CB terms: making, breaking, operating duty

These terms define the ratings of a circuit breaker as per IEC 62271-100 / IS 13118.

i) Making capacity

The peak value of current that the CB can make (close on to) at rated voltage when closing on a short circuit, including the DC offset of the first cycle. The contacts must withstand the large electromagnetic force at this first peak.

Making current = 1.8 × √2 × I_b = 2.55 × I_b (peak)

ii) Breaking capacity

The largest current the CB can break at rated voltage under specified conditions. Expressed as the rms symmetrical breaking current I_b (kA) or as MVA:

Breaking capacity (MVA) = √3 × V(kV) × I_b(kA)

Asymmetrical breaking current includes the DC component at contact separation.

iii) Operating duty

The sequence of operations the CB must perform at rated breaking current, with given time intervals. Standard duties:

• O – t – CO – t′ – CO, e.g. O – 3 min – CO – 3 min – CO (non-auto-reclose duty)
• O – 0.3 s – CO – 3 min – CO (rapid auto-reclosing duty)

Here O = open, CO = close followed immediately by open, t = dead time. The CB must keep its rated breaking capacity throughout this sequence.

iv) Rated current

The rms value of normal current the CB can carry continuously at rated frequency without the temperature of any part exceeding its permitted limit. Standard values: 400, 630, 800, 1250, 1600, 2000, 2500, 3150 A, etc.

Example: a 33 kV CB rated 1250 A, 25 kA, duty O–0.3 s–CO–3 min–CO has breaking capacity √3 × 33 × 25 = 1429 MVA and making capacity 2.55 × 25 = 63.75 kA peak.