A law that makes it an offence for the police to use torture to extract confessions can be used on others who employ such methods, including vigilantes.

Explaining why he jailed for 15 months a 42-year-old man who had tied up a teen and beat him to wring a confession out of him, District Judge Kenneth Yap referred to a High Court precedent from 21 years ago that involved a similar beating by three cops.

The High Court had said then the main aim of the relevant Section 330 of the Penal Code was to prevent torture by the police, but it also said a benchmark two-year jail sentence may be appropriate where non-law enforcement officers were involved, other factors being considered.

Judge Yap said the offender, whose name was redacted from the judgment, should not be treated any more leniently just because he had acted upon a mistaken belief that his victim had hacked into his phone when he restrained and beat the teen up.

"He had taken calculated steps to extract a confession in open violation of the criminal process, and should be punished in the same measure as any vigilante or abusive law enforcement officer," the judge said in grounds issued on Monday.

The man was sentenced last month after he admitted to tying up and assaulting the boy into a confession. An additional charge of wrongfully confining the teen in his Tampines flat for almost two hours on Nov 18 last year was taken into consideration for sentencing.

The ITE-trained electrical engineer was trying to unlock his Google account outside his flat when he became paranoid that somebody had hacked into it.

Spotting the victim at the void deck, he suspected him to be the culprit and took him to his flat for questioning. This was some time after midnight. The boy, who was high on Ice, was said to be afraid he might be a policeman.

Dissatisfied with the boy's answers, the man tied his wrists and legs. He also slapped and punched the boy until the commotion roused his family members. The man's son, who is also a minor, called the police at around 2.10am.

The boy was treated at Changi General Hospital.

At the hearing last month, the man's lawyer Chow Weng Weng urged the court not to be bound by past precedent and argued that the relevant section in the law was meant mainly to prevent police brutality, and the High Court's remarks on sentencing for non-law enforcement officers were made in passing.

He said the accused and his wife had been victims of cyber hacking for some months and were upset.

Deputy Public Prosecutor Joshua Jeyaraj called for an 18-month jail term as a starting point, given the seriousness of the offence - caning had been added as an option in 2007.

Judge Yap, saying he was "mindful of the spontaneous nature of the offence" and the accused's remorse in compensating the victim, among other things, scaled the jail term down to 15 months. "While the accused was motivated by a delusion, his willingness to take the law into his own hands was real and deliberate. This is the nub of the offence."