Bus bar short circuit calculation

http://cr4.globalspec.com/thread/25959/doubt

On what basis is the contactor stating that the sizing is correct?

Apart from cable sizing, this question is probably the most frequently-asked one in the CR4 electrical section.

Surely there must be standards for this sort of thing?

Dear sir,

As per standard, the rating of bus bar 25 KA is correct. But i want to know how to derive the value 25 KA. Could you please send me any model calculation regarding bus bar short circuit calculation.

Kailash (INDIA)

Please note that calculation of fault level withstand capacities of the distribution boards is dependent on the system to which they are being connected. It involves calculating the fault levels the entire power distribution system is likely to experience in case of electrical fault.

A book by the Title "Power System Analysis" by WD Stevenson is the one you should lay your hands on and figure out the complexities involved. In India a book by Sunil S Rao will also be readily available.

Remember the related subject is a field in itself in electrical engineering.

>NEW QUESTION<

Dear Sir,

how to find the % impedance of a motor, for short circuit current calculation? And, if several numbers of different capacity motors are connected in a panel, then how to find the short circuit current of the motors. Actually motors are the source of short circuit current. Here several different capacity motors are connected in a panel. whether all motors are considered as short circuit current source or higher capacity motor is consider as short circuit current source. And without knowing % impedance value how to find the short circuit current? Whether With these short circuit current can only find the SC rating of the bus bar of panel or any other method is available to find the SC current rating of the panel. Kindly clarify and give more details.

Thank you.

Regards,

Kailash

Hi,

The Transformer which is feeding the Panel is source of Short Circuit Current. To be in safer side the fault level is calculated by following formulae

Fault MVA = Base MVA / per unit impedance

Base MVA = Rating of Transformer feeding the Panel

per unit impedance = Impdance of the transformer in % / 100

Note : All the imedance downline are ignored thereby adding to more safety.

Example :

Suppose the Transformer rating is 33/0.415 kV, 2000 kVA with Z = 7%

Fault MVA = 2/0.07 = 28.57 MVA

Fault Current = 28.57 / (1.732*0.415) = 40 kA (approx)

<...As per standard...>

Which standard? State it! <sigh>

Excuse me sir,

The contractor also dont know the standard. From his experience only he stated. Could you please say the standard or your suggestions.

<...says as per standard...> and <...dont know the standard...> don't really fit together, do they?

Is this any way to do business?

There are multiple different factors affecting busbar sizing.

You really should try to search the web first and see how short circuit current is calculated. Try this link for example:

http://www.bussmann.com/library/docs/Edp-1.pdf

Thank you sir,

It is very useful. And i have one bus bar design data. in this bus bar arrangement, they are considering 7 factors and the transformer short circuit current is 35 KA. In the first one,

1) Main bus bar : W'100*T'10*2(Cu)

Assumed sym Rms short circuit Lk = 31,500A

Asym Rms short circuit = 78,750 A.

I couldnt understand, how they consider the sym value as 31,500 and asy value as 78750. Because as per calculation short circuit current is 35 KA.

In the second one, For finding Electromotive force for bus bar, They take short circuit current as 78,750 Asym peak

In the third one, For finding mechanical strength (Insulator pending load), The derived value is 477.75kg. But the value from insulator test report is 1182 Kg. Two times higher than the derived value. Is it correct.

I request you to kindly clarify this. Also Could you please send your e-mail id for further discussion.

Regards,

Kailash (India)

Hello,

Busbar short circuit calculation can be done according to IEC 60865.

Here 31500 A rms is assumed value. From that peak value can be derived. Pe value depends on cos Phi. Here PF considered is 0.2. For this PF the rms to peak ratio is 2.2 and hence 35000x2.2 ~ 77000.

Insulator strength should be more then the force exerted due to short circuit current. Otherwise insulator will break under electromagnetic force during short circuit. Hence the insulator strength is higher then the derived value.

Regards.

Ahmed Syed

WOW! very nice collection usefull stuff. GA

Dear sir

My name is deepak....i am working at muscat....i have some doubt..please clarify following doubts... i cannot ask to anybody ...please help me.

1) how to calculate the busbar short circuit calculation...for examble...we have 16OOA as a incomer and customer requirements 50ka/1 sec short circuit current.

now i have to calculate the busbar size. what is the formula i have to use for short circuit....how i have to use...please explain .

thanks & regards

deepak.

Please prvide the incoming supply details, the transformer details or any utility service details. Without incoming supply details you will not be able to find out the rated full load current as well as shortcircuit current.

Pls ; If you got reply for your question forward to me

i would like to ask the same