Electric Cables in Water?
Using cables buried or installed in underground ducts in waterlogged conditions
Contractors need to think carefully about the type of cable sheath selected for LV (<1kV) or MV (1kV-33kV) cables if the cable is to be buried in waterlogged ground or installed an underground duct subject to flooding.
While Prysmian’s Low Voltage and Medium Voltage armoured cables, with both LSOH and PVC sheaths, are suitable for installation in ‘free draining soils’ they are not recommended for installation in permanently waterlogged ground, such as in clay soils or below the water-table. In free draining soils a cable can be expected to achieve Prysmian’s estimated minimum design life of 25 years for a cable fully and continuously loaded. However, in permanently waterlogged ground a cable’s design life will prematurely shortened by water migrating through the cable sheath and potentially corroding the cable’s armour.
PVC and LSOH Sheathed Cable
Where PVC or LSOH sheaths are used in applications where they are likely to be saturated for significant periods it would be advisable to have a regime for periodic testing. Additionally, where the cable’s armour is the sole means of earth protection then the addition of a separate copper earth is highly recommended. For similar reasons a limited number of specifications may also require copper wires to be included in the armour.
MDPE Sheathed Cable
Medium Density Polyethylene (MDPE) sheaths have up to 20 times greater resistance to the migration of water through the cable insulation. They can be considered for installation where wet conditions exist because they are more likely to achieve a full design life. However, the applications for MDPE sheathed cables are limited: MDPE is excluded from use in all LSOH cables and LV cables to BS 5467. What’s more, the poor fire performance properties of MDPE may make it unsuitable for installation inside of buildings.
In instances where water has penetrated a cable’s insulation it should be noted that cross-linked polyethylene or ethylene propylene rubber grades of insulation have good resistance to voltage failure in the presence of water
MV copper wire screen cables
For single-core distribution cables with rated voltages of 11kV to 33kV, an MDPE sheath is used in most cases since BS7870-4.10 precludes the use of a PVC sheath. Where this is the case these cables will have significantly more resistance to water penetration and be more suitable for wet conditions. In addition their copper screens are less susceptible to corrosion than that of steel or aluminium armour. However, it is still likely that wet ground will reduce the life of the cable below that of a cable installed in dryer ground conditions.
Where contamination is present then further investigation may be required, which could include chemical analysis of the surrounding soil. Depending on the results, it may be the case that none of the sheath materials mentioned above will be suitable. Where this is the case, talk to Prysmian. In the case of contamination by hydrocarbons, for example, then cables with lead sheaths specified to the Engineering Equipment and Materials Users Association (EEMUA) publication 133 may be a suitable alternative.