Boxer start when reservoir partially full
The Boxer 10 bar, 3 HP single phase is stretching the limit for running on domestic power supply. 3HP demands 2 1/4 Kw which is more than the heaviest electric heater you can now buy but unlike a home heater the start cycle may exceed the capacity of a 13 amp fuse. When the regulator senses lower differential setting the unloader valve (beneath the regulator) closes as the switch energises the motor and the cold start valve alongside the cylinder block may shut before the motor reaches full speed. If your fuse blows there are some tips which might ease the problem.
Check your voltage at the plug. If you have a looooong supply line from power entry into your home/workshop you may need to upgrade the cabling to minimise resistance losses up to the point where you plug your machine in. Avoid extension leads. Reduce or switch off any other energy demands on the same or nearby circuit. You can enlarge the "buffer" capacity between the cylinder outlet and the non return valve into the reservoir. This enlarged buffer space will enable the motor to run up to speed before the pressure builds to stall the compressor and blow a fuse or trip the overload. The cold start valve outlet (1/4" BSPT) is common with the upstream unloader feed pipe. Either can be connected to a supplementary buffer tank or even 20 feet of air hose with the cold start valve relocated at it's end. When the compressor reaches its upper setting you will hear the unloader discharge for a far longer period of time than a regular installation. The cylinder head gets very hot so any air hose connection at the discharge manifold will have to be high temperature to stand the heat.
As a last resort you may wish to upgrade the regulator with a unloader valve delay built in.