Almost all single point instantaneous water heaters use a pressure switch (not a flow switch), which is by far, a very good idea.
Take a look at the specs of a heater made by the manufacturer of my current heater. It looks different but it's the same animal. I copied this info directly from the site so grammatical errors do not belong to me. My comments are in yellow.
Link:
Water Heater FRESH 4.5
1. Heater Properties.
The heater is made of a combinations of Nylon66 and Fiber, rust resistant, can withstand high temperature (< 200 C) and pressure (< 6 bar), and a good insulator.
2. Rated Power.
FRESH 4.5 rated power = 4500 Watts.
3. Safety System.
ELCB - prevents electric shock. It is installed within the machine and can cut the electricity at 0.1 second at both lines (line and natural) after the electricity starts leaking. This has been tested with EMC magnetic under UL (IEC Standard). This way you can be sure of your safety while machine is operating.
I don't know which IEC Standard this company is referring to but it is a requirement that any earth leakage device is able to operate within 20 milliseconds (0.02 seconds).
4. Points of Use.
For single of use.
5. Temperature Control.
The temperature is adjusted by electronic system.
6. Shower.1 function shower head to suit your need and relaxation during shower.
7. Installation.
Installed by MAZUMA experienced staffs.
8. Guarantee.
Heater - 2 year warrantee. Unit - 1 year warrantee.
9. Other Properties.
8 level of safety system;
1.
Pressure Valve with Micro Switch - tool for controlloing and testing water pressure at water inlet. It sends electrical signal to the heater via micro switrch. The system will be cut off automatically once no water is coming in.
2. Linear Control and Adjustment; control temperature by using the button to adjust the electricity given to heater.
3. The heater is made of a combinations of Nylon66 and Fiber, rust resistant, can withstand high temperature (< 200 C) and pressure (< 6 bar), and a good insulator.
4. Thermostat is the device which prevents the temperature from exceeding 57C. It operates by controlling the point where electric current is being distributed to the heater. Thus you can be assure of no scalding.
5. ELCB - prevents electric shock. It is installed within the machine and can cut the electricity at 0.1 second after the electricity starts leaking, even if the current is < 15 milliamps.
The minimum requirement is 30 milliamps for domestic purposes. This 15 mA rating is useless without a fast trip time (i.e. less than 20mS).
6. Wires are covered with PVC which can withstand temperature as high as 105 C, withstand electrical current as high as 600 V without getting burnt.
7. Inlet and outlet pipe - made of a combinations of Nylon66 and Fiber, rust resistant, can withstand high temperature (< 200 C) and pressure (< 6 bar). It is a good insulator so you can be 100% sure of no electrical leakage. It also has filter at inlet pipe to filter out dirt from water before running into the machine.
8. The lid is made of grade ABS plastic which is very durable, high temperature resistant (up to 120C) and fire resistant. These properties make the lid to be a good insulator when there is electricity leakage.