Earth required or not on heat call wiring center

I was setting up a heatmiser UH8 wiring centre tonight and the boiler has a 230v output terminal for connecting to ac devices. The call for heat on the boiler is L/SW Live on another terminal and is volt frree

My plan was to use a 5 core from the boiler to the wiring centre, but when I looked at the heatmiser wiring diagram, its showing an earth between the boiler and wiring centre for the heat enable connection.

Is this earth necessary for the boiler enable? I've seen other wiring centres don't have this - even other heatmiser wiring centres dont.

Cheers

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