Re: A few fusing questions on UK gear here in the US
The attachment is a data sheet on BS1362 fuses. It is worth noting that, like most small fuses, their characteristic curves start at 1.6 times the rated value, For example, a 10A fuse will blow at 16A, but take 700 seconds to do that at this current. The time gets much shorter as the current rises and the fuse blows in 0.1 seconds at 180A.
180A is not at all an unrealistic current; the maximum permitted earth loop impedance for a ring circuit protected by a 32A Type B circuit breaker is 1.37 ohms at which 168A would flow. In most cases ELIs lower than 0.5 ohms will be normal, so the prospective fault current would be 460A. Lower ELIs still are not unusual.
Because of these high prospective fault currents, mains fuses within equipment should be high breaking capacity types as they will have to carry this order of current in the event of a short circuit fault. The equipment fuse should blow first if it is correctly chosen for correct selectivity (discrimination) with the upstream protection.
Fuses are not very precise devices, and they can't be individually tested, so a rule-of-thumb of simply using twice the rating for 230V for 120V is not unreasonable.
As this would take most equipment fuses used on 120V, to a range where they would be less likely to discriminate with the plug fuse and it's not needed on US standard circuits, it probably best to eliminate the plug fuse.
PMM
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