Must I guarantee what I damaged of my neighbors' belongings as a result of an unintentional error during my electrical repairs, or am I not held accountable for that given that I am a specialist?

If you fix something in a building without the permission of its residents, then you are liable for any damage that results from your action, whether you were negligent or not; because "what results from an unauthorized act is guaranteed." However, if they give you permission to fix it, then you are not liable unless you transgress by doing what is not permissible, or you are negligent by omitting what is obligatory.