Should I turn the power off before connecting the electrical wiring for the HBL503?
Yes — you must turn power off at the service panel and lock the service panel before beginning any electrical work. This warning applies both during cabinet preparation and during the electrical connection step. Failing to do so poses a serious risk of electric shock or injury. (Page 3)
From the official Bosch HBL503 500 Series Single Wall Oven manual, page 3
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