Upgrading from a 2 wire outlet with 2 prongs and no ground prong to a 3 prong outlet seems like a simple enough task; just swap the outlet out and viola, now you can plug your 3 wire appliances in. This is a mistake and to understand why it is dangerous you must understand what the 3rd wire does. When an appliance that is manufactured relatively recently (within the last 20 years) has a 3 wire cord and plug installed on it it means parts of that appliance could become energized or contain residual energy while/after operation of the appliance. The 3rd wire is there to bond those parts of the appliance that may become energized.
What happens if you continue to use that 2 to 3 prong adapter or incorrectly wired 3 prong (grounding) outlet? instead of the electricity finding a path to the ground through its 3rd wire it will very likely try to find its ground path through your body. This is the impetus as to why GFCI’s were invented. Even though there is a ground path in modern homes the additional “supplementary ground” provided by GFCI’s is required in areas that may become wet. What GFCI’s are are devices with a differential current transformer, that is when the amount of current going out of the GFCI does not equal the amount coming in it “detects” the “leak” and shuts off the circuit.
This leads me to the last bit of information about upgrading from 2 prong to 3 prong outlets. You can do the change legally and safely with a GFCI. A GFCI will provide a “supplementary ground” that your device lacks normally called an “equipment ground”. Outlets using this method will need to be indicated with the blue stickers that come with the GFCI that read “No Equipment Ground”.
At Safe and Sound Electric we always recommend having any 2 wire circuit rerun back to the panel and having the panel upgraded if needed as that is the current level of protection the National Electric Code says is acceptable for new homes. Having a ground at your outlets is the only way to ensure 100% safety for yourself, your family and the equipment that may require the ground!