QUALITY OF TRANSMISSION IN COMMUNICATIONS CABLES

The presence of water (humidity) inside the communications cables is the main cause of deterioration in the transmission quality of outside telephone plants.

The origin of that humidity can be the result of different factors that may range from the manufacture of the cable, its manipulation and installation, and splicing tasks, to the usual malfunctions of the pressurisation systems that can inject humid air, together with accidents, which due to damage or other factors may allow the penetration of water.

Water’s mere presence inside the cable, whether as room temperature steam (humidity) or as liquid, with its high dielectric constant (80 times more than dry air) alters the mutual capacity between pairs and generates dielectric loss which increases together with the transmitted frequency.

But also, depending on residence time and its location, humidity or water generate the deterioration of the electric insulation and the unbalance between the wires of the pairs, limiting or preventing the use of XDSL systems.

Therefore, it is very important to reduce the content of water inside cables to the minimum, and there is a practical limit below which the cable can operate without any problem.

"When low electrical insulation is verified, resident humidity is superior to 2g/Kg (with the capacity of reaching values superior to 10g/Kg.). When that humidity is removed and the value falls down under that limit, the cable regains its original manufacture electric insulation values."

SEC-VAC studied this behaviour and developed two methods called DACTIF and RACT. With these methods it is possible to regain the transmission quality of a multipair cable that has been affected by humidity or water inside it, while keeping it functioning, that is to say without interrupting the service to the customers.

The implementation of RACT on a cable in operation enables to remove the humidity or water inside it, recovering in this way the spare pairs that were not in use and putting them in active service, and taking the working pairs to their original manufacture levels.

This operation is carried out with specially designed equipment plugged to the cable through the valve on a splice. In this way, resident humidity or water is removed in the form of steam.

This steam condenses and the amount of water removed is measured in the time unit which enables to monitor process development and possible changes in the operation mode.

The implementation of the RACT method requires a previous diagnosis which will determine the locations (splices or blocks) where the drying equipment will have to be used.

DACTIF is the method that delivers that diagnosis through the measurement of a group of thermodynamic parameters such as humidity content, pressure and temperature inside the cable, obtained from all the splice valves and other access points. 

The cable diagnosis is complemented with the routine electrical measurements carried out by the telephone operator (insulation and capacity resistance) on a representative sample of spare pairs.

Humidity, pressure and temperature measurements are taken with equipment designed and built by SEC-VAC in a non-invasive routine that produces no disruption in the service to customers or the conventional pressurization system.