3 Pole vs 4 Pole Circuit Breaker Usage 2

[romanskat]

Hi Mates,

kindly illustrate the difference between 3 pole circuit breakers with 4 pole.

one quite significant difference i aware is on the price between 3pole vs 4 pole based on same rating.Because 3pole cheaper, so we always facing recommendation to use 3 pole rather than 4 pole.

Therefore need your help to understand more on the difference in application between 3 pole with 4 pole.

thanks.

 

[jghrist]

The usual system in the US is TN-C-S. The utility service has a combined neutral/ground conductor that is grounded at the utility transformer secondary and at the service disconnect. From the service, there is a protective ground (equipment ground in NEC terms) run along with the phase conductors. If there is phase-to-ground connected load, there is a separate neutral (grounded conductor in NEC terms) that is not grounded except at the service disconnect.

If there is another ground source, such as a delta-wye transformer or a generator, it is a "separately derived source" and it's neutral is grounded at the source. NEC prohibits grounding the neutral at more than one point in each separately derived system. If you have two separately derives sources connected to a transfer switch, then you need a four-pole transfer switch to switch the neutral so that it is only grounded at one point in each separately derived system. If there are breakers connecting two separately derived system, then they need to be four-pole breakers for the same reason.

This doesn't seem too different from what the common European practice is, based on references in previous posts.

You can have a generator connected to a three-pole transfer switch, but the neutral cannot be grounded both at the generator and at the service disconnect. The problem with this is that if the neutral is grounded only at the service and the system is running on the generator, ground fault current would have to flow through the protective ground to the service and then flow through the neutral to the generator source. This increases the impedance and can increase fault clearing times.

 

[HCBFlash]

And so in an uncharacteristically "catty" discussion we unequivocally determine that there certainly are applications where using a 4-pole breaker to disconnect what the US "NEC" officially calls "the grounded conductor" is appropriate.

Where is the installation to be? What with pond-sides and all, that may well be the dominant factor. Right guys?! A 4-pole breaker may be used in other instances where 4 phase conductors are to be disconnected simultaneously.

Thanks to some of our regular contributors for continuing to share some of their expertise and insights (and links!!!) with us. (bunkers notwithstanding)


I know I've learned a bit from this thread!

 

[slavag]

TN-C-S and TN-S is not so big difference.
More or les both of systems have same advantages and disadvantages.
We use both of system, but deciding is case by case.
Regards.
Slava