Discrimination

No
If in the below graph the B and C curves did not intersect at any point then full discrimination would be achieved, as the B and C graphs only separate over a short section there is a very limited discrimination.

That was my thought as well, would i be right in saying if it was a TT earthing system with 1 sec disconnection time it would be ok? (this is on a TN-S on the site)

Glad I didnt have to plot that one out ...... though Im sure its a very useful graph!

Bosses are always right, untill proven wrong at thier expense.

sparky6078 said:

That was my thought as well, would i be right in saying if it was a TT earthing system with 1 sec disconnection time it would be ok? (this is on a TN-S on the site)

Click to expand...

Only if you could guarantee that your fault current would always be between 220A and 360A would there be discrimination between those two MCBs, other than that there is the chance that any one could go before the other.

With mcb's under short circuit fault there's little chance of having discrimination amongst any values of mcb to mcb .... from memory i believe a 6amp mcb gives full discrimination to a 63amp most other rating against each other will at most only achieve partial ...as fault currents can be in the 1000's and partial discrimination may only be up to a few hundred amps then its a free for all 'pot luck' to which will trip if not both together.

To provide short circuit discrimination usually requires larger cable install with upfront devices like 250amp mccb giving discrim' to 63amp down stream devices ... if your putting 2 mcb's in series regardless of rating this is usually poor design but the installation denotes the need to ensure discrim' as incoming services may be too small to achieve any full discrim' like a domestic.

So as you can guess its rare discrimination can be achieved in domestic and at most only partial so the 40amp mcb feeding the garage board with a 6 and 16 mcb's fitted will at most only achieve partial so a fault can still trip the upstream device but in a domestic environment this is not a big issue ... in a computer office it is.

sparky6078 said:

My boss has asked me to fit some MCB's that i feel doesn't offer enough discrimination, a 63amp c type mcb feeding a submains then out that submains is a 63amp b type feeding a circuit.

is this enough discrimination?

Click to expand...

Check your Ze and see your fault current, if it exceeds 1000amps then all mcb's 50amp and under will have zero discrimination between the types mcb with the same rating .. 63amp actually achieves a small partial discrimination if fault is over 630amps but less than 1260amps between a (C) and (D) ....as it is only partial either mcb is likely to trip with currents over 1260amps.

Any short over 630amps which is most installs will have no discrimination between your type (b) and (c) you are fitting

TBH if you are fitting too 63amp mcb in series i would say its poor design ... hope you have been slightly educated here and think its time your boss learns how to design a circuit to attain full discrimination.