What's missing without a ground
The Consumer Product Safety Commission's GFCI fact sheet explains the risk: if a bare wire inside an appliance touches its metal case, the case becomes energized, and anyone who touches it while also touching something grounded, like a faucet, can be shocked. A ground wire gives that stray current somewhere to go. Without one, a damaged appliance with a metal case has no safe path for it.
Don't fix the fit problem by snapping off the round pin or using a cheater adapter. A U.S. Department of Energy electrical safety guide says never to remove the third prong to fit a two-prong outlet, because it can lead to shock.
Option 1: a GFCI outlet or breaker
A GFCI watches the current going out and coming back on a circuit. The CPSC says that if the two differ by as little as 0.006 amps, it cuts power fast enough to prevent a lethal shock, and that it works even on two-slot receptacles. That's why a GFCI receptacle can replace an old ungrounded outlet. It also protects outlets further downstream on the same circuit, and a GFCI breaker in the panel protects the whole circuit.
The catch is that a GFCI protects people, not equipment, and it doesn't create a ground. The CPSC says a GFCI installed on an ungrounded circuit must carry the label "No Equipment Ground, GFCI Protected" so the next person knows. If an electrician installs GFCIs this way, ask for a list of which outlets are protected and where the controlling GFCI is, since a tripped device in one room can kill outlets in another. Our guide to GFCI versus AFCI protection explains how these devices differ.
Option 2: run a real ground
If you need an actual ground, for example an appliance or piece of equipment whose instructions call for a grounded outlet, the job is wiring work, not swapping the outlet face. Ask the electrician to explain how they'll provide a ground path to each outlet, whether the existing wiring can be grounded or needs to be replaced, how much wall and ceiling opening that means, and who handles the patching afterward.
A three-slot outlet on its own doesn't prove a ground is connected behind it, so ask the electrician to test any three-slot outlets in the same rooms during the visit.
Where Philadelphia codes now want GFCI protection
Philadelphia L&I's 2021 code briefing says the updated rules require GFCI protection for dwelling receptacles up to 250 volts in bathrooms, garages, outdoors, crawl spaces, all basement areas, kitchens, near sinks, laundry areas and other damp or wet spots. That applies to new or altered work, but it's a useful map of where shock protection matters most in an old house. Those are good rooms to start with if you can't upgrade everything at once.
What to note before the electrician comes
Walk the house without removing covers and list which rooms have two-slot outlets and what you plug in there. Note plugs that fall out easily, wall plates that are hot or discolored, and outlets with too many things plugged in. The DOE guide says loose-fitting plugs can cause arcing and fire, and that a hot plate means shutting the circuit off and having it checked. Our guide to warm outlets and switches covers those warning signs.
Ask the electrician whether the work needs a permit. In Philadelphia, our guide to when electrical work needs an L&I permit covers the basics. When the job is done, ask for a list of which outlets now have a verified ground and which are GFCI-protected only.
Test each GFCI after installation, at least once a month and after a power outage, as the CPSC recommends: plug in a lamp, press Test, and the light should go out; press Reset and it should come back. If it doesn't, call the electrician back.