MCCB application in TNS System

I think you are referring to IEC 60364 (312.2.1.1) for the TNS system.

In the TNS system, there are 3 line conductors, 1 neutral conductor (N) and 1 protective conductor (PE). In the electrical system, only the current carrying conductors are required to connect to the breakers for protection. So, the PE is not connected to the MCCB; however the neutral (N) conductor may or may not be connected to the MCCB. If your system has many non-linear loads and/or the loads are merely unbalanced, there is possibility of having 3^rd harmonics current, as well as the unbalance current flows through the neutral. It may happen that the neutral can carry even more current than the line conductors in this case and it is good idea to connect the neutral to the MCCB so that the neutral circuit is also protected. Use 4P MCCB in this case. Otherwise, you don't need to connect the neutral to the MCCB and 3P MCCB is sufficient without the neutral protection.

Also see these threads:

http://cr4.globalspec.com/thread/53968

http://cr4.globalspec.com/thread/43708

Other useful link: http://www.schneider-electric.com/documents/technical-publications/en/shared/electrical-engineering/protection-devices-monitoring/low-voltage-minus-1kv/ect114.pdf (page p.9 for TNS configuration)

By the by, is this also your post?

- MS

The rating of MCCB to be selected based on neutral current (i.e)generally 2 times load phase current.Then protection of the circuit will not be in order.It is therefore better to use 3 pole MCCBs for such circuits and provide neutral busbar connectors ,cables for neutral rated 2 times load current.

Thanks MS,

Considering that the connected loads in the TNS system are of combination of non-linear load( inductive loads, motors, Air conditioning unit, chillers, AHUS, lighting), there is possibility that the load currents in the system will be unbalanced. Please clarify then 4P MCCB main breaker is the best suited. How about installation of load break switch 4P in the TNS system is the load is unbalanced?

Richwin