I bought a lincoln 225/125 welder and the book said to use 50amp fuses to feed the receptacle. What are the fuses for?
The fuses are to protect the wiring feeding the receptacle if a short circuit occurs.
The welder contains its own protection to prevent damage to the welder.
I thought welders did have protection built in but, the manufactor's direction said to have the plug coming from a super lag fuse box
"the manufactor's direction said to have the plug coming from a super lag fuse box"
That does not mean there is no protection in the unit.
If I have the plug coming from a super lag fuse box, what feeds the fuse box, the breaker??
"If I have the plug coming from a super lag fuse box, what feeds the fuse box, the breaker??"
A breaker in the main panel.
The manufacturer may be specifying a particular overcurrent method since testing was only performed with that type.
For may years central air condensers required fuses is the outside disconnect to comply with the manufacturers requirements. Many units are not rated to operate from HACR rated circuit breakers and the disconnect at the condenser is only that, a disconnect with no overcurrent devices of any type.
The breaker and branch wiring are still sized in accordance with article 630, and then any manufacturer required devices are added.
The use of "super lag" fuses is likely based on the large surge currents welders develop when starting an arc, and the voltage surges that can occur if the device has an emergency shutdown and the magnetic field in the transformer suddenly collapses.