400v 3 phase questions

[Floody]

Hi guys,
got a few questions about 400v 3 phase installations.
I'm currently working as a mate with a firm who have had a contract re-wiring Boots stores. The installation is 400v 3 phase. I don't think I've done anything on 400v volts before, can anyone give me a quick rundown of how it's different and if the circuits are different etc??
Cheers

 

[D Skelton]

In all honesty mate there's very little difference. The circuits you will be working on will most likely all be 230V single phase with the exception of lifts, escalators or HVAC. The only difference will be at the board in this instance where the busbar will be split across all three phases. So MCB1 is on L1, MCB2 is on L2, MCB3 is on L3, MCB4 is back to L1... And so on. There is a common neutral bar and a common CPC bar and all circuits are run in brown and blue as usual.

With regards to any 400V Circuit that requires three phases, the main difference will be that the overcurrent protective device will be a three pole one and of course the line to line voltage is 400V. Remember the line to N or line to CPC voltage is still 230V. Also, any three phase circuit will be identified using three colours, Brown, Black and Grey for L1, L2 and L3 respectively. If a neutral is required for a three phase circuit then this is obviously still blue.

 

[Floody]

Cheers Skelton. As I won't be touching the board I ca honestly say I need not worry about it then. Just pull everything in as normal and let the sparks do their thing at their end

Thanks mate

 

[SirKit Breaker]

If you are going to get involved in testing, it isn't much more involved either.

Test as you would do for single phase, just repeat the testing on each phase.

Take the higher of the Ze readings.

Take the higher of the PSSC/PEFC readings and double it, dont try and take this test between phases, it isn't clever, especially if your tester wont do it, it will all go bang, and if you are lucky, you will be left with bits of tester in your face, if you aren't so lucky, you maybe (not) staring at the inside lid of a wooden box.

R1+R2 as per Ze, do all 3 and record the highest.

Phase rotation, plug the tester or the phase rotation meter in and see if it is correct.

Job done.

Cheers...............Howard

 

[mr7526]

In a poly-phase system, the voltages are 360/n degrees out of phase with one another, where n is the number of phases. There is no theoretical limit to the number of phases you can have in a system - in fact some aeroplanes have 5-phase systems. However, three-phase has been settled on, and is much more convenient for two main reasons:

1) It is the most economical transmission system
2) Reversal of three-phase motors only requires swapping two phases, whereas with higher phase numbers, reversal is much harder.

As can be seen, the voltages are thus 360/3=120 degrees out of phase with each other.

In a balanced system, the currents in each phase balance out the currents in the other two. This means that there is no neutral current, and thus there is no need for a neutral conductor. The practical upshot of this is that you can transmit three times the amount of power as a single phase system does with only one extra conductor. This, as you can imagine, is a great cost saving when designing transmission and distribution systems.

I imagine that you will be predominantly dealing with unbalanced three-phase systems. These DO have a neutral current because there is a different current going down each phase and some (but not all) of the current will return along the neutral wire. Note that this occurs when the phases are together (ie, on the incoming side of the three-phase distribution board). This is something for the distribution company to worry about.

You may be wondering where the 400 volts comes from, and how you get 230v out? Basically, if you draw the phasor diagram, as below:



and measure the distance between the ends of any two phases (red-blue, blue-yellow, yellow-red), you will get 400v. This is the line voltage. There is still 230v between the end of each phase and neutral, this is the phase voltage. Note that the diagram uses the old wiring colours.

HTH,

Matt