Breaking down three phase

[wade88]

Hey guys,

Can someone help me get my head around breaking three phase down into three seperate circuits please.

If i wanted to break down a three phase N + E SWA for example, into three seperate 20amp commando radials say, how would i do this when i only have one N and one E but three circuits?

 

[topquark]

You are creating the 3 circuits from L1+N+E, L2+N+E and L3+N+E. On the neutral (and possibly earth in the event of a fault) you will have all 3 phases present (but 120 Degrees out of phase with each other). So you will wire your DB and take the L1, L2 and L3 to each of the circuits in turn.

 

[wade88]

I understand the first part Topquark, What i cant understand is how i take a N and E with each circuit when i only have one of each. I know this sounds ridiculous but im not so good with three phase and want to understand it.

Let me try and draw it so i can alleviate the confusion...prepare yourself

 

[topquark]

preparing myself ... in the meantime:

You just take the supply into a Neutral Bar and Earth Bar and take a feed for your circuit from there (possibly via a safety device(s), depending on your requirement for multipole safety). It's not really any different (from a purely mechanical perspective) to single phase.

 

[wade88]

Is that too small? But am i on the right lines there so to speak and just repeat for each circuit?

 

[topquark]

Yep. Repeat for each circuit. Main concern would then be balancing current demand across the phases.

As you're providing for 20A from each circuit, your phases would be roughly balanced (assuming the current draw was similar).

 

[wade88]

Ok thats how i thought it worked so glad i wasnt miles away.

Thanks for your help buddy.

 

[topquark]

No worries. You should also connect the L1, L2 and L3 in sequence of rotation (I'll mention it now as it won't complicate the scenario ... just before someone comes in and "corrects" it ).

With using phases individually that's pretty much it.

If you have machinery that uses all 3 phases then you have to worry about using the phases connected in sequence (often called roation).

 

[wade88]

Why is that important to connect them in sequence quark? By this i gather you mean, L1 first L2 second etc. As the circuit is dead whilst installing of course, why would it make a difference?

 

[topquark]

Sorry Wade88 that wasn't very clear. I wasn't suggsting that you connect them in that order (although it does read like that). You connect the cables so that, in the DB, Phase 0deg is on L1, Phase 120deg is on L2 and Phase 240deg is on L3.

It's convention. You normally connect to L1, L2 and L3 in phase sequence so that anyone taking a 3 phase circuit from your DB can easily connect the phases in the correct sequence. This is typically for a motor, so it's important to get the phase sequence/rotation correct.

 

[wade88]

Ah right, so how do you identify which one is 0 degrees, 120 degrees and 240 degrees?

 

[topquark]

This would be a live test. You need a meter capable of indicating the phase rotation. I use a fluke T140 voltage indicator; it can indicate the phase rotation (ie which comes first) between two phases. So you just do a number of comparisons in order to ascertain the order. There are more expensive devices that will test all 3 phases at the same time etc.

 

[wade88]

Yes i have a T140uk also, (R1) indicates L1 is L1 and L2 is L2 and (L2) indiciates that L1 is L2 and L2 is L1 correct? or something like that. So after the test, if i have determined that L1 is L1 etc, i can denote that L1 is 0 degrees, L2 is 120 degrees and L3 is 240.

 

[topquark]

You'll see it has two LEDs on it marked R and L. R will be lit when you have L1 and L2 in the correct rotary sequence, with the lead on L1 and the meter on L2.

It doesn't (yet) mean that you have identified L1 and L2, it means that the phasing is L1 first L2 second. It could be that you have L1 and L3 at this point. You will need to do a further test to confirm which you had (and another to confirm the correct overal sequence is safest, as someone may have given you two Phase120's and a Phase240 to play with, so be warry).

 

[topquark]

You'll see it has two LEDs on it marked R and L. R will be lit when you have L1 and L2 in the correct rotary sequence, with the lead on L1 and the meter on L2.

It doesn't (yet) mean that you have identified L1 and L2, it means that the phasing is L1 first L2 second. It could be that you have L1 and L3 at this point. You will need to do a further test to confirm which you had (and another to confirm the correct overal sequence is safest, as someone may have given you two Phase120's and a Phase240 to play with, so be warry).

If someone would care to check the manual, I have a feeling that it does if R is lit!

 

[wade88]

Nice one man, muchos gracias

 

[wade88]

I lost the manual for mine, but just checked the downlaodable one from fluke website and you are correct...so was I, in a back handed sort of way. makes a change.

 

[topquark]

Nice one man, muchos gracias

De nada, but check your manual for the details!

 

[wade88]

Manuals are for Germans...

 

[wade88]

One other quick thing quark, if i was to use a three pole breaker, the process would be no different would it, i would just be isolating all three circuits from one breaker instead of three seperate mcbs.

Im afraid i have to ask these no brainers to be sure to be sure...as some Irish fella said

 

[telectrix]

NO!.each single phase circuit must have it's own individual breaker, so as to be able to isolate 1 circuit without isolating all 3

 

[topquark]

NO!.each single phase circuit must have it's own individual breaker, so as to be able to isolate 1 circuit without isolating all 3

^^^^^ As telectrix has said each circuit must be isolatable individually. The only time you would use a 3 pole device is if it was feeding one (3 phase) circuit.

 

[wade88]

Its a good job i did ask then! Makes sense. Thanks again lads

 

[telectrix]

in a 3 phase board, you can replace a 3 phase breaker with 3 1 phase, no sweat.

 

[wade88]

So i can pop that 3 pole out and replace it with 3 single pole mcb's as we have gone through above and convert it to 3 seperate circuits etc. cool cool. Thanks lads