I believe they are talking about the Lockout/Tagout procedure. When repair or maintanance is being done on a particular equipment it should be tagged and all source of energy should be locked out. What this means is, an "out of service" or "do not use" tag is placed on the equipment. This is "tagout" then the "lock out" procedure starts. Usually the person performing the lockout procedure will have a lock (either a combination or regular lock and key) that he places in the main switch control door locking the energy source so noone else goes to turn it on while he is doing repairs. This is call "Lockout". In spanish this is called "Los procedimientos de Cierre y Etiquetado".

I would translate it...

El interruptor principal de desconección debe ser cerrado.

Hope this helps! Good Luck!