When it is necessary to use dissimilar metals in contact, these shall, if possible, be so selected that the potential difference between them in the electrochemical series is not greater than 0.5 V. If this is not possible, the contact surfaces of one or both of the metals shall be electroplated or otherwise finished in such a manner that the potential difference is reduced to within the required limits. If practicable, the two metals shall be insulated from each other by an approved insulating material or a coating of approved varnish compound