I grew up calling them industrial plugs..
Depending on who I’m talking to though, also call them commando plugs..
At wholesalers will ask for them by colour, voltage, current and number of pins... they still get it wrong haha
So there has been 4 isolators in 4 years..
Your connections in isolators have been ok..
You seem to be happy with the rating of the isolators..
So I would be now checking out the shower..
How do you know for certain they haven’t been switching it on/off under load?
I would test shower element...
If I went to a job where a shower was up to a 4th isolator in as many years and I knew my previous connections were good then I would also inspect the shower and iland test it for any problems.. especially if the shower is only 25 amps
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