The phase to neutral supply voltage is controlled by the power generation companies and utilities by using transformers with on-load tap changers. Depending on the region you can find different supply voltages.
Europe is following the IEC 60038, mainly with distribution in TN-systems. The voltage is defined with 230 V ±10 % and the frequency is 50 Hz ±0.2 Hz. Many other countries in Africa and Asia are following the same limits.
Considering the European limits, the voltage can vary between 207 Volts and 253 Volts. With increasing voltage, the power for resistant load is rising. (P = I2 * R)
It’s not harmful, the water in the water cooker is faster boiling. The used energy is the same. Home appliances are designed to operate within the voltage limits.
Exceeding the limits can lead to damage or malfunction, e.g. conventional light bulbs might stop to provide their originally intended function earlier. I can’t prove this, since several years all bulbs in our region have been changed to compact fluorescent bulbs. These bulbs come together with introducing additional harmonics in the grid, but this is another story.
What happens when the voltage drops?
The water cocker takes less power and needs more time. All resistive loads have this behavior. Inductive loads e.g. electro motors will pull more current. This leads to additional heat and can damage the home appliance when the voltage is below the specified limits.
Industrial motors have under voltage protection. For refrigerators and air-conditions, it depends on the manufacturer.
CLOU energy meters with supply control switch can offer additional protection against damage due to over- or under voltage. These meters are programmable for the limits (default is ±20 %) and for the disconnection time. The meters reconnect after the given time if the voltage is within the limits.
The meter display or the indication on the user interface unit is like this. It toggles every two seconds with the remaining waiting time for reconnection.
Thank you for taking a look.
The Over- and Under Voltage Disconnection