4 Pole Circuit breakers for DG Incomers on LT Panels

hello !

same difficulties continues ..... with time !

1. 4 pole acb is used to isolate the neutral of power which is not connected to bus o/g.

2. If 3 pole acb is used, then neutral contactor or manual switch isolator for neuttral disconnection in both the incomers are necessary to perform the same function of neutral isolation.

3. If both the incomer's neutral are solidly connected to bus, without the above provision, then .....

unbalanced load currents shall be circulated in bothe supplies i.e. stand by dg alternator winding & supply transformer secondary winding. this may cause burning of winding, generally of the alternator, as normally, this is of smallaer capavcity.

Again, during the earth faults / short circuits, fault currents shall flow thru neutral of alternator, even though it is idle. causing damages to its winding.

Yes, as long as the load currents are balanced, and no short circuit faults, there will be no damages to the alternator windings. as circulating currents ( un wanted ) shall be zero.

Hello,

"1. 4 pole acb is used to isolate the neutral of power which is not connected to bus o/g."

Qu.1: Can you say what is meant by "bus o/g"?

"2. If 3 pole acb is used, then neutral contactor or manual switch isolator for neuttral disconnection in both the incomers are necessary to perform the same function of neutral isolation."

Qu.2: Can a 4-Pole Automatic Transfer switch(ATS) between Grid Transformer and DG be used to accomplish neutral isolation?

"3. If both the incomer's neutral are solidly connected to bus, without the above provision, then .....

unbalanced load currents shall be circulated in bothe supplies i.e. stand by dg alternator winding & supply transformer secondary winding. this may cause burning of winding, generally of the alternator, as normally, this is of smallaer capavcity."

Qu.3: I can see unbalance current flow in neutral of both DG and Grid Transformer, with Grid Transformer in service and DG in Standby. How can the windings of the DG be subjected to circulating current flow under these conditions if the three poles of a 3-pole ATS(assumed installed) are open while the Grid Transformer is in service?

"Again, during the earth faults / short circuits, fault currents shall flow thru neutral of alternator, even though it is idle. causing damages to its winding."

Qu.4: I agree with neutral current flow through the neutral bar of the Generator under ground fault conditions, but how can the DG phase windings be damaged by the ground fault current flow if ATS is open with DG in standy mode?

Qu.5: Is the use of 4-pole main ACB's an NEC Code requirement for 3-phase, 4-wire Distribution panels supplied by Grid and DG via 3-pole ATS? For example: 120/208 or 277/480V 3-phase, 4-wire distribution panels.

use neutral isolating contactors with appropriate controls to ensure correct switching sequence of poles and neutral.

It means, If both the sources are running in parrelle or in isolation, but without neutral Isolation, only in that can unbalance neutral circulating current will flow from Neutral of the suce (DG / Trafo).

But, If I am using DG as only stand by functiion means it will start only when there is no Power from Trafo. I can run it. For it I must ensure that only 1 DG has to be on the system.

Am I Correct?

But Stil I have queries,

How unbalnce current circulates between both the nuetral?

Normally, we go for 4 Pole ABC in case of DG as source. But if we are using 2 nos of Transformer as 2 different suorce, we are using 3 Pole ACBs in general practice. Why 4 Pole ACBs not being used for 2 Trafo Systems?

Dear Ajay Patel ,

Let me make your question a bit simple.. and answer too..

1) for LT panels: If you have 2 transformers and 2 incomers with 1 tie breaker(buscoupler), then 4 pole ACB is not required in the incomer provided:

a) tie breaker is also 3 pole

b) the neutrals of the 2 transformer are not solidly connected.

If tie breaker is 4 pole and u intend to have a common neutral bus throughout the length of the panel, then the incomers should also be 4 pole.

4 pole breaker has the advantage of total isolation. that means.. suppose the neutral of transformer is not earthed properly and suppose if maintenance of breaker or panel needs to be carried out, then , if the maintainence personal comes in contact with neutral bus, then he may get a shock depending on the unbalance neutral voltage. hence it is always advisable to have 4 pole breakers specially if they are connected to transformers directly.

some manufacturers use 3 pole bcos, its is cheaper than 4 pole and to get the order, technicality is compromised. 3 pole can be used in down stream panels or directly to the load.

2) Now coming to the generator part : if there more than 1 DG need to be synchronized, then it is advisable to consider 3 pole breaker with switched neutral contactor or motorised MCCB because if neutrals of both DG's are grounded, the circulating current will flow to the neutral and create unbalance.

3) suppose in a LT panel, only 1 transformer and 1 dg set incomers with tie breaker is suggested, then it is advisable to consider incomers with 4 pole with one 4 pole buscoupler. 3 pole with switched neutral is not required for isolated DG set having AMF logic since the other end is connected to infinite grid.

for further information, please type DG neutral earthing on this forum and try to get information from forum experts.

i hope i am not wrong in the above conclusion and i request our forum experts to provide some more info and correct me if have made any mistake in the above info.

Thanks nd regards

SIR,

I am having no tie breaker (buscoupler) in the panel .

and 1 incomer (transformer), other DG incomer . in this case please explain me why we need 4 pole ACB.