Working on Energized Underground Cables

DOE-HDBK-1092-98

19. All cables and insulated wires that do not have grounded conducting sheaths or shielding

should be treated as bare conductors. They shall be considered energized unless approved

methods have been used to determine that they are deenergized. Barricade or cover these

conductors with protective equipment or devices that will be within reach of a worker's

position.

20. Where multiple cables are present the cable to be worked on shall be identified by electrical

means unless its identity is obvious. [See 29 CFR 1910.269(t)(5)]. Where cable has one or

more abnormalities that could be an indication of an impending fault, the defective cable shall

be deenergized, except when service load conditions and a lack of feasible alternatives require

that the cable remain energized. In that case, employees may enter the manhole if they are

protected by the affects of the failure by flash blankets or other devices capable of containing

the adverse effects of the fault. [See 29 CFR1910.269(t)(7)].

7.10.2 WORKING ON ENERGIZED UNDERGROUND CABLES

In general, work should not be performed on energized underground cables. However, strictly

external work, not requiring an appreciable change in location of the cable, may be performed under

direct supervision. Energized cables that are to be moved shall be inspected for defects. [See 29 CFR

1910.269(t)(6)].

7.10.3 TERMINALS OF UNDERGROUND CABLES (POTHEADS)

Before work is started, the overhead line connections to a cable terminal upon which work is to be

performed should be either:

1. Deenergized and grounded or

2. Disconnected and covered with protective equipment.

7.11 FERRO-RESONANCE

Ferro-resonance can generate overvoltages of up to 12 times line-to-ground source voltage upon

opening of a single-phase device or a poorly synchronized three-phase device. Violent failure can

occur, exposing personnel to the high-voltage failure and accompanying conditions. Ferro-resonant

conditions can result in damage to lightning arresters, switching devices, buried cable, transformers,

and associated equipment.

Ferro-resonance can be initiated when all of the following elements are present and the switching

means at dip point or takeoff is either a single-phase device or an unsynchronized three-phase device

that does not operate all phases within 1/2 cycle:

1. System grounded at the source but with no ground at the transformer bank, such as a

transformer or transformer bank connected delta on a grounded-wye system.

2. Shielded cable or overhead conductor length sufficient to create the capacitance necessary.

7-27