If an NYPD bomb tech is going to make a mistake attempting to defuse a truck wired with 250 pounds of highly explosive fertilizer, he ought to make it here.

The Hazardous Devices School, a 450-acre campus in Huntsville, Ala., jointly run by the Army and FBI, is where newbie “bomb techs” and old pros assigned to the NYPD’s 108-year-old bomb squad come to make the errors that would be fatal in real life.

Under the guidance of military vets and FBI agents, the city cops practice piloting bomb-disposal robots through a subway car and a bus aisle.

They learn to defuse a bomb like the one al Qaeda terrorist Faisal Shahzad tried to detonate in Times Square two years ago.

There is a mock-up of a military recruiting office — a target of terrorist plots around the country from Times Square to the South.

The cops also repeat dismantling pipe and dynamite bombs modeled after devices found in New York.

PHOTOS: HAZARDOUS DEVICES SCHOOL

“One of the best things about the school is you have the chance to make a mistake and walk away with your fingers,” said Denis Mulcahy, an ex-bomb tech who attended 10 sessions at the school over his 20 years in the NYPD squad.

“If you make a mistake in class, you will never forget it and never make it again in a real-life situation.”

The lesson is drilled home when instructors demonstrate what happens in even a minor explosion. They load a surgical glove filled with hot dog fingers and detonate it.

Aspiring NYPD bomb techs spend six weeks at the academy in the Redstone Arsenal, with classes in chemistry, engineering and physics and training in robot control, disrupting bombs with water and defusing a time bomb by hand.

The school, offering the only recognized certification program of its kind in the country, has a 90 percent graduation rate.

“The devices are devices, whether in a tall building or on the crowded streets of Times Square or a church in the Midwest. Once you identify the bomb, then you have to deal with it the same way,” said Paul Carter, chief of the school.

Bombs of New York lore are found throughout the course.

There is a replica of the Nissan Pathfinder Shahzad wired with alarm clocks, fertilizer, M-88 firecrackers, propane tanks and gasoline. His device failed to detonate.

“The actual explosion would have caused a lot of structural damage,” said supervising instructor Thomas Epperson. “It would have gone down the block and the shock wave bounced off the walls out and up. It would have blown out windows blocks away.”

The techs also learn how to defuse a pipe bomb hidden in a KFC box — a design nearly identical to one the Puerto Rican terrorist group FALN detonated at 1 Police Plaza in 1982.

Sometimes they do this wearing a heavy — and hot — Kevlar bomb suit.

A mock-up village offers techs a real-world setting.

There are several subway scenarios. One has them maneuver the arm of an HD1 robot to get to a satchel bomb under a seat.

In another, the tech uses a robot to defuse a suicide-bomb vest on a mannequin.

Other missions include bombs stashed in an airplane seat, a movie theater, a high school, a house of worship and a shack similar to Unabomber Ted Kaczynski’s.

“This isn’t Hollywood. We’ll show what is real,” said Floyd Pirtle, a supervising training instructor. “You only get one take in real life.’’

Mike Walsh, who retired from the NYPD bomb squad five years ago and is active with the International Association of Bomb Technicians and Investigators in the private sector, said one never knows when the training will be useful.

“In school, we cleared a booby-trapped lane of entry to a [mock] marijuana field. I thought I’d probably experience that one day,” he recalled.

Then in 2002, he had to clear “Viet Cong”-style booby traps — improvised explosive devices with sophisticated mercury triggers — that a disturbed Russian immigrant had laid in his campsite in Pelham Bay Park in The Bronx.

“There were 14 IEDs out there that day. There I was in Pelham Bay Park, poking in the ground,” Walsh said.

The most common find in New York are homemade pipe bombs of the match-head and gunpowder variety, NYPD spokesman Paul Browne says. Ten to 20 are found each year.

NYPD bomb-squad commanding officer Lt. Mark Torre has his squad train several times a year in defusing IEDs common to Iraq and Afghanistan in case they turn up on city streets.

Also on their minds is the trend of planting multiple devices to kill fleeing civilians and arriving first responders.

Cops are also trained to see the dangers in a home lab where the explosive triacetone triperoxide (TATP) is being made. Subway- bomb plotter Najibullah Zazi was working to make TATP when he was busted in 2009.

The feds cover the $50,000-per-student tuition, plus weeklong refreshers every three years.

The school, which opened in 1971 and renovated in 2004, runs on a shoestring budget of $7 million a year.

Nationwide, there are 3,189 active members of police- or fire-department bomb squads who have trained there, and nearly 30,000 graduates overall.

For security reasons, the NYPD keeps the size of its bomb squad under wraps.

larry.celona@nypost.com

