Disconnection times

[Wilson12]

Hello I understand why you need to find out the pfc to ensure breaking capacity is adequate.
But I'm struggling to understand minimum fault current.
Tncs e.g 230/0.35 =657 amps
I think it's if the system has enough fault current to trip the breakers in time e.g a ring would be 3-5 times 32 amps but why do we need to know the external figures to work out this. Isn't that external to the property so has no influence on the protective device within the property.

 

[Darkwood]

You circuits in your house are part of a larger circuit going out to the DNO Sub-mains tranformer, the ZE determines the fault that will flow to operate your protective device, your protective device for your circuit is downstream of the supply Transformer in the same way a fuse in a plugtop is downstream of your fuseboard but all these short circuit protective measures rely on the external earthing system as well as the internal wiring to operate within a tabulated time.

 

[Wilson12]

for a tt system say the impedance is 20 ohms which is effecting the amount of current flow isnt that the to earth and not the external line conductors resistance.

 

[Darkwood]

When you are take a Zs or Ze it is your L conductor resistance and the Earth path together (like a R1 + R2 but includes the complete circuit to the substation or pole TX and done live) regardless of the system connected it will show the path resistance and sometime the current that will flow (meter dependent), in the instance of a TT you would have your current flow down the Line through the point of fault back down the earth wire, through the earth rod, through true earth to the transformer where it is center tapped to earth, because your using an earth rod then it all depends on the conductivity of the ground from the rod to the transformer which in your example would be 20ohms including line resistance.