34 Pa. Code § 39.143
All conductors over 750 volts, and ungrounded bare conductors over 300 volts to ground shall be isolated by elevation as required by § 39.106 (relating to isolating live parts by elevation), unless guarded in accordance with § 39.144 (relating to guarding conductors), so that no person may inadvertently come in contact with them. Buses and bus structures and line connections thereto, installed in accordance with §§ 39.104 and 39.160 (relating to working space about electrical equipment; guarding live parts of switches and automatic cut-outs not installed on switchboards) in suitable locations specially arranged for such purposes, shall not be required to be so isolated.
34 Pa. Code § 39.143
This section cited in 34 Pa. Code § 39.185 (relating to guarding live parts).