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# short-circuit capacities for MCB and RCCB

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#### isurunalaka

##### Member level 5
hi, MCB and RCCB have different short-circuit capacity values. when we install a RCCB and MCB in a panel board, how to coordinate RCCB and MCB with different short-circuit capacity values?
can we install 6kA MCB with a 10kA RCCB without having bad protection for RCCB and MCB?

what are the preferred short-circuit capacity value for domestic/industrial MCB and RCCB?

The short circuit current is determined by the impedance of your local mains input (distance from transformer, gauge of wire etc). The again within your premises, the impedance of the internal wiring to the protection device. So a factory might have a 400A incomer with 25 kA but with a 6 A 2.5 kA breaker in the porters office. In the UK 4 kA is good enough for most domestic situations.

isurunalaka

### isurunalaka

Points: 2
The short circuit current is determined by the impedance of your local mains input (distance from transformer, gauge of wire etc). The again within your premises, the impedance of the internal wiring to the protection device. So a factory might have a 400A incomer with 25 kA but with a 6 A 2.5 kA breaker in the porters office. In the UK 4 kA is good enough for most domestic situations.

is there any standard for domestic and industrial applications?
how to coordinate with RCCB and MCB kA ratings?

I don't actually understand what you mean with "coordination" of short circuit capacity in this case.

If a short circuit occurs, the MCB will trip anyway. So you can probably accept a lower short circuit capacity of the RCCB, at least it's not primarly required for circuit safety. In case of a massive short, the contacts of a RCCB with insufficient capacity can weld, but the MCB would still disconnect the short.

Preferably both devices will be rated for the expectable short circuit current.

You rate the short circuit capacity of a trip based on the PSC at that point in the circuit considering all the upstream impedances and the effect of things like large induction motors and PFC banks (both of which can INCREASE the PSC value for devices electrically close to them).

There is no coordination as such between the devices in this matter, it is strictly about the impedance at the location where the device is installed.

Coordination applies to sensitivity and response time (sometimes I^2T as well) and is used to ensure that only the trip just upstream of the fault opens, at least most of the time, it is hard to get it perfect in a real installation.

Some of the big stuff gives two short circuit capacities, one for a level at which you can just reclose and carry on, and one for a single shot event where the damage is contained by the breakers case, but the breaker is ruined.

A UK note: In a domestic install we have a rather weird getout clause that allows a smaller then expected short circuit rating on the assumption that a fault is unlikely within the first few meters of downstream cable, this allows the use of a 6KA trip in a consumer unit.....

Regards, Dan.

Noted. Can we use 6kA RCCB together with a 10kA MCB?

Obviously, provided you make sure the PSC is lower then the appropriate limit at the point where you install the devices, so either less then 6KA, or less then 10KA at the MCB AND less then 6KA at the RCCB (Which practically means sufficient cable between the two to raise Zs sufficiently).

Now the exact details do tend to vary depending on your local electrical regs, but I cannot see an issue in the UK.

Regards, Dan.

Obviously, provided you make sure the PSC is lower then the appropriate limit at the point where you install the devices, so either less then 6KA, or less then 10KA at the MCB AND less then 6KA at the RCCB (Which practically means sufficient cable between the two to raise Zs sufficiently).

Now the exact details do tend to vary depending on your local electrical regs, but I cannot see an issue in the UK.

Regards, Dan.

i mean 10kA MCB needs a 10kA RCCB or a 6kA RCCB?

10kA MCB needs a 10kA RCCB or a 6kA RCCB?
The MCB doesn't "need" a specific RCCB rating, the grid conditions do.

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