I'd also be concerned that the insulation appears to be caught under the terminal - cant be sure at that angle but needs checking to ensure a good connection.
Its got to be the potential difference between natural earth and the pme earth supplied at the income.
I think the only way around this is to change the hot tub earthing system to a TT arrangement and make sure the RCD/rcbo is double pole.
Even if it were ok at a 300mm depth (and I don't agree it is) how does the cable enter and exit the ground, is this part mechanically protected? Also, did the electrician cover the cable with yellow hep tape?
They can be a very useful tool if used sensibly and nor wholly relied on.
Edit: Only thing is in the video, he didn't prove/test the unit before applying it.
For the amount of cable shown in the video, tray would have been ideal, admittedly not normally required for residential. Have used it in the past with a large house with many cable runs leading back to the CU.
Edit: having said that, it didn't stop wet pants laying his unclipped plastic pipes...
I get where you are both coming from and you are probably right, but there's always a chance a firefighter may be required to pull down such a ceiling to access a fire - maybe over the top, but the time and cost it takes to install a couple of clips is worthwhile IMO.
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