Electric showers heat cold water on demand via an integral heater tank.
Should the water inside the heater tank become excessively hot, a safety device known as the thermal switch is activated. This isolates the electrical current to the heater tank and allows the water to cool. Once the water has cooled to a safe temperature, the heater tanks electrical supply is restored, so the shower will again deliver heated water.
Should you live in a hard water area you will experience limescale formation on certain parts of your shower, particularly the shower head.
A build-up of limescale in the shower head restricts the flow. Therefore, water cannot escape quickly enough through the shower and remains in the heater tank for longer than necessary. The water will then overheat and, as explained above, cause the thermal switch to operate.
To remedy this problem you should ensure that the shower head is descaled on regular basis, using a suitable descalent solution.
A kink or blockage in the shower hose will also restrict the flow and result in the same symptom.
Alternatively, the temperature control dial may have been set on too high a level and is therefore causing the water to overheat.