An electrical fire in the unfinished Tenleytown Metro station on upper Wisconsin Avenue NW yesterday knocked out power to 1,000 stores, houses and apartment buildings, disrupted morning rush-hour traffic and blacked out a television and radio station for much of the day.

The blaze began when two 13,000-volt power company cables just under the street surface at Wisconsin Avenue and Albermarle Street burst into flame. It was quickly extinguished by city firefighters by 7:20 a.m.

But a third cable went out an hour and 33 minutes later, spreading the blackout northward on Wisconsin Avenue.

Several television and radio stations in the area lost power momentarily until their emergency power systems kicked on. But WTTG-TV (Channel 5) and WASH radio, both of which are at 5151 Wisconsin Ave. but lack backup power, were blacked out until late yesterday afternoon when Potomac Electric Power Co. restored regular power to the area.

D.C. Fire Department and Pepco oficials said yesterday they do not know what caused the electrical cables at the Metro station to catch fire. A Pepco spokeswoman said the incident is under "extensive investigation."

Frances X. Flaherty, 5th Battalion fire chief, said the two-alarm fire was reported at 6:54 a.m. when pedestrians observed dense smoke billowing from under the wooden decking over the unfinished Metro station.

With the help of a construction crane, firefighters pulled away the decking and extinguished the flames. Flaherty traced the fire to the Pepco cables suspended temporarily under the decking. Thick insulation around the cables caused acrid black smoke, which hampered firefighters for some time. There were no injuries.

Traffic was routed around the Wisconsin-Albermarle intersection, slowing commuters and disrupting bus schedules for several hours.

Outages caused by the fire spread ina zigzag pattern along the Wisconsin Avenue commercial corridor from Harrison Street south to Van Ness Street and then west toward Ward Circle and the 4200 and 4300 blocks of Massachusetts Avenue.

Pepco said about 1,000 customers in the area were affected, most of them commercial and apartment properties. Pepco spokewoman Marcia Schneidler said the outages were "spotty," affecting some customers but not others immediately adjacent.

Janney Elementary School near Wisconsin and Albemarle, for example, lost its power (classes went on anyway), while the big Sears Roebuck & Co. store across the street kept its power.

At WTTG and WASH employes worked with flashlights and candles after battery power emergency lighting expired. The nearly eight-hour TV blackout started at 8:53 a.m. during the Bullwinkle cartoon show. WTTG viewers thereafter missed such Monday regular shows as "The Dating Game" "Panorama" and "Mayberry RFD" until power was restored at about 4:30 p.m.

WRC-TV (Channel 4) reported it lost 10 to 15 seconds of airtime at 8:53 a.m. and WDVM-TV (Channel 9) about 45 seconds when their diesel-fueled emergency generators switched on to replace the lost Pepco power.