water logged buried 220 cable

[Michael Roberts]

What do people believe the life expectancy is of a 220v power cable that has been buried in a pvc water logged tube for 4 years in the tropics?

During the rainy season the tube is full of water and the dry season it is subject to humidty as I assume the tube dries out.

Does anyone have experience how these cables age? Will they leak intermittently or will they leak continually?

Sorry about the generality of the question.

 

[waross]

Generally a wet cable may last for years in a wet or damp location.
The submersible pump in my deep well (about 500 feet) has lasted for over 30 years. Normally about 100 feet of cable is submerged.
On the other hand, a cable may experience degradation and early failure when subjected to wet or damp locations.
Some high voltage cables experience a type of water degradation when energized at a high voltage in the presence of water or moisture. This is called "Water Treeing". The same cable may last indefinitely at lower voltages.
What is the type or designation of the cable?
Is the cable rated for dry locations or rated for wet or damp locations?

Does anyone have experience how these cables age? Will they leak intermittently or will they leak continually?

Generally the leakage will increase until the overload or overcurrent protection operates.
The failure is often progressive and once appreciable leakage has started, will progress to complete failure quite quickly.
But the progressions of failures of this type are seldom documented.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BlackJackJacques]

From your perspective, you have to assume it will leak when it is the most dangerous to do so. Treat it like a gun -it is always loaded. Once these cables become water-logged, there is always water trapped within.

Best

Jim

 

[itsmoked]

Power it thru a GFI breaker and don't worry about it further. When it repeatedly trips the GFI it's time to replace it. (GFIs sometimes trip just because they feel like it - hence - repeatedly)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Depending on more than one factor, the capacitive leakage on a long wet underground cable may be close to or more than the tripping current of the GFI.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

In our factory, we laid 600 V power cables (for 3-ph, 415 V system) underground enclosed in a 4 inch PVC pipes. The pipes were pre-drilled with 10 mm holes at the bottom every 1 meter to drain any collected water and act as a vent for any trapped moisture. That was more than 30 years ago. No issues whatsoever. But at much higher voltages, as Bill says, water treeing will become a problem at some point.

Bill - At 220V, what sort of capacitive discharge can happen?

Muthu

http://www.edison.co.in

 

[waross]

Not capacitive discharge but the charging current for the capacity to ground of a long conductor. If the PVC has a lot of water in it water has a high capacitive constant.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Bill; Why would water have anything to do with capacitive leakage?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BJC]

Any cable installed underground can be assumed wet. In conduit or direct buried it's going to be wet sooner or later. If it's not rated for wet locations assume trouble.

 

[waross]

Water has a very high capacitive constant.
If it's dirty, grounded water, it will provide a good ground plane surrounding the cable.
Consider the capacitance to ground of a cable sitting on a wet grounded surface with the capacitance to ground of the same cable immersed in and surrounded by grounded water.
If the water is contained in the PVC in such a way that the water is not grounded, it will increase the capacity to ground compared to cables in a dry PVC tube. (but not as much as the cable surrounded by grounded water.)
The moisture content of wood may be determined by placing a sample of lumber to be tested between two plate electrodes and measuring the capacitance. The capacitive constant of water is so much greater than the capacitive constant of dry wood or air that the moisture content may be accurately determined by the change in capacity cause by the water content in the wood sample.
We don't know how long the cables are. It won't take much charging current to trip a GFI.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

I've seen instances where a 120V circuit wired with THHN/THWN would trip a North American GFCI (5 ma) while the same circuit reconductored with XHHW wouldn't. Some insulations don't resist the capacitive charging current nearly as well as other insulations.

 

[itsmoked]

OK Thanks Bill. I hadn't thought about dirty ion filled ground-planey water. Makes sense in that condition.

Still the quality and length of the cable could support a GFI and if it did there'd be nothing more to talk or worry about.. until there is.

Interesting too David.


Michael Roberts; How long is the cable?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I am not sure of the exact settings but in parts of the world, GFI breakers are used for equipment protection. The trip setting is much higher than the life safety setting that we are used to in North America. Some countries use a GFI breaker as the main service entrance breaker, but with the higher trip setting that I mentioned.
Possibly the higher setting GFIs are available to the OP.
David, I had an application supplied from an old stepped square wave inverter. I used GFI receptacles. I tried to use the feed through feature to other receptacles. The stepped square wave charging even 15 or 20 feet of conductor to the second receptacle was enough to trip the GFI.
And designing/wiring hospital operating theatres, some circuits must be fed from isolating transformers, with leakage monitoring/alarming equipment.
The allowable ground fault leakage of any circuits in reach of a patient on the operating table are so low that the transformer capacity may be limited to a single circuit and cable insulation capacitance may use up quite a bit of the allowable leakage.
Human skin has a much higher resistance than internal tissue. A voltage that may be low enough to be undetectable by touch may be enough to be fatal if directed to internal tissue.
All issues with cable insulation capacitance.
--------------------
"Why not the best?"
Jimmy Carter