3ph run to garage.

[JohnW2]

I’m using an example of a 60A submain so a 5 second disconnection time.

Ah, that makes sense then. As you probably realised, I was thinking about 0.4 (or 0.2) sec disconnection time. As you go on to say ...

The difference is explained when you look at the fuse graphs, as the operating current for a fuse is linear, and a circuit breaker is not as you need to operate the magnetic part of the circuit breaker to achieve the required disconnection time.

Indeed, as you imply, with a breaker the Zs will be the same for 0.4 and 5 seconds (since both require magnetic tripping), but with a fuse the Zs can be much higher for 5 seconds. It's an interesting point, and not something I've really had to think about very much. The only significant submains I've had to deal with are those in my house and, being a TT installation, they get their fault protection from TD RCDs.

Kind Regards, John

 

[RF Lighting]

Ah, that makes sense then. As you probably realised, I was thinking about 0.4 (or 0.2) sec disconnection time. As you go on to say ...
Indeed, as you imply, with a breaker the Zs will be the same for 0.4 and 5 seconds (since both require magnetic tripping), but with a fuse the Zs can be much higher for 5 seconds. It's an interesting point, and not something I've really had to think about very much. The only significant submains I've had to deal with are those in my house and, being a TT installation, they get their fault protection from TD RCDs.

Kind Regards, John

Maybe it’s just me but I find it strangely satisfying that a good old fashioned bit of fuse wire still performs just as well, if not better than a fancy modern circuit breaker.

 

[JohnW2]

Maybe it’s just me but I find it strangely satisfying that a good old fashioned bit of fuse wire still performs just as well, if not better than a fancy modern circuit breaker.

Yes, in many senses I feel the same - and I doubt whether anyone reading this will still be around if/when cutout fuses cease to be fuses!

Whilst it's undeniably true that 'flipping a lever' is 'more convenient' than replacing a fuse, fuses (should!) blow so infrequently that it's hardly an issue (provided, I suppose, one has a spare fuse {or fuse wire!} to hand!). Mind you, returning to the misunderstanding that caused me to question you, if, much more generally, one is talking about 0.4s disconnection times, then breakers may offer an advantage in terms of Zs requirements.

Kind Regards, John