> double fused double switched
We must draw a distinction between Service Equipment (the building fusebox) and Appliances (our boxes).
In the USA, we don't fuse or switch the White (groundED, "neutral") in the fusebox. If we did, and it went open, a "dead" circuit would still be 120V to the nearest radiator, which could surprise someone.
If it appears to be necessary to break both sides of a 120V-0V circuit, we must use a ganged fuse (bet you never see one) or a ganged circuit breaker. However that is so unusual that I doubt it is ever done for a 120V-0V circuit.
US-type "240V" (120V each side of Ground) circuits are complex. We generally use a 2-gang circuit breaker, though individual fuses used to be legal IF they were mounted so you had to take both out at once. Since US-culture 240V boxes are generally above DIY scale, I'll ignore them.
General power in the US is 100V-120V because we electrified before good insulators, and 100V is less lethal than the higher voltages we would prefer for reduced copper-cost. Most of Europe was electrified later, when decent insulation was available, and general outlets are 230V, apparently single-ended (one side grounded in fusebox or power-pole). Looking at where you fuse and switch, a EU 230 system works the same as a US 120V system, just check your voltage taps. Since one side is supposed to be earthy and "safe", we can just switch/fuse the other "live" side and have a safe "dead" mode.
It is presumed that the Service Entrance will be wired correctly 99% of the time.
Now when you get to the Appliance, 60% of the outlets in my kitchen were wired with Black and White reversed (and all the Green wires cut off). This is almost typical in US wiring: we had millions of 2-pin unpolarized outlets installed 1910-1960, and even if someone has put polarized or 3-pin outlets in their place, they are often wired wrong. Inspectors do pay attention to the Service Entrance, and most power companies won't connect until the panel is Inspected, but no inspector can check ALL outlets and many outlets are maintained without inspection. And then there are miswired power cords.
Inside the Appliance, you must assume that either wire may be HOT (or even both, as in bcarso's odd tale of +/-60VAC). However you can also assume that only Qualified Persons will be inside the box while it is plugged in (see big warning on all commercial boxes). If your wiring is done perfectly, especially if any exposed metal is properly Grounded, it "should" be adequate to fuse one side, and Black is the preferred side.
Two fuses are usually cost-prohibited in US-market commercial production (however I do have a Cisco router with a warning that it has a "fused Neutral"). They may be a good idea, and VV says they are required under his regulations in some cases. (This may reflect the reality of the New Eurpose: while UK, West Germany, and many other areas have generally good wiring, some backwaters and ex-USSR areas may be wired as bad as much of the US, and at more-lethal voltage and/or more-hot circuit power limits.)
In DIY, where wiring may be imperfect, and fuse-cost is negligible compared to your labor and your life, I see no reason you can't fuse both sides of the line, so long as you remember that "does not light up" is NEVER the same as "dead inside".
Always ASSUME the inside of a box is LIVE and DEADLY until proven otherwise. If it is plugged in, it can kill you. If it was plugged in, and has energy storage (capacitors), it can kill you. When you open the box, always assume it is full of angry rattlesnakes until you check it every which way. I generally work with one voltmeter on the main power cap all the time: I need to see voltage to be sure the meter works, and I need to see low/no voltage before I put my flesh on conductors. Often I just work without letting my flesh contact any conductor.
Yeah, yeah: keep one hand in pocket, stand on a slab of glass.... my shocking experience is that these methods fail as soon as you get excited or careless. And despite incredible luck (I'm still alive), I have permanent damage from a one-hand shock (600V through my little finger).
