This is the way it’s done here.
The incoming service cable is for practical purposes unfused. There are fuses at the substation where it originates (pole transformers are rare here) where a delta-star transformer steps down from 11kV to the 400/230V to people’s homes, shops, offices etc, but they are too large to protect the section of service cable from the 3-phase one under (usually) the street to the consumer – if anything goes wrong with it the electricity company would rather it burns out than have it blow a fuse which blacks out out many customers.
It goes into a locally fused thing called a cut-out, these days it has a cartridge type fuse. Typical ratings have crept up over the years – I think 80A is probably still the most common, 100A fairly prevalent, as are 60As in older installations. The electricity company will not replace it unless you keep blowing it or ask them for an uprated supply for your new particle accelerator or whatever. Some companies are putting in 125A ones on new-builds to allow for EV charging points.
Customers are not allowed to pull the fuse. Quite apart from the dangers, obviously once it’s out the opportunity to steal electricity becomes easier. There are seals on the fuse carrier and the outgoing terminal covers. Electricians are also not allowed to pull the fuse, although some suppliers do give training on the more modern types to registered electricians and let them. In theory if anybody needs the supple de-energised they arrange for the supplier to send someone, and then to return later to replace the fuse once the work (e.g. replacing the consumer unit) is done. Unsurprisingly many people, particularly electricians with a ticking clock, find this inconvenient, but fortunately there is a never-seen but always present creature called the “fuse fairy” who magically arrives at just the right times....
From the cut out to the meter, and the meter to the CU, are single-core double insulated cables known as tails – probably 25mm² (just under 3AWG) or could easily be 16mm² (5AWG) in older installations.
It’s in those where you’d put blocks like I showed earlier to “split the tails”.
The isolator switch shown isn’t always/often there. Having one makes it really easy to isolate the supply so you can change or move the CU, or split the tails without needing to pull the service fuse. Some suppliers will install one (for a charge, natch). If someone has come to change the meter and you’ve got everything ready (switch already fixed, lengths of meter tails cut to size) he might do it. Or an electrician or DIYer does it after the fortuitous fuse fairy visit.
Now – as to why a lot of people prefer to keep a garage or workshop or other outbuilding supply completely away from the house CU, it is partly because it “feels” better, although I fully accept that that’s an emotive reason. But there is, or at least for a long time there used to be, a practical reason – RCD protection. (RCD = Residual Current Detector = GFCI). For some time here it’s been required at the circuit level for an ever-expanding list of circuits, and until relatively recently it was done by having 1 or 2 or 3 RCDs in the consumer unit covering some or all of the circuits, i.e. [RCD] --> [a bunch of circuit breakers]. So the breaker supplying your outbuilding would be on an RCD in your house CU. Any earth fault out there could easily trip that, so you’d lose other circuits in the house and you’d have to schlep back there to reset it. If your outbuilding is a rickety wooden shed, or a gazebo, or if you’ve got outside lights or sockets supplied from it, earth faults might be quite possible. Or conversely a fault in the house suddenly deprives you of power when its dark and you’re using the spinning finger chopper.
But the long-term practical reason is, it is true, declining. Increasingly people are using RCBOs (Residual Current Breaker with Overload protection, i.e. GFCI breaker), as they have dropped in price a lot in recent years. (How that will change as requirements arise for RCBOs other than Type AC or A remains to be seen....). So if you’ve got an all-RCBO CU you could put a plain old circuit breaker in (if you use armoured cable then the
cable to the garage doesn’t need RCD protection) and feed your garage CU that way.
Given US rules, if I were designing an installation there and I knew I was going to have, or might well end up having, different panels in different places I’d like for my “main” panel to have nothing but breakers supplying the other panels.
