Hi Mark 5
Where any kind of short circuit/overload protective device (in this case an MCB) is used for earth leakage protection (no suitably rated RCD), consideration has to be given to the earth loop impedance at the furthest part of the circuit (Zs)
The circuit designer has to ensure that the MCB will operate within a given disconnection time.
This is done by checking the maximum permissible Zs charts within BS7671 for each MCB type.
When an MCB is changed from say a type B to a C, the maximum permissible Zs is much lower and if less than the present circuit Zs it’s possible that the required disconnection time can’t be achieved.
It’s just one reason why testing is so important with additions or alterations to electrical installation.
I hope that makes sense.
Edit:
I’ve just noticed the last part of your question.
If you are replacing a BS:60898 (MCB) with say a BS:61009 (RCBO) then as long as the RCBO is suitably rated for the application, it will provide the earth leakage protection.
To check the maximum permissible Zs charts, you just use the protective device BS number & type then refer to the charts within the regs book.
Where any kind of short circuit/overload protective device (in this case an MCB) is used for earth leakage protection (no suitably rated RCD), consideration has to be given to the earth loop impedance at the furthest part of the circuit (Zs)
The circuit designer has to ensure that the MCB will operate within a given disconnection time.
This is done by checking the maximum permissible Zs charts within BS7671 for each MCB type.
When an MCB is changed from say a type B to a C, the maximum permissible Zs is much lower and if less than the present circuit Zs it’s possible that the required disconnection time can’t be achieved.
It’s just one reason why testing is so important with additions or alterations to electrical installation.
I hope that makes sense.
Edit:
I’ve just noticed the last part of your question.
If you are replacing a BS:60898 (MCB) with say a BS:61009 (RCBO) then as long as the RCBO is suitably rated for the application, it will provide the earth leakage protection.
To check the maximum permissible Zs charts, you just use the protective device BS number & type then refer to the charts within the regs book.