TELEPHONE LINES Electronic equipment connected to telephone lines are very sensitive and they do withstand a very low voltage. CPT solutions are available for formats such as DIN rail, aerial, KRONE type and 19" Rack. Aerial protectors - RJ connector The MCH series of surge protective devices has been designed for protection against transient overvoltages in aerial telephone lines with RJ connecting terminals which provide for insertion of the SPD to the connecting cable itself.