When I turn my electric shower off, there is no mains pressure inside the unit past the solenoid valve.
I considered, perhaps erroneously, that my shower by example is effectively vented and so didn't consider it to be at mains pressure.
I understand that now. I am more familiar with low power units that store 10 litres or so of water.
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Yup, there is a table of comparable outputs for different sizes of heater here:
When you consider that the total capacity of many domestic electrical installations in the UK is 24kW with a single phase supply, you can see why three phase is required.
Electric showers have a safety a pressure release system that operates in fault conditions - albeit one time operation.
And possibly the heater cannot be energised unless there is sufficient flow of water
When it is "off" then the shower head side has no pressure - but the inlet to the shower is normally a cold water main. (some can run from tanked supplies of cold, but that is not the norm)
Yup, those are often small unvented cylinders with an immersion heater. (you can go upto 15 L and not have to meet all the controls requirements for unvented cylinders)
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