Hello neutral_cook. While electric kettles are designed strictly for water, you can certainly use one to heat standard pre-cooked sausages if you follow a few precautions to protect both your health and the appliance.
First, the most reliable and safest method is to boil the water first rather than cooking the sausages while the kettle is actively heating. If you place sausages directly inside while the kettle boils, the fats and proteins can leach onto the heating element, causing burnt residue that can damage the internal components or trigger the dry-boil safety switch prematurely. Instead, fill the kettle with enough water to fully submerge the sausages, bring it to a rolling boil, and then unplug the appliance. Once unplugged, gently lower the sausages into the hot water, close the lid securely, and let them steep in the residual heat.
Regarding the material of the kettle, stainless steel or glass models are vastly superior to plastic ones for this purpose. Plastic kettles readily absorb meat oils and odors, which can permanently impart a sausage flavor into your future hot water and morning tea. Metal interiors are non-porous and will clean up much more reliably.
For standard precooked hot dogs or frankfurters, letting them sit submerged in the freshly boiled water with the lid closed for about 6 to 8 minutes is generally sufficient to heat them completely through to a safe serving temperature. If you are using slightly thicker sausages, you may want to leave them for 10 minutes, but avoid leaving them too long so the casings do not burst.
Cleaning the kettle immediately after use is crucial. Do not submerge the kettle base or electrical contacts in water. Empty out the cooking water, add warm water with a few drops of mild dish soap, and gently sponge down the interior walls. Rinse thoroughly, and then fill the kettle halfway with clean water and a couple of tablespoons of white vinegar or lemon juice. Bring this mixture to a boil once and discard it. The mild acid will break down any remaining grease film and completely neutralize any lingering meat smell.