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OSP Magazine MARCH 2007

A reader of mine asked the following question by email:
“I was wondering if you knew what the 2005 National Electrical Code was talking about in Article 310.13 FPN. It states “Thermoplastic insulation, where used in DC circuits in wet locations, may result in electro-endosmosis between conductor and insulation.” What is electro-endosmosis?
Thanks, John

John,
Sure I do. First, the definition: in electrochemistry and physics, electro-osmosis, also called electro-endosmosis, is the motion of polar liquid through a membrane or other porous structure (generally, along charged surfaces of any shape and also through non-macro porous materials which have ionic sites and allow for water uptake, the latter sometimes referred to as “chemical porosity”) under the influence of an applied electric field.

Now, just what the heck does that mean to you? In the 2005 National Electrical Code, Article 310.13 FPN is referring to entrance cables to buildings, but electro-osmosis affects all air-core cables where there is an ingress of water into the cable core and DC voltage is present.

When water enters a cable sheath, the air spaces between conductors is replaced with water. Plastic insulation has miniscule bubbles in the insulation that appear in the manufacturing process. These miniscule bubbles occur every foot or so in the insulation on both Tip and Ring conductors.

The first services affected by water are the DSL services. As soon as enough water enters the cable core, service is affected. An example is when a cable is transporting ADSL, ADSL 2+, VDSL, T1, and HDSL at the same time. T1 and HDSL must be placed in binders separate from any ADSL, ADSL2+, or VDSL. With water present, coupling occurs from sub-unit to sub-unit and service is disturbed and in many instances wiped out.

On the Plain Old Telephone Service (POTS) side of the house there is typically a six-month interval between the ingress of water to customer complaints. The first complaint from the customer is, “I have static on my line. It sounds just like frying bacon.”

Electro-osmosis begins when a DC battery is placed on a cable pair where there is water in a section. The POTS battery supply is set up with the negative side of the battery attached to the Ring conductor and the Tip conductor and ground connected to the positive side of the battery.

Water is pulled through the insulation by the minus potential on the ring conductors that are working. When enough air bubbles fill up with water electrolysis begins. Metal from any and all positive conductors is sacrificed to working ring conductors.

If the cable is single sheath with exposed aluminum or copper alloy, the shield is sacrificial to all working ring conductors. The shield will be eaten away until there is no shield continuity and then associated Tip conductors will be affected and eventually go open.

If the shield is aluminum, the aluminum will become free floating aluminum oxide. That is the milky white substance a technician sees in the water when a wet section of air-core PIC is opened.

If the cable is a PAP or PASP cable there is a plastic barrier between the conductors and the shield, so tip conductors will be sacrificed and the affected POTS customers will lose dial tone.

Example of Electro-Osmosis
Following is an example of electro-osmosis in a wet section of air-core PIC in a distribution plant:

Water enters the core of the cable and electro-osmosis starts. It takes around six months for electrolysis to begin. When the pair balance is lost, service is affected and customers complain.

A service technician is dispatched and isolates the fault to the distribution plant and his pair shows indicated battery on the Ring conductor and the Tip conductor clear of faults. The technician moves the customer to a tested good distribution pair.

Electro-osmosis starts on the pair that the technician cut to and ceases on the pair that the technician cut from. Within four days to two weeks the customer calls back and complains, “I had static on my line, someone came out here and fixed it, and now it is back!”

Cut to Clear Isn’t the Right Solution
Using the cut-to-clear policy, telcos will drive customers into a black martini rage with unacceptable service for three to five years before they finally replace the wet section. A proactive approach must be used and the wet section of cable replaced as soon as possible.

The pattern of symptoms for a section of PIC cable is battery on Ring conductors only. Tip conductors test clear. If you see battery on a Tip conductor and the Ring conductor clear, that pair was reversed. For some odd reason, battery doesn’t know color.

When the problem is isolated down to a section and the bonds are removed from the shield at both ends, battery from -15 to -30VDC will be present from the isolated shield to ground.

Use an open meter to determine the percentage of water in a section. The open meter will show an increase in footage due to the increase in capacitance. In a partially wet cable, this increase exists only in the wet area.

To analyze a suspected wet section, measure and record the actual distance of the section. Next, measure and record the length of an open pair with an open meter. Subtract and divide the difference by 2. For example, if a 700-foot section of cable showed 1,100 feet on an open meter, the 400-foot difference would indicate that there is 200 feet of water in that section if it is full of water.

A time domain reflectometer can then be used to find the start of water from each end indicating the dry sections and the difference from the actual section length is the exact amount of water.

Electro-endosmosis is a mouthful, but now you know the reason why water in sections of air-core pick affects quality service.

(My thanks to Wikipedia (www.wikipedia.org) for their definition of electro-endosmosis.)

Signing Off
Thanks to the reader who brought up this interesting question. I hope you will think of your problems and let me know about them. I won’t use your name (unless that’s acceptable) and you will be helping your fellow technicians learn from your challenges (I’ve always included women when saying “fellow”—let me know if there’s a more PC term.). Contact me at dmccarty@mccartyinc.com or 831.818.3930.

About the Author - Don McCarty

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