MCCB

[frm2005]

25Ka at the intake 2ka is our calculated pfc at one of our bedsit consumer unit.

There's 4 blocks 

A,B,C,D

block C is where the intake is the substation is on site 8 meters away from our main LV panel situated in block C. This is where we'd see the highest fault current obviously.

block A is 100m away from our LV panel we are feeding Block A (which requires a 400amp supply) with 2x 240mm 4core cables wired in parallel. This is why there is such a drop in pfc, the apartment DB is about 30m away from block a's panel the bedsit dB is another 20m away from that. 

This is why we don't believe a  mccb rated at 25ka fault current is required that far upstream.

 

[Andy™]

dont really care about the PFC at the end of the circuit. you need to know the PFC at its worst, i.e the start o the circuit

 

[apprentice87]

At the end of the day, the client can specify anything he likes.. If he wants well expensive MCCB's what is the problem, they are paying, so why not just fit the things??

Surely if they are taking responsibility for the design, why should you care?? The fault current may well be 25kA at the intake position, so you will need either fuses or an MCCB there, but. you know the impedance of the cables, just calculate what it will be at the ends of the things, and then decide what breaking capacity you need for protection from then on..

john..

 

[Sidewinder]

I think that there is some confusion here.

The DNO fault level of 25kA is reasonable at the Tx, and the first downstream DB.

Therefore, this must be catered for at this DB.

Once you are downstream from this the cable impedances will attenuate the available fault current, also, there will be backup protection, hence the concept of allowing 6kA MCB's in a domestic CU, regardless of fault current as there is backup protection by the DNO fuse, which allows this with up to 16kA fault current at the origin.

Now the OP has 25kA at the origin, has put all the calcs into Hevacomp, and come up with the theoretical fault currents and the discrimination analysis from this.

This may, or may not work, it may or may not be right, I don't have the Hevacomp calcs.

The consultant is concerned about the high fault currents at certain points in the system, so I would be asking them what they have calculated these to be to see if they are actually greater than the breaking capacity of the devices, they may be, they may not be.

I would never design backup protection into a new design of private distribution these days.

i.e. I would never fit a device not capable of breaking the full prospective fault current at the point of installation without damage.

Now, remembering that the full fault current could be phase to earth of phase to neutral or phase to phase depending upon the circuit, this would have a bearing on the device selected.