Abstract: In an oil-filled electric power cable installation, the oil is pressurized using oil tanks connected to stop joints in the usual way, and an oil-purifying unit is connected between stop joint and tank. The oil-purifying unit comprises a column or bed of a molecular sieve or other active purifying agent with particle sizes, and interstice sizes, big enough to limit the pressure drop across the unit to 20 kN/m.sup.2 at the most. This eliminates the need to provide bypass passages controlled by valves as the oil can flow both ways through the column.