Breaking Capacity (KA)

You should know the three phase symmetrical short circuit current at the location of the MCCB. Then only you can decide the breaking capacity of the MCCB. The Breaking Capacity of the MCCB should be more than or equal to the three phase symmetrical short circuit current at the point of installation of the MCCB. It is not load related.

Breaking capacity is the maximum current that the MCCB can break without damage. So, obviously MCCB with 25kA Breaking Capacity is far stronger than that with 10kA Braking Capacity.

To simplify, you cannot determine the necessary breaking capacity of a CB based on the load, it must be based on what is available on the line side.

You can get an outside estimate of the required symmetrical short circuit capacity by dividing the kva of the source transformer by it's percent impedance. As an example if the source transformer is 1000kva, 3phase at 5% impedance, expect a maximum symmetrical of 20000kva. Convert that to amperes. 36 KA is the more prudent choice.

While a higher kA MCCB would be safer, it is most likely to be unnecessary. Even if the welder is dead-short-circuited at its terminals, the impedance of the cables from the upstream breaker would limit the Short Circuit current available at the welder. Moreover, if you use the same capacity MCCB as the upstream one, there may be a loss of discrimination, resulting in nuisance blackouts. You may be ok even with a 10kA MCCB, if the cables are long enough to limit the SC.

Here is a thumb rule I follow; If this welding set is using any power electronics then 10KA; only magnetics set 25KA.

Be careful, this rule of thumb doesn't make sense. Do not use it!

This a safety issue, you cannot just throw any comment like this.

Others have already correctly said that the breaking capacity needed is evaluated by the source's impedance and the feeder cable's impedance as long as it is permanent.

Please refer to a competent engineer or master electrician who knows your local electrical code.

I fully agree with 'marcot'. Thumb rules are only for answering in interviews. They MUST not form the basis for equipment sizing in actual site conditions. You have to do a perfect calculation duly taking into account all the limiting impedances.

Breaking/Interrupting/withstanding capacity is basically the highest (short circuit)current that can pass without damaging the respective device. Since the heat effect is proportional with the current squared and the time until the upstream breaker is able to completely interrupt the circuit, the highest possible short circuit current is at the incoming/service point. This current is based on the available short circuit power of the grid. Therefore an available short circuit has to be correlated with the voltage (i.e. 65 kA at 480V but 15 kA at 2300 V)

Further down you go, the impedance of the cables will add to the total impedance of the short circuited circuit and lower the possible value of the current (at the same voltage)

On the secondary side of a transformer the possible short circuit that may appear is limited by the impedance of the transformer(as shown in a previous post). But be careful that the (even limited) secondary possible short circuit may be higher that the available short circuit current at the primary side.

Under normal circumstances further down you go on the distribution system, the lower the required IC rating (mainly because of the impedance of the feeding cables).

There is no disadvantage in selecting a higher IC rated equipment (breaker, fuse, etc) except that higher rated pieces are bulkier and definitely more expensive.

A 40A, 10 KA rating of a breaker indicates that that particular breaker can withstand a short circuit of 10 kA (kilo-amperes) while a 40A, 65kA breaker can withstand 65 kA.

The IC rating is usually given considering the voltage range that breaker is designed to work.

The 40A value is to be selected based on the load to be protected.

Calculating short circuit kVA is not that easy as everyone think. I am still struggling to calculate for some installations where Local electricity boards substation is too far from the installation. In your case, you can calculate the short circuit current may occur at your breaker by checking the %impedance of welding machine. %impedance of welding machine might have written on the name plate.

Please don't follow any thumb rules for calculation of short circuit current as some one mentioned above. Short circuit current can damage you installation by installing an under-rated breaker.

You Utility should be able to tell you the Short circuit level on the High side of the stepdown transformer. You can than used the impedance of the Transformer to determine the fault level. If you are close to a major substation chances are your fault level are high and gven a choice between 10kA and 25kA select 25 kA. A higher rating will not hurt. Be carefull that the fault level is not more than 25 ka.

Regcan80,

Bussman manufacturing has on their web site a couple good papers that include the explanations and worked examples for the short circuit calculations you need. They are based on some assumptions that will keep the results on the high side of the range of possible real answers. I recommend their materials to you as a beginning resource.

--JMM