As a HVAC journeyman, if while troubleshooting a heater with a cracked heat exchanger, or a faulty/leaking gas valve, it is my obligation to disable the furnace. I then note said danger on my invoice with an accompanying bid for the repair, signed by the customer. By this prudent procedure, I have done my do diligence and should be held harmless.
Likewise, if I note a safety issue with a customer's electrical wiring, I also note it on my invoice and bid. However, I have yet to disable anyone's electrical service.