It makes sense to me that the shower would have some means to protect itself from damage if there is no water. How else do you test things like this rare as it may come up..
Mains fed shower assumed.
Yes, you can do a test by having the power switch to High (normal setting) and the temperature contol to coldest, switch on the shower, the shower temperature should be ~ 35C (at 8LPM flow and 18C mains), then keep opening other cold water taps slowly until the water runs cold, this will happen at ~ 3.5 to 5.0LPM ish, there are also two thermal cut outs, (TCOs) one, self resets at 48C, the other, to protect the shower, operates at 75C and is non resettable, the TCO has to be replaced, the 48C TCO will (on most showers now), switch out both elements) and at the present 18C mains temp will operate at a flowrate of 4.3LPM (9.0kw shower) so may switch out the elements before the pressure switch does, if neither this or the pressure swich doesn't do the switching off then the second TCO will switch off the elements at 75C.
Edit: Might be better to have the power setting to Eco, one element on, with the temperature control to coldest for the above test, the shower temperature will then only be 26C but the flowrate would have to fall to 2.15LPM before the TCO will operate at 48C so the pressure switch should operate well above that flowrate and switch out the element.
When I installed a 9KW Triton T80Z, 2 years ago, on doing the above test, the elements switched out at 4.1LPM and back in at 5.0LPM.
