Isolator versus protection device on main incomer

[Richard100]

Hi, i'm only a student so please forgive me for the basic question:
Say you have a generator or power transformer serving a main mccb panel, why can't the main switch of the panel be just an isolator? I've only seen the main switching device as a mccb or acb.
I know that in a house you'll have a service head and then a main isolator integral to your consumer unit, or separate. This is what I'm trying to compare with a commercial arrangement.

 

[westward10]

Sometimes they are used as circuit protection for the supply conductors to prevent excessive overcurrent.

 

[Richard100]

Sometimes they are used as circuit protection for the supply conductors to prevent excessive overcurrent.

Thanks but I know that cables coming from a substation would come from the transformer's own circuit protection, acb or lv panel. Surely this would give the protection to any excessive overcurrent? It's the same with a generator, the cables are from the generator's circuit protection devices

 

[westward10]

True but your distribution panel could have a maximum rating of say 1000A so you install a main switch rating to provide protection for this.

 

[Richard100]

Thanks for your replies westward100, i'll have a good about your answers.

 

[Bobster]

It's also done to limit fault current.

The unit/panel will have a kA rating, without upfront protection they would have to deal with the full fault current from the transformer.

 

[Richard100]

I'm really seeing the bigger picture, thanks.
An onsite generator or onsite transformer will be so close to the main panel that the fault current will be much higher at the panel than if the supply was coming from a remote ukpn substation