Power shower installation

[koster]

Staying at a friend's house over Christmas, I remarked that it was annoying how you couldn't rinse the bath with the shower because the shower hose was 'tethered', i.e. it went through a hole in the wall assembly. The shower had been recently installed, and the plumber had said that they had to do it that way, but that it could easily be changed (by the householder, after the plumber had gone) by unscrewing the shower head, taking the hose out of the hole, and reattaching the shower head.

I then remembered reading something in the installation instructions for my own power shower, saying that it had to be installed so that the shower head, at its maximum extent, was a certain distance from the bottom of the bath.

What is the reason for this regulation?

[koster]

Ah, I found the relevant paragraph:

"To comply with water bylaws, the height of the shower should be such that when the sprayhead is hanging loose on the flexible hose, the sprayhose does not drop below the spill over level of the bath or shower tray which could lead to contamination. However, there may be installations when this requirement cannot be met. If the sprayhead can be placed within a bath, basin or shower tray, then a double check valve, or similar device, must be fitted in the supply pipework to prevent back-flow."

But I still don't really understand. I mean, even if you dipped the sprayhead into the bath water, is the bath water really going to rise up the hose and into the public water supply?

[scotman]

It is highly unlikely but not impossible, hence all the safety features. All items running off mains pressure water should have these fitted.

[magicmerlin]

Sometimes, when showers are switched off they can temporarily reverse flow of water, this could then create a syphon effect if shower head is in water. Hence the need for double check valve.

[buildersmate]

There also used to be a BT regulation that only BT technicians could touch the wiring of your phone. That was pretty unenforceable too, as well as taking risk management rather too far.
I'm aware of the regulation but just ignore it - it looks really silly having that tethered arrangement.