The issue is not that the floor will become energized but rather that it will become conductive enough to serve as the return pathway through the earth, back to the main bonding jumper, in the Service Disconnecting Means' enclosure (Read the main panel); and thus back to the secondary of the Utility's transformer from whence it came. That path only has to carry the 30 or more milliamperes which is needed to stop your heart. The current to cause this is less than 1/15000 of the current it would take to open the Over-current Protective Device (OCPD); the fuse or circuit breaker. Example: A high resistance neutral connection elevates the frame of a dryer; which is bonded to the neutral of a three wire circuit, as was previously allowed by the National Electrical Code (NEC); to 20 or 30 volts. You're late for whatever and come down, fresh out of the shower with bare feet, to grab some needed clothing item out of the dryer. You touch the dryer, which is sitting on it's plastic leveling legs, and take a shock through the trunk of your body from your hand to your feet and right past your heart. Your heart's control rhythm is disorganized and your heart muscle starts contracting in the disorganized way that is called fibrillation. [Your spouse hears you call out and fall, runs down to see what's happening, calls 911, and starts CPR. That is because your whole family HAS taken Family and Friends CPR. HAVEN'T THEY. The first responder fire department team applies an automatic External Defibrillator (AED). You are transported to the hospital and survive a very unpleasant but educational experience. "Experience is a dear school but some will learn in no other."] To prevent this entire sequence dryers are now required to be supplied through 4 wire circuits which contain a separate Equipment Grounding Conductor (EGC) to connect it's frame back to the main bonding jumper. The high resistance in the neutral will cause only slower drying and or drive motor overheating, which will open it's overload protector making you call someone to service it.
We install the GFCI receptacles because they will open the circuit at 6 milliamperes of current leaking from the circuit which is well below the 30 milliamperes of current which would stop the organized beating of your heart. That is why the NEC requires GFCI protection of receptacle outlets in areas which would have a conductive floor such as unfinished basements and garages.
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Tom Horne