A police officer is never really off duty, even when he's in plain clothes after attending a trial in City Hall and then on his way to work.

Which describes Sgt. Ralph Galdi on March 31, 1986, when he was shot and killed by a fleeing drug courier.

Yesterday, Galdi's family joined elected officials, police, fire brass and District Attorney Lynne Abraham during a plaque-dedication ceremony at 5th and Spring Garden streets in memorializing Galdi.

"This was a sergeant coming from court and going home or to his job at the time he got involved," said Police Commissioner Sylvester Johnson. "There's something very special when you have a person with 20 years of service, almost off duty, and observed something happening and gets involved. There's nothing more special than that."

Slain a day before his 46th birthday, Galdi was in his 21st year with the police force.

He was assigned to what is now the Special Victims Unit when he and Officer Vincent Wiley chased Pedro Vega after Vega caused a collision at 5th and Spring Garden streets.

The Daily News reported at the time that as Galdi was about to frisk Vega, he turned and shot Galdi twice in the abdomen.

Wiley shot Vega, while citizen George Listner, who was involved in the accident, was able to track him down and hold him for the police.

Yesterday, Listner said that although he didn't know Galdi, something prompted him to intervene.

"I didn't know what was going on, but after the killer struck my car, the next thing I knew, I heard gunfire and I just jumped out and started going after him," Listner said.

"I never knew the man who was killed, but afterward, I heard so many great stories about him, what a wonderful family man he was - a great father, husband. I'm glad I was able to help put his killer behind bars."

One of Galdi's sons, Ray, 16 when his father was slain, remembers him as a firm but fair man.

"He was a truthful, honest guy," said his son, who joined his mother, Elizabeth, his two sisters, Donna and Liz, and his brother Michael, along with other family members at the ceremony.

"He worked a lot of hours, but when he was there, he was the boss, a great dad," Ray said. "I hope I can be the same dad he was. I have two children now, and hopefully, I can raise them the way he raised me."

This was the 33rd plaque dedication in honor of fallen police officers, and although the ceremonies are poignant and tasteful, at least one elected official regrets that there was ever a need for them.

"I wanted to show my respect for the Galdi family and for all the men and women in blue, who make a great contribution to the safety of all our citizens," said City Councilman Jim Kenney.

"It shows there's appreciation from all citizens of Philadelphia for those people who make the ultimate sacrifice for all of us."

For more information on the hero plaque program or how to become a citizen sponsor, contact attorney James Binns at 215-275- 3000. *