When twin and earth is installed in a wall chase it is almost always laid flat on the wall.
Suppose the chase was cut just wide enough that the cable sat in it edge wise and the installer took care to ensure the Neutral was upper most. Then if someone did drill into the cable the drill would hit Neutral first, then Earth and hopefully the RCD would have started to trip before the drill reached the Live.
Maybe if there was no RCD then the drill would short Live to Neutral and/or Earth and trip the MCB
Suppose the chase was cut just wide enough that the cable sat in it edge wise and the installer took care to ensure the Neutral was upper most. Then if someone did drill into the cable the drill would hit Neutral first, then Earth and hopefully the RCD would have started to trip before the drill reached the Live.
Maybe if there was no RCD then the drill would short Live to Neutral and/or Earth and trip the MCB