Fuse to Circuit Breaker Equivalence

[SparkyAus]

Replace of 45amp Fuse with 50amp CB on 10mm2 cable?
Is this okay because of the 0.9 derating of fuse to overloads and because 10mm2 can handle 50Amp current. Installation in air equivalent double brick unit?

 

[bill01803]

Depends on the type of fuse and type of MCB and what Zs you have

 

[littlespark]

Depends on the type of fuse and type of MCB and what Zs you have

OP in Australia.

I don’t know if they use Zs like we do, or maybe call it something else.

Generally speaking, it won’t be a problem.

 

[SparkyAus]

Yes, we have fault loop impedance Zs, but mostly irrelevant when using RCDs or RCBOs,
Which is required now on all domestic final sub circuits.

 

[littlespark]

Yes, we have fault loop impedance Zs, but mostly irrelevant when using RCDs or RCBOs,
Which is required now on all domestic final sub circuits.

Interesting.

We still use it for the overcurrent side of an RCBO, as well as mcbs.

 

[SparkyAus]

The reason it's irrelevant is because the tripping time of RCD will be quick, less than 0.4s if there is any earth fault so no need to worry about tripping current to trip circuit breaker to clear an earth fault.
What we still have to worry about is the fault loop impedance when determining prospective short circuit current Ipf to size correct kA rating of MCB or RCBO. However our service rules (depending on state) also state to take kA of domestic installation at transformer as 10kA for commercial industrial as 25kA or if greater the actual Transformer fault current.