CB ratings: 2500 A, 1500 MVA, 33 kV

The ratings of a CB follow from its breaking MVA and rated voltage.

Definitions and formulas used

• Rated symmetrical breaking current I_b = rated breaking MVA / (√3 × rated line kV), rms.
• Rated making current = 1.8 × √2 × I_b = 2.55 × I_b (peak), since the first major peak of an asymmetrical current can be 1.8 times the peak of the symmetrical current.
• Short time rating = the current the CB can carry while closed for the stated time without damage; it equals I_b for the given duration.

Given

2500 A, 1500 MVA, 33 kV, 3 s, 3-phase oil CB.

(i) Rated symmetrical breaking current

I_b = MVA / (√3 × kV)
    = 1500 / (√3 × 33)
    = 26.24 kA (rms)

(ii) Rated making current

I_m = 2.55 × I_b
    = 2.55 × 26.24
    = 66.92 kA (peak)

(iii) Short time rating

= 26.24 kA (rms) for 3 seconds

(iv) Rated service voltage

= 33 kV (rms, line to line)

Answer: (i) 26.24 kA rms; (ii) 66.92 kA peak; (iii) 26.24 kA for 3 s; (iv) 33 kV rms (line). Rated normal current is 2500 A.