( ( ( ( ( Rubber ( is ( a ( good insulator ) ) ) ) , ) and ) ( that ( 's ( why ( rubber ( is ( used ( to ( ( cover wires ) ( in ( an ( electric cord ) ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NNP Rubber)) (VP (VBZ is) (NP (DT a) (JJ good) (NN insulator)))) (, ,) (CC and) (S (NP (DT that)) (VP (VBZ 's) (SBAR (WHADVP (WRB why)) (S (NP (NN rubber)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB cover) (NP (NNS wires)) (PP (IN in) (NP (DT an) (JJ electric) (NN cord)))))))))))) (. .)))
Rubber is a good insulator, and that's why rubber is used to cover wires in an electric cord.
(ROOT (S (NP (JJ Electrical) (NNS wires)) (VP (VBP are) (ADVP (RB usually)) (VP (VBN covered) (PP (IN with) (NP (NN plastic) (CC or) (NN rubber))) (S (VP (TO to) (VP (VB insulate) (NP (DT the) (JJ electrical) (NN wire))))))) (. .)))
Electrical wires are usually covered with plastic or rubber to insulate the electrical wire.