Re: sizing load ,cable& breaker

Spir Georges GHALI

Dear Mr. Ally ;


Regarding the highlighted points, please find below the explanation :

– Ssc : 500 MVA : The IEC defined the maximum value of Short Circuit Power for each level, exp. for ” 11kV  &  20kV ” this value is till now ” 500MVA “, but if in your country the Ssc is smaller, you can either use this value or the defined value in your country. Some persons prefer do not calculate the RMV & ZMV, and they consider that the Ssc value is too big ( infinitif value )( making the calcultaion more easier ).


– RMV / ZMV : The value of this ratio have been also defined by IEC as the value of ” XMV ” is too bigger than ” RMV “, noting that in the past this ratio was ” 0.2 ” but as the MV networks is composed now of thousands of KM it's better to taken into consideration ” 0.1 “


– The value ” 1.05  & Cmax ( 1.05 ) : it's the voltage's flactuation of the medium voltage network.


– The value of ” X ” : as this value is depending on the Cable's Length, the IEC defined this value accordingly to the kind of cable and the method of installation, that means if the kind of cable is different than ” Touching – Flat ” the X will be also different. for exp. for Multi cores cable ” X = 0.08 mΩ ” wahtever the installation method.


– The value ” Cmin ” : as there's a Voltage Drop when a short circuit happen that will be in the most of cases around 5%, and we want to calculate the minimum value of the short circuit current, we should take it into consideration. For me, when I calculate the minimum short circuit current for the final circuits in the network I prefer to take  it into consideration even smaller than ” 0.95 ” ( 0.80-0.85 ).