Grounded Electrical Outlet
Only connect your dryer to an electrical outlet that has been grounded and also conforms to local codes and ordinances. The rule applies whether you use a three- or four-prong power cord to supply power to your dryer. An outlet that has been grounded is designed to reroute an electrical current away from the appliance in the event of a power surge. If the outlet isn't grounded and a power surge occurs, it can short circuit the appliance and start a fire.
Even though certain gas clothes dryers still come with power cords, they're rarely included with most electric clothes dryers. At the time of purchase, an appliance salesperson will probably tell you whether or not you need to also purchase a power cord. Due to recent electrical safety code changes put in place over the past 10 years, he will probably encourage you to obtain a four-prong power cord that works for most clothes dryers. However, using a four-prong cord also requires that it's plugged into an electrical outlet with four posts -- one for each wire.
Never try to make a four-prong power cord fit into an electrical outlet with three posts. It not only won't work, but you'll probably damage the outlet by trying. Rather than downgrade to a three-prong power cord, replace the outlet's terminal block to one with four posts. The terminal block is the strip in the center of the outlet that connects the wires to electricity. If you're uncomfortable making the switch, contact a qualified electrician to perform the task for you. However, never downgrade the outlet to use a three-prong power cord; the change might violate local codes and ordinances.
Recommended Vs. Required
You don't always have to purchase a four-prong power cord to operate a clothes dryer. If you have a three-post outlet that has been properly grounded, the dryer might perform just fine with a three-prong power cord. Confirm which power cords are acceptable for operating your dryer model in the user guide. However, if you plan to reuse an old three-prong power cord, inspect its condition first. Never use a power cord if the wires are exposed or frayed.