When an addition is made to an existing electrical installation which has been installed in any approved system of wiring, if the existing installation is found to be in a safe operating condition it will not be necessary to rewire the existing part. In case the existing installation is found to be not in a safe operating condition, the unsafe wiring must be removed and rewired in accordance with the provisions of this chapter before a final certificate of approval will be issued. (Prior code § 16-1-21)