Aberdare Cables warns that ‘graphite coated’ and ‘anti-electrolysis’ cables are not simply standard cables with graphite spread onto the outer sheath. ‘Graphite coated’ cables are required to undergo a sheath integrity test at manufacture and are designed with material radial thicknesses specifically for that purpose.
It should be noted that testing the sheath integrity at elevated voltage levels in excess of 5 kV dc on a cable not designed for that purpose will very likely cause serious damage to the cable.
This brief provides some background to the requirements and techniques applicable to this type of cable.
When to use anti-electrolysis graphite coated cables
When electric cables are to be installed in areas where the presence of ground currents is suspected or known to exist, it is important to ensure that these ground currents do not enter the electric cable.
The metallic elements in the cable (i.e., armour or lead sheath) would normally provide a lower impedance path than the ground. As a result, any holes in the outer sheath of the cable may allow these ground currents to flow in the lead or armour of the cable in preference to the ground. Currents flowing from the ground into the cable lead sheath or armour, and then exiting at some point further down the cable, may cause severe corrosion of the lead sheath or armour.
In order to guard against this danger, electric cable may be ordered with what is called an ‘anti-electrolysis design’. Such cable is often referred to as a ‘graphite coated’ cable. (Note: not all graphite coated cables are necessarily of an anti-electrolysis design). Graphite coated Anti-electrolysis cables differ from standard cables in a number of respects:
- The bedding and sheath are both designed to withstand voltage across itself, as well as normal mechanical considerations.
- The bedding thickness on graphite coated cables is normally specified thicker (2 mm minimum) than the bedding thickness on standard cables. The reason for the enhanced thickness is to increase the mechanical protection offered.
- The sheath thickness on graphite coated cables is normally specified thicker (3 mm minimum) than the sheath thickness on standard cables. The reason for the enhanced thickness is to reduce the chances of the sheath being damaged by stone penetration or other damage likely to occur during installation.
- As the name implies, graphite coated cables are coated with graphite.
This graphite coating is applied in order to allow the manufacturer to test the electrical integrity of the sheath while the cable is on the drum. Once the cable is installed underground, there is less need for the graphite, because an outer electrode is provided by the ground in which the cable is buried.
During factory testing, a voltage of 10kV dc is connected between the cable armour and the graphite layer. A voltage of 4 kV dc is applied in a separate test across the bedding, between the cable metallic screens and the cable armour.
The graphite ensures that the sheath voltage is applied evenly across the whole cable circumference and length. This voltage is maintained for one minute, and any leakage current is recorded. Leakage current is an indication that the sheath is damaged, and that current is flowing from the armour to the graphite through the damaged portions of the outer sheath or bedding.
Note: Graphite coated cable types of a non-Anti-Electrolysis design have standard bedding radial thicknesses and only undergo a voltage withstand test of the outer sheath in the factory.
Sheath integrity testing
It is important to note that this test cannot be carried out on energised cables. A sheath integrity test is normally carried out after installation, and sometimes at 6-month intervals thereafter. After installation, the cable armour is disconnected from earth at both ends, and a dc voltage of 5 kV is applied between the cable armour and the sub-station earth or an earth spike. (Depending on cable type, there is some danger of damage to the bedding, and caution should be exercised when carrying out such testing).
In order to prevent overvoltage to the bedding layer, all metallic layers under the bedding should be raised to the same voltage as the armour. Connecting the copper tapes of XLPE cables or the lead sheath on PILC cables to the armour, may achieve this. After applying the dc voltage, the leakage current flowing through the outer sheath is recorded. (Note: The graphite coating must be cleaned off the outer sheath for a distance of +-300mm from both cable ends in order to prevent tracking).
When a sheath integrity test is to be carried out on an already installed cable, the precautions above must be observed, i.e., the cable must be de-energised, and the armour and termination earths must be disconnected. After making the necessary connections as described above, the voltage of 5 kV dc may be applied. The leakage current is again recorded, and the reading can be compared with the original reading taken after installation. Any major current leakage, or an increase in the value recorded, is an indication that the sheath has been punctured. Fault location and repair to the outer sheath must then be carried out as soon as possible to stop damage to the lead sheath or armour layer.
If a 10 kV sheath integrity test is carried out on the outer sheath of a standard cable, irrespective of whether it is coated with graphite or not, it will most likely fail. (i.e., the outer sheath will be punctured). Not only will it fail, but the applied voltage will in all likelihood burn holes through the standard PVC sheath, and portions of cable may even catch fire. Note that these failures may occur, irrespective of whether graphite is coated onto the cable or not.
Clearly this is not an acceptable situation, especially where anti-electrolysis graphite coated cable was specified because of known ground currents.
NB: It is important to completely remove the graphite from cable ends to avoid contamination during jointing and terminating.
Enquiries: +27 (0)11 396 8107