,.. in their pressurized cables to keep water out in case of a crack. Ever see those orange and red cylinders at the base of a pole? That's a temporary measure until the leak is found and fixed ... then the nitrogen tank is removed and the compressors in the central office take over. Ideally, 9 pounds of pressure is pumped in at the C.O. For every pound of pressure maintained in the field (or underground) will keep out something like 3 feet of water.