Question regarding eddies currents 3 phase

[SparkBull]

I have to supply a 3 phase + neutral circuit but I have to run it through a contactor. The contactor is 3 phase no neutral and will be bushed onto the left side of the board with 2 bushes (one for feed, one for load).
The other side of the board is bushed onto some trunking where the load will go out of.

Am I right in thinking that I can just take the 3 phases no neutral in and out of the contactor trough the separate bushes, and take the neutral from the load straight into the DB busbar and not run it through the contactor? So the outgoing load with 3 phase & neutral all go through the same bush, but the cables in and out of the contactor that don’t require a neutral don’t need a neutral through the same hole?

Apologies if this isn’t explained well enough it’s hard to put into words without a drawing.

 

[pc1966]

What sort of load is it?

How much current?

If it is a well-balanced 3-phase load then you ought to have no neutral current, but if you have a mix of loads (so unbalanced) or a non-linear one that generated odd harmonics then the neutral current could be significant and so a potential issue for eddy currents.

I'm guessing it would be a non-compliance to be justified/documented as well.

 

[David Prosser]

Why can't you use a four pole contactor ?

 

[pc1966]

Or simply route the neutral through the contactor board?

 

[SparkBull]

It’s a 32a supply, there’s 3 of them through 3 separate contactors .

I could use a 4 pole contactor or route my neutral through it I’m just asking if it’s necessary?

From my understanding eddies occur when the lines and neutral go through different holes, so if there is no neutral required there would be no eddies?
If this isn’t the case can someone explain why 3phase supplies not requiring a neutral (certain motors) don’t create eddies currents?

 

[pc1966]

The eddied are created by current flowing in a loop around a magnetic core that is conductive (as most magnetic materials are) so acts like a "shorted turn" on a transformer.

If your bunch of conductors has all phase & neutral then there is no net current, hence no driving of the shorted-turn-transformer formed by going through a hole in a steel enclosure. Same ideas as an RCD, but there the current imbalance would trip the breaker instead of heating the enclosure.

In a balance 3-phase circuit like a delta connected motor, etc, that has no neutral there is perfect balance between the 3 phase supplies so again no net current when looked at as the bundle of phase wires.

But if you have a TP&N load you might have neutral currents from either an imbalance of the loads, or harmonics that don't sum to zero on the neutral, so if you separate the neutral from the phases you could see a net current flowing around them.

Simplest, safest, and cheapest would be to use your 3-pole contactor and just route the neutral through the same entry and exit holes as the bundle of phase wires use. That way you don't have to worry about the magnitude of neutral current.

Within reason, if very non-linear you might need to look as harmonic sum could exceed the phase current!

 

[David Prosser]

What is the purpose of the contactor ?

 

[westward10]

If it is just 32A I would have thought all lines would pass through a single suitably sized coupler. I personally don't think your set-up is an issue you see it all the time on multi-gang lighting grid switches with three phases present, you don't take all the neutrals through the switch array.

 

[SparkBull]

thanks that makes a lot more sense RE TP no neutral and net currents.

It’s a contactor for a 3P&N heater, it would be hardly any extra effort to route the neutral through the enclosure I was just wondering if (and why) it was needed as I couldn’t find anything on google regarding a circuit that had TP&N end of line but only needed TP at a certain point.

also would this mean I’d have to take my earth through the contactor aswell?
The contactor is already earthed through the control circuit singles

 

[pc1966]

thanks that makes a lot more sense RE TP no neutral and net currents.

It’s a contactor for a 3P&N heater, it would be hardly any extra effort to route the neutral through the enclosure I was just wondering if (and why) it was needed as I couldn’t find anything on google regarding a circuit that had TP&N end of line but only needed TP at a certain point.

also would this mean I’d have to take my earth through the contactor aswell?
The contactor is already earthed through the control circuit singles

I would expect a TN&N heater would be balanced, but it might have a heat setting that just switched 1..3 phases on as a simple means of control, so the neutral current could get as high as one phase worth.

Earth should not normally have any current on it so any (sane) route is fine as it won't contribute to eddy current heating. All that matters for that is bonding impedance is low enough and it can handle the PFC.

 

[Lucien Nunes]

As the CPC doesn't normally carry current, it doesn't need to go through the same bushes. Only in very high fault current scenarios do you need to consider the magnetic effect of CPC current.

you see it all the time on multi-gang lighting grid switches with three phases present, you don't take all the neutrals through the switch array.

But you do have the switched lines carrying away the same current that the permanent lines brought in, which have the same effect as the neutral in balancing the current and hence eliminating the MMF, provided they go through the same bushes. If the PL and SL of each circuit do not go through the same bush, then there is a current imbalance and hence a net MMF. Typically the currents are small enough not to create any noticeable effect.

Although there is a regulatory requirement for it and it is good practice, in reality there are seldom any noticeable effects over short lengths of cable / conduit e.g. one bush or gland plate, for cables carrying less than 100A or so. Higher unbalanced currents passing through extended runs of ferrous containment or armour are likely to cause problems.