Before you do anything that could cause damage, determine appropriate precautions. Note that this implies that this step could actually be step 2 (before reproducing the symptom), step 3(after getting symptom description, if that action doesn't jeopardize anything), or step 4 (after reproducing the symptom, if doing so doesn't jeopardize anything). In no case should corrective maintenance be done before determining the proper precautions. The sooner a damage control plan is made, the safer. Unless you have a good reason to do otherwise, make this step 2.
The following are SOME examples. The list is no way comprehensive. YOU are responsible for safety and damage control.
Wear proper clothing.
No long hair or jewelry with machinery.
Eye and ear protection where there's a risk.
Make sure you're not creating a fire or explosion hazard.
Limit current if possible and short circuit damage a risk.
Screen incoming materials to prevent damage where appropriate.
Never remove fuses without limiting current another way.
Take any precautions including diagrams to assure correct re-assembly.
Back up programs and operating system.
Back up all at-risk data, including configuration data.
Back up anything at risk in automated systems.
Limit the number of parts entering a malfunctioning machine
If practical, feed the malfunctioning machine scrap parts.
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