Why normal household power supply must be single-phase?

[thanhvu94]

Hi everyone,

I'm taking an electrical course at the university and the professor asks us a very tough question: "Why we use single-phase for power supply while 3-phase is much safer, more efficient? What happens if we replace all the single-phase power supply by the 3-phase one?"

So, why normally household power supply must be single-phase?

All the search on Google so far doesn't satisfy me enough because it cannot give me the benefits in terms of power or technology. So, I hope that someone here can really enlight me .

Thank you so much.

 

[Leesparkykent]

It doesn't have to be single phase.

 

[thanhvu94]

Yes, but may I ask why normally people use one-phase power supply? Why don't we just make every house three-phase power supply at the beginning? There must be some persuasive reasons for this, right?

Thanks.

 

[Leesparkykent]

Cheaper to install from the DNO's point of view and the calculated maximum demand of a typical house suggests the 3P isn't required.

 

[Pete999]

Don't see how a 3phase 4 wire supply would be safer than a single phase supply, op could you explain your reasoning on this please. Mind you it would curtail all these so called Electricians who have not or can't work with 3 phase, may be thats your reasoning for a safer supply lol, tin hat on, trenches dug.

 

[sparksburnout]

It isn't safer, it is more dangerous due to the higher voltage between phases. It is not usually provided for domestic installations as the maximum demand would not require it, so it is cheaper to install. Why would you install a cable with more conductors and a 3 phase meter when it reality you would never get near the maximum capacity of a single phase? There is no real concern regarding efficiency, domestic networks are pretty much unbalanced anyway due to the way they have been installed and maintained.

 

[andyb]

I think your 'professor' is pulling your plumbs.

 

[Darkwood]

To note!...Some houses do have 3ph supplies but in our country nearly all domestic appliances are 1ph due to the standard practice of single phase,

Very few domestic items could be connected using 3ph although they do exist with multi-voltage options like some ovens and hobs.

 

[Dizzy_Maskell]

Well it would make life easier for me a TP&N supply as I'd have more options to workshop tools for my garage,
Been to look at a few jobs where they have bought an eBay job and turns out three phase mate wanted make a garage in his house bought a car lift three phase etc etc

 

[NDG Elecs]

I think your 'professor' is pulling your plumbs.

Quite possibly Andy but there are plenty of academics out there who can't see the 'volts for the amps'. They get way laid in the theoretical side of things whereas in practise a lot may be completely non sensical.

Although saying that I had a tutor on the 2330 that was a complete tawt and would often tell us incorrect things to throw us all, such as ohms law not being used in AC systems.

 

[Flanders]

May be the prof is going down the lines that thers willbe less amp drawn down each phase if 3 phase is used as aposed to single phase , as it is the amps that kills you not the volts , but thst would be on the dno side , i can't see how it would make the house electrics any safer

 

[Darkwood]

If you have 3phase throughout the domestic arena then the incidence of floating neutrals could increase as more appliances would run on 3ph and we already have a market flooded with inept trades people who cannot tighten the terminals on the main switch correctly, god knows what they would do with extra terminals -

Other issues associated with 3ph often exclusive to commercial and industrial then become a domestic issue too, loss of a phase wouldn't just simply cut all power but could see integral trips operating on 3ph appliances thus costing households a fortune in callout to check and reset equipment, this is all hypothetical but based on the commercial and industrial scene.

 

[Lucien Nunes]

It isn't safer, it is more dangerous due to the higher voltage between phases

Not necessarily true. If the line voltage were the same as the present phase voltage (i.e. 230/132 instead of 400/230) and single-phase loads were connected in delta, then the equipment wouldn't consume any more current but the voltage to earth would be lower, therefore safer. Of course there is a competing factor that both wires are live to the touch instead of only one (normally). As it happens, this system was used quite widely in the past, and it is still standard for shipboard supplies. The reason Germany for example adopted the un-polarised Schuko plug was that at the time, 3-phase delta was in use and both pins could be lines, so it didn't make sense to distinguish them. These days, 3-phase supplies are still in use in continental houses but star, rather than delta.

My take on it is the difficulty of handling large loads on small systems. If you have a 100A SP supply, you can run a 50A shower load, given diversity, without any specific complications. A 3-phase supply of the same total capacity would not support such a large load on any phase, so it would have to be 3-phase, which is more expensive. It is harder to utilise diversity on say three 25A main fuses than one 60A.

less amp drawn down each phase if 3 phase is used as aposed to single phase , as it is the amps that kills you not the volts

These two true statements are totally unrelated to each other!

 

[Marvo]

I know a lot of countries in that neck of the woods have very small domestic supplies available, sometimes as little as 10Amps even. With very small supplies if you provided it as 3-phase and of the same KVA rating as 10 or 20 Amp single phase for example it would be almost unusable due to the low amount of Amps per phase.

 

[Geordie Spark]

It isn't safer, it is more dangerous due to the higher voltage between phases. It is not usually provided for domestic installations as the maximum demand would not require it, so it is cheaper to install. Why would you install a cable with more conductors and a 3 phase meter when it reality you would never get near the maximum capacity of a single phase? There is no real concern regarding efficiency, domestic networks are pretty much unbalanced anyway due to the way they have been installed and maintained.

Aye, I'd rather have just one phase belting up me arm than three of the buggers. :hump:

 

[YoungScud]

May be the prof is going down the lines that thers willbe less amp drawn down each phase if 3 phase is used as aposed to single phase , as it is the amps that kills you not the volts , but thst would be on the dno side , i can't see how it would make the house electrics any safer

I haven't a bloody clue what you're saying here. The current drawn by the load is irrelevant when you touch a live conductor. When that happens, it's the current YOU draw as a result of your body resistance which is vital.

 

[YoungScud]

As far as DNOs are concerned, the only time a three phase supply is required in a domestic installation is when load demands it, that is when there is a load which exceeds a 100A supply (around 27 kVA). Above that, load has to be split across multiple phases. Other than that, occasionally a posh household with a need for specific multiphase equipment (swimming pool, large electric gates etc.) would obviously require such a supply. Given a 230/400V three phase, as is standard across the UK for LV supplies, it's hard to see how it could be thought of as safer.

 

[telectrix]

it's geneerally safer as the Electrical Trainee.s would not be let loose on it.

 

[sparksburnout]

So if a 3P supply had been provided in a premises way back when (Lets say a commercial one), would the supply cable have had smaller CSA's per phase than if only a single phase had been installed, meaning that it would be limited on its single phase operation (to 60A?)

 

[Flanders]

I haven't a bloody clue what you're saying here. The current drawn by the load is irrelevant when you touch a live conductor. When that happens, it's the current YOU draw as a result of your body resistance which is vital.

Sorry i was trying to think of a reason why the prof thinks that 3 phase is safer , but of course as you say , your body is the load when you touch the line so it want make any difference with 3 phase or single , i must engage my brain in future