The extension of Metro's Red Line from the Rhode Island Avenue station north to Silver Spring will be delayed until next February because Metro officials recently discovered that the communications cables along the line are defective.

Late last month Metro engineers discovered that the double cables - which run the length of the six-mile stretch of tracks midway between the northbound and southbound rails - are leaking electrical current. However, they were unable to find the exact locations of the leaks.

These cables, which each measure one inch in diameter and are made up of a mesh of 50 interwined wires, carry information on the positions and speeds of the trains. Through them engineers are also able to monitor the positions of track switches, which determine the direction oncoming trains will take.

The cables do not control the movement of the trains themselves; they simply monitor those movements.

The six miles of surface track that will extend the Metro line to Silver Spring now are to open Feb. instead of Nov. 30. An estimated 20,000 new Metro customers are expected to use the new Brookland, Fort Totten, Takoma Park and Silver Spring stations.

General Railway Signal Company, the contracting firm that installed the cables, has paid the $100,000 bill for the replacement cables. The cables were tested and found in good working condition at the factory where they were produced. Later, they were tested while wound on wire reels before they were installed underground, according to Metro spokesman Cody Pfanstiehl. At that time, too, there was no problem with the cables.

Defects were first noticed in late July, just before Metro began test runs of trains along those tracks.

Although the problems with the cables are not severe now, Pfanstiehl said, they will probably worsen. Therefore Metro engineer are to test cables in 40-foot sections from Nov. 21 to Dec. 23 of this year.

Pfanstiehl said that amount of time will allow for replacement of every inch of cable if necessary. If possible, only portions of the cable will be cut out and replaced. Metro's schedules don't permit testing to begin earlier he said.

Pfanstiehl said Metro officials have no idea what caused the defects. He added that General Railway has generally done very good work on Metro construction project.

Since they allow technicians at the communications center at Metro headquarters at 600 5th St. NW to monitor the trains, Pfanstiehl said, "they would be blind without the cables."

The cables allow the technicians to monitor electronically controlled diagrams which show where trains are on the tracks, where they are moving and what the tracks look like - in a manner similar to the way traffic controllers track airplanes by radar.

A separate system automatically controls the trains movements and can prevent two trains from colliding. Technicians can also instruct an engineer if the train stalls or if track telephones or utility stations are malfunctioning.