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C.B Ratings

C.B Ratings

C.B Ratings

(OP)
Dear Friends

My question is as follows:

In medium voltage switchgears , why are designers rating their service breakers very much higer than maximum rated current required for full load.
If they intend to supply a 1 MVA load at 11KV they might use 630A breackers or higher ?
Actually we do'nt see less than 400A breackers.

Regards

am1960

RE: C.B Ratings

Physical size is dictated by mechanical strengths required for withstanding forces during a fault, insulation, and clearances required by the high voltage application. Therefore they inherently are capable of handling certain amount of current. So they are rated accordingly.  Modern technology now affords 1200A as the base ampacity for 15 kV class breakers. You select the CTs and protective devices according to your needs.
 
Economics of manufacturing and demand plays a role too, while deciding typical standard ratings. Designers only select from what is available.

Rafiq Bulsara
http://www.srengineersct.com

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