CB Making Capacity Rating

Making current is an asymmetric one with a peak which can be as high as 220% of the steady state RMS current that the CB may be called upon to break. It may happen that the CB is unable to close and latch onto such a current. Some modern compact CBs, which cannot make and latch onto such high currents, have a "Making-current-release" which delatches the mechanism above the CB's max making capacity.

Ok, thank you. I'm getting the drift. Would appreciate if you may refer readings on how short-circuit generates this electro-dynamic forces, does this have to do with the contact voltage. What indeed would the contact voltage be if there exist a fault?

Thanks.

If you study the paper ECT162, you may quickly lose interest that it is all about busbars, and not about the contacts of the CB. Please look this image near the end of the paper...it gives the calculation of the 'crowd effect repulsion' that happens between butt contacts. (Curiously, the Ragnar Holm formula is missing in the English version, it is there in the French version)

You made me chuckle with your comment about the screwdriver. While I was an apprentice we had just installed a new panel, the bars had been split in to sections during testing. Testing finished the fishplate joints were replaced, the foreman decided (wrongly) to energise without a final test. As I was the newbie I was given the honour to close the ACB. So I charged the breaker and pressed the close P/B, the room lit up! While we were in recovery mode one of the sparks walked in and came out with the classic question, "anyone seen my 3/8^th spanner!" "No but we know where it ******* was!" He beat a hasty retreat.

That vindicates my stand! Thanks, Tony!

The making current of the circuit breaker is always more than the breaking current. To understand why making current capacity of the breaker should be more than its breaking current capacity, let's discuss how fault current behaves,what is the characteristics of the fault current.
When the fault occur in the electrical network ,the AC and DC current is present. The DC current magnitude is about 60 to 80 % of the AC current.

The peak value of the current during sub-transient period is
= 1.414 x I x 1.8
= 2.55 x I

Read More:https://sdvelectrical.blogspot.com/2019/06/why-making-current-of-circuit-breaker-is-more-than-breaking-current.html

The calculation is not as simple as you have projected.

It depends upon the system X/R ratio too.

Thanks for the responses.

My confusion is that, we have breaking capacities since the current would want to persist in the form of arc, but why does the same current "resist being made". When contact is open, no current exist, current will only flow once contact is made, once it is made and a fault still exist, it now needs to be broken (interrupting capacity).

Hope you may enlighten.

That a breaker can "close onto a fault of its maximum breaking capacity and break it" is fundamental - because the age-old inclination when a trip has occurred is to re-close the breaker, to see if it was a transient fault e.g. lightning strike.

Closing capacity is easy if it is a resistive load, but for example, a motor load may routinely take many times running current each time it is started, so a closing rating good for thousands of closures becomes necessary.

Making capacity of a circuit breaker is its capacity to close the circuit breaker under fault condition. This is expressed in terms of peak value (first cycle peak) and comprises of both AC and DC component.

i could not resist amplifying this aspect of making capacity, especially important for ACBs which need to have a 1 second short-circuit withstand capacity Icw (for time-based discrimination). The big, robust (and expensive) old ACBs could make, latch and trip right up to their maximum SC capacities with no problem. As technology improved, many things changed. Short circuit levels went up. Space becoming more expensive, compact devices became essential. New materials (plastics and metals) made this compactness possible. Microprocessor-based relays made the ACBs much more intelligent.

The compactness came with a compromise. The ACBs could withstand high SC currents for 1 sec due to some very intelligent force-compensating contact designs. But the ACBs could not close and latch onto the same high SC current !!! The closing energy input to the system was carefully designed for low bounce, long life etc, so it is usually inadequate to latch the ACB. So, if the ACB closes onto a high fault(higher than its max making capacity), the contacts will keep getting repelled and try to reclose, resulting in possible destruction of the ACB. Such ACBs are fitted with Making-current-releases, so that the mechanism is delatched as soon as the contacts touch at such high fault levels.

Here is an extract from Schneider ACB catalogue. Note that the "Make-and-latch" value of the SC current is lower than the 1sec Withsstand value. (Meaning that the ACB, if already closed, can withstand the SC, but cannot close onto that SC)

There are cases where a breaker is combined with fuses, such that the breaker can make/carry extreme fault currents - but not break them, that task is done by the fuse. The two being co-ordinated, so that the CB safely clears faults for which the fuse does not blow.

Making current capacity is about 2.55 times of the breaking current capacity. When fault occurs the current waveform is not symmetrical about its axis and during sub transient period the current has AC and DC components. If the circuit breaker is closed at fault the breaker must be capable to clear the fault.b The link may be useful for better understanding about making and breaking capacity of the circuit breaker.

Making and Breaking Capacity of Circuit Breaker

A key element, mentioned in post #2 by SHAMITHGAMBHIR1, is that the contacts will attempt to repel one another under high fault conditions. This phenomenon is actually useful in opening a breaker during a fault because it speeds up the process of getting to where the arc is quenched. But if closing into a fault, that can cause the contacts to bounce and make matters worse. So the closing capacity has a lot to do with the ability of the breaker to withstand those physical forces on the contacts themselves

Breaking capacity has has more to do with opening time and arc quenching capability while holding together under the physical strain of the magnetic forces taking place between poles.

The circuit breaker breaking capacity is expressed in RMS value. The circuit breaker of 21 kA (rms) breaking capacity can interrupt that much fault current safely. However, if the breaker is again switched in fault condition. The Peak current will be very high and it will be about 2.55 times of the breaking current capacity. The breaker should be capable to interrupt the fault current if it is switched on when the fault exist downstream of the breaker.

Making current capacity of the breaker(kA-Peak) = 2.55 x Symmetrical breaking current capacity

Read More:Making and Breaking Capacity of Circuit Breaker