The biggest safety precaution to take when de-scaling an electric kettle with vinegar is to ensure that the heating element does not get wet. In addition, some kettle manufacturers note that vinegar should not be boiled in the kettle, but instead added to the water after it has boiled.
In most cases, letting the vinegar and hot water mixture sit in the kettle for a few hours should be enough to remove all but the most extensive scaling. For more severe scaling, it may be necessary to let the solution sit in the kettle overnight. If this still does not remove all of the scaling, the inside of the kettle can be scrubbed gently using a non-metallic pad. However, it is important to avoid scrubbing the heating element inside the kettle, as this could potentially damage it.
In addition to white vinegar, other methods can be used to de-scale an electric or stovetop kettle, including lemon juice or citric acid. All three methods use a fairly strong acid to loosen and dissolve the mineral deposits that cause scale buildup.
It is generally recommended to de-scale a kettle every one to three months, depending on how hard the local tap water is and how often the kettle is used.