Interesting. The key word that caught my eye was 'slot'. I had a hunch you were referring to tube fuses as we know them. They have never been used here for residential service but have and still are for commercial and industrial installations, particularly three phase service. Our residential service is normally 120v, 100 amp with two hots and a neutral. All the 120v appliances come from a single leg and 240v for motors above fractional HP and high wattage heating elements by using both hot sides but are still single phase.

This topic made me think of some of the OLD home installations I've seen. Some of the earliest were 'wire fuses' in a wood box lined with asbestos sheet and mounted on the wall with normally 2 circuits for the 20 amp service. It was nothing but two screw terminals connected with a small lead alloy wire intended to melt. Same principle as the replaceable flat strips inside the porcelain or single use fiber tube fuses I believe you were talking about. We moved from the wire fuse to round glass threaded plug with a visible link inside to see which had blown among the 4 to 8 circuits on a 60 amp service. Sometime near the 1950's they started using breakers with 100 amp service to all homes.

240v and 480v, 3 ph with 400amp and 600amp industrial service had a different evolution.

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