The protective earth (PE) is a fail safe circuitry with the only purpose to protect persons against potential electric shock. In contrast to the functional earth the diameter of the protective earth wire must never exceed the diameters (square area) of the neutral and line wires of the cable: for a 5x10mm² power cable with N, L1, L2, L3 and PE, all five inside wires have the same diameter and wherefore the same square area. For large gauge cables like 50, 70 or 95 mm² the protective earth wire can be theoretically even smaller than the other four wires. For that reason and the different connecting points the protective earth cannot work as a good functional earth for an audiovisual system or any studio application!
|symbols for earth / ground|
|protective earth PE||protective earth PE|
|functional earth FE||chassis ground||signal ground|
The functional earth is used for interference suppression. It is a low-impedance current path between the circuitry and earth and is only used to improve the equipment's performance and not (!) for protective purposes.
Functional earth should be established for the low frequency and the high frequency range. At the low frequency range the most important factors for a satisfactory functional earth are the cross section area of the earthing cable and the cable length to ensure a minimum over-all cable impedance. At the high frequency range a short cable length, high outer conductor surface (skin effect) and low inductance are important. Also important is a large connecting surface between the circuitry and the earth cable. The cable length is more critical in the high frequency range than in the low frequency range.
Because the outer conductor surface is more important for the high frequency range than the actual cross section of the cable, flat cables are more effective than round cables.
For regular interference suppression for audiovisual equipment and installations the functional earth wire should be 95 mm² with the shortest cable run to the building's earth connection.
|functional earth cable 95 mm², |
|flat functional earth cable for
high frequency interference suppression