: Phase to phase loads
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Created On: 06 December 2013 03:48 PM
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 06 December 2013 03:48 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Hi there,

Do the wiring regs prohibit a single 400V phase to phase load and if so could someone give be the reg number. I can't find any obvious problem with the concept although I know it's not commonly applied in the UK.

Thanks, Tim

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Everyone loves a fireman - but hates the fire inspector.
 06 December 2013 03:55 PM
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Parsley

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See note 1 page 42.

Regards
 06 December 2013 04:26 PM
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OMS

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It's perfectly acceptable and might be more common than you imagine - I can think of a number of heating and water heating applications that use phase to phase loads

There are also plenty of lighting systems that operate phase to phase - area flodlights being common with two phases across the HID lamp ballast

and then we have anything that's delta connected

Regards

OMS

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Failure is always an option
 09 December 2013 01:40 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Ok thanks for the references and comments,

So say you had some single-phase loads connected across line-to-line are there any special considerations that would need to be looked into even though It's perfectly acceptable to BS7671.

The reason I ask is that I'm looking into the design of the laundry and galley equipment supplies for a ship however the Korean designers normally use an isolated earthing system - even for LV non-critical appliances. Given that we have requirements for easy fault-finding via automatic disconnection of supply we've requested that RCDs/RCCBs be installed for each circuit and for the laundry and galley equipment to be on a separately derived solid earthing system (TN-S). Rather than come back with a 3-phase 4 wire system per standard LV onshore arrangement they are proposing a 3-phase 3 wire system with no neutral and single phase loads connected in delta fashion across lines (line-to-line voltage being 230V). Although this is non-standard I can't find any reason to reject this proposal. The only issue I can think of is that the feeder cables would need to be sized with 33% redundancy in case there is an unbalanced load. Given the star-point of the transformer will be solidly earthed to the hull of the ship the line voltages will remain at a fixed reference.

Maybe I'm having a bad brain day but it got me thinking why domestic systems are not wired this way. If we can connect single phase loads in a delta fashion (line to line - no neutral) as long as we isolate each pole it should be no problem unless the extra cost of double pole isolation favors a 4-wire system? Or maybe it's that the 4-wire system allows 230V single-phase loads but 400V 3-phase delta connected motors etc. at a higher voltage. I just want to make certain that there isn't any major issue with a system base on line-to-line connected loads. Next I will need to see if double-pole RCDs will operate correctly.

Cheers, Tim

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Everyone loves a fireman - but hates the fire inspector.
 09 December 2013 03:11 PM
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broadgage

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A system as is proposed with 230 volts between phases is fine if correctly installed.
All OCPDs need to be double pole, and switches should be double pole in many situations.
If the installation has a solidly earthed neutral, but this is not distributed, then the voltage to earth from any phase will be fixed at about 133 volts.
If the system has no earthed neutral then the voltage between any phase and earth could vary between zero and 230 volts depending on the degree of leakage.

The public supply in parts of Southern Europe was like this until recently, and a few bits may still survive.

The system does not find much favour these days for public electricity supplies as it restricts the load that can be supplied by a given cable size.
By changing from a 230 volt only, 3 wire, 3 phase system to a 3 phase 4 wire system at 230/400 volts, 70% more energy may be transmitted for only 33% more copper.
If the limit is not conductor heating but voltage drop, then the increase is greater still. Premises wiring is slightly cheaper and simpler with an earthed neutral.
 09 December 2013 03:20 PM
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AJJewsbury

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I've a feeling that unearthed systems are common even on UK built ships - there might be a good reason for that. There are some nautical types here, so might be waiting to see if they have any advise to offer.

The public supply in parts of Southern Europe was like this until recently, and a few bits may still survive.

I gather than one of the problems was that single phase customers, generally supplied via a single meter, discovered they could obtain "free" electricity (if at reduced voltage) by connecting between the 2nd (unmetered) line and earth....

- Andy.
 09 December 2013 06:14 PM
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Inrush

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You just need to remember that current flowing in a phase - phase connected load will be +/- 30 degrees out of phase with any loads supplied between the same phase/s and neutral.

That's before the load power factors are considered
 09 December 2013 06:15 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

ok, makes sense, I can see how the metering issue would be a problem and that a 230/400V system is more efficient on copper.

A lot of ships with electric propulsion (especially tankers) are isolated earth to prevent conduction of fault currents through the tanks and to provide resilience 1st earth faults (don't want your propulsion motors tripping out during maneuvering in port etc) however recommended practice is for the non-essential services (laundry and galley etc) to be provided via solidly earthed systems for ADS and easy fault finding.

Do any of you know if there's a problem with using RCDs on line-to-line single phase loads. As long as they're 2-pole devices I don't see why it would be a problem - just that it's a non-standard application. Can I just sub the N for another Line?

Cheers, Tim

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Everyone loves a fireman - but hates the fire inspector.
 09 December 2013 06:21 PM
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Inrush

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Do any of you know if there's a problem with using RCDs on line-to-line single phase loads. As long as they're 2-pole devices I don't see why it would be a problem - just that it's a non-standard application. Can I just sub the N for another Line?


Assuming there're double-pole devices and the voltage is 230V (50Hz) between phases I wouldn't see a problem, could be worth checking with the manufacturer though.
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