3ph to Single Phase

[funinacup]

Hi all, just took over a 2nd unit at work which has 5x 32amp 3ph sockets on the back wall which will be our workshop. I think 3x are 4mm cable (3c+n+e) and two are 2.5mm.

I don't need any 3ph power but do need 20 and 32amp single phase power.

Is it OK to make up a lead with some 4mm / 6mm flex with a 3ph plug on one end and a single phase socket on the other, to allow my equipment to be plugged in?

Which of the live pins would I connect in the 3ph plug?

I'll only have one machine operational at a time. Any other questions ask away and I'll do my best to answer!

Many thanks
Michael

 

[sxturbo]

The way I understand it, you'd want 1 phase core and neutral and earth for single phase.

However the frequency may be out of kilter. If your equipment is not sensitive to the frequency then this is acceptable.

This will work on your equipment that requires 32 Amps.

 

[funinacup]

The way I understand it, you'd want 1 phase core and neutral and earth for single phase.

However the frequency may be out of kilter. If your equipment is not sensitive to the frequency then this is acceptable.

This will work on your equipment that requires 32 Amps.

Thanks. Isn't it 50hz regardless of single / 3 phase?

 

[sxturbo]

Thanks. Isn't it 50hz regardless of single / 3 phase?

Yes but 3 phase is slightly different and the phases are different pulse widths.

In a single-phase system power is supplied through two wires: one delivers the current, the other provides the completes the return path. During one phase cycle power delivery fluctuates, with peaks and dips in voltage. In a single-phase system the power wave peaks at 90⁰ and 270⁰. This means that at two points in a cycle power delivery is at maximum. At other times, the power delivered is less than optimum.

In a 3-phase system, the load is shared across three power wires. The three power wires (A,B and C) are arranged to be out of phase with each other. All three phases of power have entered the cycle by 120⁰. By doing this, the three phases of power peak in voltage at different times during a complete cycle. By supplying power this way there are no peaks and drop-offs. Sharing the load between three wires means power is supplied constantly.

 

[Chunky19]

All very true but that’s got nothing to do with using a single phase and neutral.

 

[Harry Bloomfield]

Yes but 3 phase is slightly different and the phases are different pulse widths.

It is still 50Hz, but the phases /the peak will be 120 degrees out /delayed from the other two phases. To use a single phase, then a neutral will be required.

The 'pulses' or sine waves of each phase are otherwise identical no difference between them at all.

 

[Jackrae]

What school of electrical engineering did that theory come from.

A single phase supply is readily available (in sinusoidal form at 50Hz) from any of the 3 phase lines and the neutral.

You should fit a suitable means of fused isolation unit between the 3-phase source and the intended single phase outlets rather than making a 'botch' conversion lead.

 

[Notch7]

I believe houses are fed 3 phase supply in the street and adjacent houses get the 3 phases to balance load.

In a commercial unit, if taking single phases from the Distribution unit I believe precaution is needed where there it’s possible different single phases are run around a factory - not sure why.

 

[Harry Bloomfield]

I believe houses are fed 3 phase supply in the street and adjacent houses get the 3 phases to balance load.

In a commercial unit, if taking single phases from the Distribution unit I believe precaution is needed where there it’s possible different single phases are run around a factory - not sure why.

Correct! As it progresses along the street it, to each house in sequence, it would be Ph1; Ph2; Ph3; Ph1; Ph2;Ph3 and so on all the way along. It could even be two adjacent houses on each phase, the idea overall - is that there should be a near equal load placed upon each of the three phases. A perfectly balanced load means there would be a minimum of current flowing in the neutral. Often the neutral will be smaller than the phase conductors, because due to the load balancing - it isn't needed to carry that much current. Should that neutral fail (and it has been known), it then becomes possible for a lightly loaded house compared to others, to find its supply rising to near 415v.The neutral is the common conductor which will go to every house in the street.

Phase to neutral the voltage is 240v (yes I know230v, but this is the UK), phase to either of the other two phases it is 415v. In factories, offices and similar where 3ph is often available, care needs to be taken to ensure areas which have a 240v on one phase, are reasonably segregated from areas in which another phase is in use. What you don't want is a cleaner pushing one faulty vac around on Ph1, also grabbing a second faulty vac which is plugged into another phase - otherwise, they get 415v across them.

Ph1 in one factory, might not even be the same ph1 conductor in the adjacent factory. All that really matters is the 'phase rotation' to the factory owners, or the 'firing order' of the phases in relation to each other, for 3ph motors. Get a wrong combination and the motor goes backwards, with a 50/50 chance of getting it right or wrong.

Easy to correct on a small motor by simply swapping two of the phase wires over, but on some of the big stuff I regularly worked upon - you checked the phase rotation and made sure the motor went the right way first off, because they were so difficult to change heavy cables over. Even then, some machines mechanically should not be rotated the wrong way, so if in doubt - you disconnect the coupling.

 

[funinacup]

You should fit a suitable means of fused isolation unit between the 3-phase source and the intended single phase outlets rather than making a 'botch' conversion lead.

What would this look like?

At the moment there is a row of 5x rotary isolators on the right hand side. Would it be better to rewire the isolator so it is single phase cable coming out and run it to a single phase socket, instead of trying to convert what's already there?

 

[ericmark]

There are two problems taking single phase from three phase, one is balancing the load, the other is is there a neutral? In theory a three phase socket with a neutral pin should have a neutral running to it, in practice this is not always the case, in theory it should be wired so every socket has same phase rotation, but nothing forces this, so if we call the phases red, yellow, blue then if red phase is on pin one with one socket it should be pin one with them all, so making three converter leads one should use pin one, next pin two, and next pin three so the phases are even loaded, but does depend on how wired in the first place.

I would not think the loading from what you say would really be a problem, the main thing is ensure there is a neutral.

Since there is 400 volt phase to phase in most places one will only supply one phase to each room unless using three phase equipment, so in spite of saying balance the load on each phase, it may be with safety in mind only one phase is used, normally the safety officer will do a risk assessment.

 

[AndyPRK]

You could just change the 3 phase socket, to a single phase socket.

Put the spare 2 phases into wago blocks.

Regarding fusing. I guess what is being said is if you fit a 16A socket, maybe you need to fit a 16A MCB at the distribution panel? or maybe you don't. I'm not sure.

 

[flameport]

Would it be better to rewire the isolator so it is single phase cable coming out and run it to a single phase socket,

That may be an option, however depends on many things such as the cable size, whether a neutral is present, protective device, what you intend to connect to it, and also whether what's there was installed properly in the first place. The photo only shows a row of isolators, so presumably the sockets are elsewhere - that in itself is an unusual arrangement.

Regardless of that, this is not DIY work. You need an electrician to assess what's there and install the appropriate equipment for your requirements.

 

[davelx]

Aren't those 5 sockets mounted in a line on that back wall?

 

[AndyPRK]

I assumed the sockets were on the far metal wall.