(Adds quotes from lawyers, details on court ruling)
By Bill Cotterell
TALLAHASSEE, Fla., Sept 26 (Reuters) - A woman sentenced to 20 years in prison after firing a "warning shot" during an argument with her abusive husband was granted a new trial on Thursday in a case under Florida's controversial self-defense law.
The aggravated-assault conviction of Marissa Alexander, 32, touched off a furor last summer when her supporters compared it to the self-defense case of watchman George Zimmerman, who was later acquitted.
In Alexander's case no one was injured, but because she fired a gun Florida's mandatory minimum sentencing guidelines required a 20-year prison sentence.
A state appeals court ruled that Alexander deserved a new trial because the judge failed to properly instruct the Jacksonville jury in north Florida regarding her claim of self-defense.
"I'm absolutely ecstatic," said Michael Dowd, a New York lawyer who has defended domestic violence victims since the 1970s, and is representing Alexander pro bono.
The conviction "was reversed on a legal technicality," said a spokesperson for the State Attorney's office in Jacksonville, who indicated that prosecutors intend to re-try the case.
The case of Alexander, who is black, drew criticism from civil rights groups concerned about self-defense laws and mandatory minimum sentencing rules.
Alexander's fate received little attention outside Jacksonville until her May 2012 conviction in the wake of another Florida case involving Zimmerman, who said he shot an unarmed teenager, Trayvon Martin, in self-defense.
Under the so-called "Stand Your Ground" clause which was added to Florida's self-defense law in 2005, people who use deadly force to defend themselves from serious injury - rather than retreating to avoid confrontation - can be immune from prosecution.
Zimmerman was acquitted of murder charges in July, although he never sought immunity under "Stand Your Ground," instead relying on standard self-defense law.
The 1st District Court of Appeal in Tallahassee said in Thursday's ruling that the trial judge made a "fundamental error" when he instructed the jury that Alexander was required to prove self-defense beyond a reasonable doubt.
"The defendant's burden is only to raise a reasonable doubt concerning self defense," Appeals Court Judge Robert Benton wrote for the court.
However, the three-judge panel ruled that Circuit Judge James Daniel properly rejected Alexander's plea for dismissal under the "Stand Your Ground" clause, arguing that her actions did not meet that self-defense standard.
Daniel's mistake was to instruct the jury that Alexander had to prove "beyond a reasonable doubt" that her husband had attacked her, which was a separate issue under common self-defense rules.
Alexander's "Stand Your Ground" claim was rejected because she left the house during the confrontation to retrieve a gun from her car, returning to fire a shot near Gray's head.
A slightly built woman who stands 5 feet 2 inches (1.57 meter), Alexander said her 245-pound (111 kg) husband Rico Gray, was about to attack her when she fired into a kitchen wall during the August 2010 incident. He had previously been convicted on a domestic violence charge for attacking her.
Gray's two children were at home, in the living room. Prosecutors said the shot endangered Gray and the children.
At the time, Alexander had an active restraining order against her husband and she carried a concealed weapons permit.
(Marissa Alexander v State of Florida 1D12-2469) (Writing by David Adams; Editing by Grant McCool and Richard Chang)