Have read a lot of stuff here and other places on RCD's and the like and it's all good. 30 mA or less cut-off seems to be a common recommendation.

I understand that if there's a residual current of say 30 mA and it lasts for 25 milliseconds then the device will switch off the power. Just say that the leak gave an instantaneous short of 1 amp. Does the device still disconnect at 25 milliseconds or does it shut down more quickly?

Still on RCD's. Have had water heater installed with an RCD switch on the power-line. It has just dawned that it may not be a safe placement as the switch is located on the wall in the bathroom, (well above the shower head). I'm wondering if this is likely to be potentially unsafe. The fact that the installer didn't connect the unit to the earth system makes me wonder.

Have read a lot of stuff here and other places on RCD's and the like and it's all good. 30 mA or less cut-off seems to be a common recommendation.

I understand that if there's a residual current of say 30 mA and it lasts for 25 milliseconds then the device will switch off the power. Just say that the leak gave an instantaneous short of 1 amp. Does the device still disconnect at 25 milliseconds or does it shut down more quickly?

Still on RCD's. Have had water heater installed with an RCD switch on the power-line. It has just dawned that it may not be a safe placement as the switch is located on the wall in the bathroom, (well above the shower head). I'm wondering if this is likely to be potentially unsafe. The fact that the installer didn't connect the unit to the earth system makes me wonder.

The 30mA or less cut-off is not a recommendation...it is mandatory in IEC countries (for "personal" protection only).

Most RCD's will trip at around 50% to 60% of their rated current. E.g. a 30mA RCD will trip between 15mA & 20mA. The 30mA rating is a "maximum" trip current whereby the device MUST trip. The trip will also vary a little. Again, the RCD MUST trip WITHIN 20mS.

It's never a good idea to have anything electrical in your bathroom due to the potential safety hazards.

Your instantaneous hot water MUST be properly earthed regardless of whether or not you have installed an RCD for it or regardless if it has a "built in" RCD. I would place this on your "high priority" list of things to get done.