Funny, but the LOTG are quite specific on this point:
Law 12: Fouls and Misconduct
SENDING-OFF OFFENCES
A player, substitute or substituted player who commits any of the following offences is sent off:
- denying the opposing team a goal or an obvious goal-scoring opportunity by a handball offence (except a goalkeeper within their penalty area)
- denying a goal or an obvious goal-scoring opportunity to an opponent whose overall movement is towards the offender's goal by an offence punishable by a free kick (unless as outlined below)
- serious foul play
- biting or spitting at someone
- violent conduct
- using offensive, insulting or abusive language and/or gestures
- receiving a second caution in the same match
- entering the video operation room (VOR)
That extract is from the FAs own current 19/20 website. It's amazing that referees aren't aware of it. Perhaps somone should ask Mike Riley if he, too, is aware of it, as there appears to be a "clear and obvious" training need required by all the employees of PiGMOL currently officiating in the Premier League.