Is it much mor awkward working on 3 phase

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Is it a lot more awkward working on 3 phase systems compared with a domestic single phase system? if so what are the main points to watch out for from an experienced electricians point of view?
 
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Is it a lot more awkward working on 3 phase systems compared with a domestic single phase system? if so what are the main points to watch out for from an experienced electricians point of view?

Just remember your safe isolation procedure involves a ten point test.
 
And voltages/currents are often much higher.
As riveralt says, remember your isolation procedures and let safety be your priority. Take care and if in doubt stop and think. You may not get a second chance.
 
Watch the voltage rating of anything used to connect a single phase circuit to the three phase system.
 
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The main difference between three phase and split or single phase is three phase has a rotation.

You can get a phase rotation meter but really for most work it is not required. In most cases one can just try the appliance and if it goes the wrong way around just swap two phases.

However connecting generators to run together for example then one would want to check direction before connecting.

Because there is direction motors don't need anything to get them to turn in the correct direction. However the currents needed to start them can be very high so many methods are used to reduce the start currents.
Star - delta start
Resistor start
Auto transformer start
Electronic soft start
Inverter control
To name a few.
Because of this doing what may be simple with single phase like pump float control can get quite complex with three phase and it could involve the rewiring of star / delta starters to include something like float control.

The other point is balance. We have to consider how much power we are demanding from each phase and we have to try to ensure an even load across all three phases.

As things get more complex we may also have to consider power factor correction although that may also be the case with single phase. However the fact that three phase is used with motors means it is more common with three phase.
 
If you are talking domestic 3Ph&N then most of the time the phases are split out into single phase circuits feeding different floors or one phase for lighting and other two divided across floors. You then need to think about 400V between rooms or between lights/sockets etc.

More recently I have seen 3Ph&N hobs/ovens (configurable options) and even a steam press but unless you are going to run a dedicated 3Ph circuit for it you are still only working with single phase.

In the commercial world you often see large array grid switch plates with mixed phases. The switches with 400V PD between them are in close proximity to one another so watch out!!

In commercial and industry you could well be into 3Ph 3 wire with motor systems etc. Power factor, phase load balancing become major factors and require experience.

Its a broad subject. One word of advice, if you have not worked with 3Ph 3 wire or 3 ph 4 wire then make sure you work with someone who has the experience first. Don't trying winging it on your own.
 
Do remember there are three systems not just two.
Single Phase.
Split Phase which is really single phase.
Three Phase.

The Split Phase has the highest voltage of 460 between lines so really the voltage on standard three phase is lower than some single phase supplies.
Split phase is used where there is high demand but no rotating machines. For example a caravan site.

The 3.3kV will often be just two wire and the transformer pole mounted with often 2 x 100A fuses. Often people refer to it as being two phase but that is not really what it is.

Also with the reduced price of inverters we are getting a lot of star / delta wound motors with 230v windings rather than 400v giving 230v delta and 400v star and are powered from 230v single phase via the inverter. I have seen a few burnt out where the inverter has failed and the electrician has connected them up direct on line without first moving the links to star position.

Mines and quarries still use a lot of 660v three phase not a clue why but again easy to make mistakes. We all tend to jump to conclusions and assume low voltage means 230 / 400 volt.

In Sizewell we had three phase 110 volt supplies where 32A three phase would supply a splitter box with around 9 single phase 63-0-63 (110v) supplies. Also had 16A versions these would allow a total of 30A to be drawn yet still no more than 16A on any one core.

As to colours well we still use the wrong colours. Other than with the three phase 110 volt supplies most 110 volt supplies have the colours Brown, Blue, and Green/Yellow yet we all know it should be Brown, Black, and Green/Yellow for a split phase supply. Blue is now only used for neutral.

When we changed the colours with flex we had two blacks and a brown and only in last few years has grey arrived on the scene. So with all white and numbers, and all black and numbers, plus two blacks and a brown we have a whole mixture of colour codes including people who use brown, black and grey to feed two way switching. As a result we have to be very careful as to what colours stand for. Where Red, Yellow and blue were used with single phase we could expect to see yellow as earth and blue neutral with red as the phase conductor. However with the new colours there seems to be no such uniformity and of course since we are using wrong colours anyway we can't really expect there to be any uniformity.

I would not expect any electrician to worry about using blue as a phase conductor with a 110v supply so why should we expect them to do any different with a 230 volt supply?

We all know what we should do but we also all break the rules and as a result working with split or three phase supplies needs far more care then with a normal domestic single phase supply.
 
The Split Phase has the highest voltage of 460 between lines so really the voltage on standard three phase is lower than some single phase supplies.
Split phase is used where there is high demand but no rotating machines. For example a caravan site.

The 3.3kV will often be just two wire and the transformer pole mounted with often 2 x 100A fuses. Often people refer to it as being two phase but that is not really what it is.

Good point ericmark, I have heard of split phase but never worked on it. Is it a delta primary with two anti-phase taps (180 deg out of phase with respect to one another)
Are there 3 taps on the secondary (L1 - L2 - N)? Or is there no neutral?
If no neutral (or centre tap) How do you get 230V from it?
 
Split phase transformers were used for working from single phase 11kV lines, i.e. HV lines with only two conductors instead of three. Cheaper in rural areas with low loadings.

So the transformer is single phase, has an 11kV primary and a 480V centre tapped secondary. I say 480V because I don't think any have been installed in the past 20 years.

Not to be confused with a Scott transformer that produces two phases at 90° to each other from a three phase primary.
 
So the transformer is single phase, has an 11kV primary and a 480V centre tapped secondary. I say 480V because I don't think any have been installed in the past 20 years.
As a matter of interest, why did they do that rather than just have two separate 240V secondaries? The only difference would be that the 480V secondary to which you refer would be broken at the centre point, but that would then remove the issue of 480V bewteen the two 'split phases'.

Kind Regards, John.
 
Split phase is used where there is high demand but no rotating machines. For example a caravan site.

It's still quite common out here in rural Norfolk, typically in the sort of place where a small hamlet of a couple of dozen houses is comparatively remote from where 3-phase lines run, since it saved money to run a single-phase 11kV spur.

Other than with the three phase 110 volt supplies most 110 volt supplies have the colours Brown, Blue, and Green/Yellow yet we all know it should be Brown, Black, and Green/Yellow for a split phase supply. Blue is now only used for neutral.

For 110V CTE it should actually be two browns rather than brown and black, according to the appropriate table, since they are just the two outers of a single-phase supply.


As a matter of interest, why did they do that rather than just have two separate 240V secondaries? The only difference would be that the 480V secondary to which you refer would be broken at the centre point, but that would then remove the issue of 480V bewteen the two 'split phases'.

The whole point of the arrangement is to get 480V between the two outers to benefit from the reduction in current in the LV distribution cables. Just as with a three-phase system, the neutral carries only the imbalance current.
 
As a matter of interest, why did they do that rather than just have two separate 240V secondaries? The only difference would be that the 480V secondary to which you refer would be broken at the centre point, but that would then remove the issue of 480V bewteen the two 'split phases'.
The whole point of the arrangement is to get 480V between the two outers to benefit from the reduction in current in the LV distribution cables. Just as with a three-phase system, the neutral carries only the imbalance current.
Fair enough, but that presumably only results in a current reduction in the neutral(s). Do they therefore distributes with the neutral conductor having a lower CSA?

Kind Regards, John.
 

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