Spanish to English Tech/Engineering Mechanics / Mech Engineering welding equipment

I'm translating health and safety information for a company regarding a wide range of different tools that an employee would use, and one of them is mobile welding equipment. One of the instructions for correctly handling the equipment to avoid electrical hazards is as follows:

"Antes de conectar la toma al casquillo se debe de desconectar la corriente."

and then

"Cuando no se trabaje se deben cubrir con capuchones la toma y el casquillo."

I cannot seem to find any reference anywhere as to what "casquillo" might refer to in the context of mobile welding/soldering equipment. Can anyone shed any light?