Over the hum of air conditioners and the glow of computer terminals, the amplified voice in the futuristic, high-security room had the edge of a challenge to it: "Dickerson's got a storm."

To the men in charge of Washington's electricity supply, Thursday's thunderstorm, which ended a three-week heat wave, meant juggled connections, downed lines and potential disruptions to the intricate net of copper arteries through which the Potomac Electric Power Co. exchanges millions of watts of power each day with neighboring states to insure the Washington area's electricity supply.

Thursday was a weltering day that broke all previous records for electricity demand here, the kind of day that couldn't help but make everyone think about the great New York City blackouts.

But nobody talked about that.

At Pepco's control center, deep underground in western Bethesda behind high, guarded fences and a maze of code-locked passageways, engineers and technicians quietly went about their business, routinely avoiding disaster. Their voices were calm, their conversatiins punctuated by numbers.

"Thirty-nine twenty-six but we don't believe it," a technician announced around 4 p.m. He was right to be skeptical.

A few rereadings later, the day's peak and an all-time record electricity demand were recorded: 3,853 megawatts, or 3.8 billion watts, enough to light up 38.5 million 100-watt bulbs, 13 bulbs for every man, woman and child in metropolitan Washington.

At that moment, Pepco's six power plants were pumping out 3,710 megawatts, far short of their 5,000 megawatt capacity. Because power was cheaper in areas north and west of here. Pepco was also buying 314 megawatts from the rest of its power pool, the Pennsylvania - New Jersey - Maryland (PJM Interconnection.

Further, it was serving as a conduit for another 300 megawatts flowing through from the north into the Virginia Eleectric and Power Co. (Vepco) system to the south.

"Vepco may be selling somewhere else . . . the power may be coming right from Baltimore Gas (and Electric Co.) or from up as far as New England," explained Dave Penders, superintendent of power generation at the Pepco control center. "You have to go a long way through the system to find out who is buying whose power."

The intricate system of grids that links the nation's electric lines is designed to keep the lights on everywhere by making power instantly available to cover any loss. Nine major connecting lines march into the Pepco area from surrounding states. If the electricity level starts to drop anywhere, computers automatically open up switches to keep the overall level steady.

On Thursday, for example, a 289 megawatt generator at Pepco's Benning power plant suddenly stopped. The problem turned out to be a fauity vacuum pump and the generator was out for 37 minutes. The grid system, however, worked smoothly - making up for Pepco's loss of power.

For Pepco's customers, "when something like this happens, nothing happens" as systems operation manager Walter Johnson put it.

The 5 p.m. readings reflected the generator situation: 3,578 megawatts coming from Pepco itself, 403 megawatts coming in on the northern lines and the same 300 flowing through on its way to Vepco's power pool, called Carva. The drop in Pepco's own production because of the generator outage was made up by increased imports.

The Pepco people were ready for the storm. The day crew was put on overtime and extra maintenance crews were ordered after a study of the 3 p.m. weather report.