VAR for Chelsea v Norwich game - how does it work?

A view of the Video Assistant Referee (VAR) system pitchside before the Carabao Cup Semi Final, Firs

A view of the Video Assistant Referee (VAR) system pitchside before the Carabao Cup Semi Final, First Leg match at Stamford Bridge, London. PRESS ASSOCIATION Photo. Picture date: Wednesday January 10, 2018. See PA story SOCCER Chelsea. Photo credit should read: Adam Davy/PA Wire. RESTRICTIONS: EDITORIAL USE ONLY No use with unauthorised audio, video, data, fixture lists, club/league logos or "live" services. Online in-match use limited to 75 images, no video emulation. No use in betting, games or single club/league/player publications. - Credit: PA

Norwich City will be subject to the use of VAR – video assistant referees - for the first time at Stamford Bridge tonight.

VAR is being trialled, having been used for the first time in a competitive match for the Brighton v Crystal Palace FA Cup tie earlier this month. The system is in use only at Premier League grounds, which have the technology to get the footage back to the league's TV HQ at Stockley Park in Uxbridge – 14 miles away.

VAR is regarded as the fifth member of the refereeing team which, tonight, will be led by Graham Scott. So how does it work?

According to the FA's official website: 'VAR will be used for clear and obvious errors, or serious missed incidents, relating to specific incidents in three 'game changing' situations – goals, penalty/no penalty decisions and straight red cards, plus mistaken identity for red or yellow cards.

'The VAR automatically checks every such incident and if a clear and obvious error has occurred the referee is informed; the referee can then confirm or change the original decision based on the information from the VAR, or the referee looks at a replay on the side of the pitch. The original decision is only changed if it was clearly and obviously wrong.'

The only person who decides whether the system is to be used is the VAR himself: there is no system in place for either manager or players to call for its use: so anyone drawing imaginary squares with their hands is wasting his time. Even the ref can't ask for help if he is in doubt: the ref has to make their decision first and if the VAR wants to get involved, he can.

The VAR steps in when the game next restarts after the VAR has spotted something.

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Incidents are reviewed in two ways: the VAR can inform the ref of an error that doesn't require the official to see the video – like a goal scored from an offside position. The other the on-field review where the VAR advises the ref to watch footage on a pitch-side monitor and reconsider a decision. In both cases the referee will make a TV signal gesture before communicating the final decision. History shows there is one on-field review in every three games.