Neutral Conductor

Not a good idea, What will happen if only you neutral breaker trips?

My understanding is that you only need to be protecting the active conductors.Your neutral conductor is always going to be less amps in a multiphase circuit, zero if balanced perfectly.Even though you did not size your breaker to your neutral cable, it is still protected by the breaker that is protecting tha active cables.

if you want to protect the neutral by a breaker, it should be a 4 pole breaker, 3 hots and a neutral, factory built common trip, not field installed handle tie.

_{My understanding here is simply a detection device rated to that of the neutral conductor and is connected to the neutral conductor which will protect the cable from heating up/melting.}

Correct. In practice you need a current monitoring relay or similar (not a multi-pole or separate circuit breaker for the neutral obviously) to ensure the reduced neutral current level is not exceeded.

Depending on the length and cost of cable run weighed against the cost and complexity of the additional monitoring circuitry and whatever associated control gear is needed to drop out the associated phase circuits, reducing the neutral size may be a viable cost saving option. High current long copper cable runs are a good example.

The previous standard AS/NZS 3000:2000 only allowed the neutral to drop to between 1/2 to the full size of the largest phase conductor depending on the phase conductor current.

This is actually a quite simple circuit, but that doesn't mean it's cheap. A current sensing relay on the neutral can be set to trip a MCB feeding the line conductors via a shunt trip.

It begs the question, why would you need such protection? The only reason I can think of is high harmonics. With normal 3Ph+N distribution the neutral current should never get near the line current.

Can you give more details?

Is this a homework question?

Thanks for the input. Its not a homework its a real situation whereby parallel 185mm²Cu conductor is used for active 3 phases and single 185mm²Cu for neutral. Fiji follows the Australian electrical wiring rule. as/nzs 3000/2007. For clarification matter if we use 1 x 185mm²Cu as mentioned in my first submission rule 3.5.2 also allow for single conductor provided current monitoring device is fitted. My understanding at first hand is incorrect as there should be a complete isolation if the neutral current exceed the capacity of the neutral cable.

Thanks

If you need to break the neutral then the MCCB must be 3Ph+N, where the neutral is early make - late break.

Actually, the sentence does not say quite what you have assumed. Just breaking the neutral could actually kill someone for example.......not in a correctly installed system though, though breaking the neutral alone is to my mind "is not a correctly installed system".

But, if say the trip was a four way trip, removing all three phases at the same time as the neutral, if ANY one of them was in an over current situation, surely that would be OK?

As in most countries the neutral is earth bonded at some point, there is actually no need to open the neutral as long as the 3 phases are opened.

A seriously unbalanced loading could cause a higher than normal current in the neutral, which can easily be spotted when measuring a high voltage between the neutral and ground. I don't remember what the legal limits are, sadly.....someone else here will know.

Which is why I run separate neutrals to each single phase leg, not a single 5 wire cable ( 3 phases plus neutral and ground), but 3 x 3 way (1 phase, 1 neutral and 1 ground). Not that I need to do it often.....not for years....
5 wire cables are a mixed blessing, but loads should be carefully distributed and no leg should use the max power when the other legs are not being used....

I think you may find that this device is a earth leakage unit (EL Unit or RCD), where by any current higher than 30ma (long ago upgraded to 60ma in some countries), becomes unbalanced against the phases will cause a trip and three phases and neutral will be disconnected, (tripped) on the EL Unit. The phases still go through a CB after the EL unit, so short circuit protection is available as well as the unbalance from the phases opposed to neutral.

Google Circuit Breaker Industries or try Group Schneider for an EL Unit. I have not checked the AS/NZ standard.

http://www.cbi-electric.co.za/products_all_circuitbreakers.php

EARTH LEAKAGE PROTECTION

Advanced electronic Residual Current Devices (RCDs) for use in residential and industrial applications. These include RCD modules, which can be attached to Moulded Case Circuit Breakers.