Two things before you use it…
First check that any socket you intend to use does actually carry an earth connection (just because it might have an earth pin doesn’t mean that there’s wire going anywhere from it). If you have a meter measure the voltage at the sockets between phase & neutral & then between phase & earth - they should be very similar. It’s worth checking that you’ve not got anything more than a a few ghost volts between neutral & earth too.
Secondly you must ensure that those sockets are somehow protected via a 30mA différentiel. In this case I suspect it will be via one that controls a number of circuits back at your main installation, unless there is one elsewhere in the stables. Check that it actually works, via the test button.
After your immediate freezer use please cut off the supply & we’ll never talk of it again 