calculating short circuit capacity for a domestic circuit

im trying to prove , k2S2 of the
live conductors are greater than the
value of let-through energy (I2t) for
the RCBO's im using on all circuits . now i only need to know the value between L-N because i know the rcd part of the rcbo will protect any earth fault, its just overcurrent i need to calculate, basically i need it up and runnind by wednesday and they have sent me type c rcbo's instead of type b's, and i want to see if they will do the job before i have to send them back and wait about a week for the type b's but i'm struggling with finding the calculations for this.all i need is to prove that the rcbo will trip within the specified amount of ms allowed by bs7671 i have the R1+R2 values, Ze , and PFC, and circuit lenghs and i know its just a matter of using these to calculate them but cant find any math on it ....PLEASE HELP.

Measure the fault current, square it and multiply it by the time in the graphs at the back of BS7671 for the given fault current. That will give you the I²t energy let through at that point in the installation.

can you give me the values you have I would like to go through the calc, for practice. Thanks

Martin

Spark123 said:

Measure the fault current, square it and multiply it by the time in the graphs at the back of BS7671 for the given fault current. That will give you the I²t energy let through at that point in the installation.

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Wouldn't it be nice if life were so simple .

That will work provided the adiabatic equation is valid for most of the duration of the fault. It does not work outside the limits of the equation - these being 0.1 second at the lower end and 5 seconds at the upper bound.

The largest errors will occur below the lower bound - that is for faults that cause disconnection in under 0.1 second (5 cycles of the supply). During this period the fault current can be asymmetric and this cannot be represented on a simple graph. It comes down to using figures obtained from actual test data because the wave shape is not a symmetrical sine-wave.

So it depends on your predicted disconnection times - if these are less than 0.1 second you need a different approach.

Take a look at the 2nd paragraph of [433.5.2].

However, help is at hand, first in the standard BS EN 61009 and also, more accurately, in manufacturer's data. This data is generally available on their web sites.

Typical data from the standard is expressed for a given rated short circuit capacity and is specific to the energy limiting class of the RCBO (class 1, 2 or 3).

Most will be class 3 and if we take a breaking capacity of 6kA the maximum let through energy must not exceed 35,000 A²s for a type B rated at 16A or less and 42,000 A²s for a similar type C.

Similar RCBOs rated > 16A <= 32A have values of 45,000 A²s for type B and 55,000 A²s for type C.

If these values are too high you would get much more accurate, and generally lower values, from the manufacturer.

laughingjester said:

i only need to know the value between L-N because i know the rcd part of the rcbo will protect any earth fault,

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The RCD will operate on an earth fault, but if this was a short between L and E, a large fault current will flow.
In most situations this will not be greater than the L-N fault current, so it usually won't matter, however this is not always the case.