Busbar KA Rating

You have to analyze the circuit. Suppose 3 Generators are connected to the common bus through 3 Individual AcBs and one or two outgoing ACB. Assume Faults at different locations and visualize the flow of current. case1: Fault occurs in any one of the incomer between DG and Main bus bar. Flow of current from Bus bar 15+ 15 = 30kA and flow of current from generator to fault 15 KA. Case 2 : fault in the middle section of Busbar: Flow of fault current will be 30 kA and 15 kA. case 3 : Fault occurs at the extreme end of the Busbar. Total fault current will be 45 kA . But this will be partly 15 kA, partly 30 kA and partly 45 kA. Best solution would be to strengthen the bus bars supporting system to with stand fault current of 45 KA. This is not very difficult. Supposing Busbar supports are given say 1000 mm interval, introduce additional supports such that support distance is only 500 mm. Any doubt contact.

Friend,

Don't forget to consider the contributions from the connected load. Transformers and large motors will supply fault energy backwards into their incoming lines, to add to the total fault current(s) at the point of a fault. Your question is not trivial.

--JMM

Thanks to all,

one of the question is to know if the upgradation of KA rating is needed for adding additional power generation if found KA rating of switch board is small.

OR my question is to know : not to change the KA rating but allow interlock of new gen with existing gen ACB to have break after make of new incoming DG. it means momentarily new DG and existing DG will be in parallel and as soon new DG comes on load existing DG will reduce and go off the switch board (not to allow long parallel running). I have seen some installation in this manner, if this is allowed - I want to know how long we can allow parallel running before we remove additional power generation connecting the switch board.

thanks and best regards

K.Parimalam

Hello pari,

If I read it right, the bus-bar should be able to withstand the highest fault kA. As you are aware the fault kA is usually rated along with time 1 sec or 3 sec. In your case, the momentary paralleing of 3 DGs can happen. The worst case fault will be a bolted fault at the busbars at this instance of all 3 Dgs on bus, though for a fraction of a second. Such an fault occurrence is very very remote and I will take a considered decision not to increase the fault kA rating of the busbars anticpating such an event. To my rememberance no indian code / rule prohibits any such lower fault kA rated during momentary paralleing. If I were at your place, i will let the system to continue as it is .

Ramesh.A

I am not in agreement with Mr Ramesh of not increasing the fault level of the board.

In my opinion cost of strengthening the bus bars to higher fault level will not be much, comparing to the disaster which will take place one day or other.

Dears,

I agree with Mr.Ramvinod's counter to my reply. But my doubt it is not just about increasing the fault kA rating of the BB by way of increasing the X sec area, it calls for providing BB support insulators of adequate size to handle the mechanical and electro-dynamic stresses to happen at higher fault current withstand situations. I am also not sure of the Arc flash test rating of the panel. This sort of fiddling i.e,just increasimg the size of BB alone in a fully type tested panel is not allowed as it defeat the integerity of panel design. Hence, the option is to either go in for a complete new panel with additional KA rating or to leave the panel as such.