Megger Reading Between Wires - Very Long Run

[cslater]

I am looking at a control wire on an airfield. The wires are #14 and go between two vaults that are approximately 2 miles apart. They carry a 120V signal to a relay that turns a set of lights on and off.

Although it sounds like a small wire for that distance, the system worked for many years. There was an ice-cube relay at the end of the run and the current draw was low enough for it to consistently switch. However, the equipment was upgraded and a lighting relay put in place that will not latch.

I am certain that the problem is too high of an amperage requirement for the new relay, but there is an open issue that I need to understand.

During the testing, megger readings were taken of the wires. Both wires test properly to ground, but when they are meggered to eachother, a reading of around 55 k-Ohms is seen. Our initial thought was that there is weak insulation between the two wires at some point along the way, but the wires have been thoroughly tested again and we're not seeing a problem.

What I'm wondering is if the two wires running parallel for such a long distance could be causing the low megger readings. I can't put my finger on what the exact cause would be but my gut says that there could be something there.

Does anyone have any insight they can shed?

 

[texie]

I am looking at a control wire on an airfield. The wires are #14 and go between two vaults that are approximately 2 miles apart. They carry a 120V signal to a relay that turns a set of lights on and off.

Although it sounds like a small wire for that distance, the system worked for many years. There was an ice-cube relay at the end of the run and the current draw was low enough for it to consistently switch. However, the equipment was upgraded and a lighting relay put in place that will not latch.

I am certain that the problem is too high of an amperage requirement for the new relay, but there is an open issue that I need to understand.

During the testing, megger readings were taken of the wires. Both wires test properly to ground, but when they are meggered to eachother, a reading of around 55 k-Ohms is seen. Our initial thought was that there is weak insulation between the two wires at some point along the way, but the wires have been thoroughly tested again and we're not seeing a problem.

What I'm wondering is if the two wires running parallel for such a long distance could be causing the low megger readings. I can't put my finger on what the exact cause would be but my gut says that there could be something there.

Does anyone have any insight they can shed?

Wow, that's a long way! Not sure if I'm qualified to comment on your megger readings but it seems obvious that the coil burden of the new system is to great as you mention. It seems that you will have to interpose a very low burden relay in front of the new system. But I would have to wonder how this worked all these years even with a low burden. It would seem that you run the risk of the relay not releasing due to capacitance.

 

[GoldDigger]

During the testing, megger readings were taken of the wires. Both wires test properly to ground, but when they are meggered to eachother, a reading of around 55 k-Ohms is seen. Our initial thought was that there is weak insulation between the two wires at some point along the way, but the wires have been thoroughly tested again and we're not seeing a problem.

What I'm wondering is if the two wires running parallel for such a long distance could be causing the low megger readings. I can't put my finger on what the exact cause would be but my gut says that there could be something there.

What do you know about your particular megohmmeter/insulation tester?

If it applies a steady DC voltage during the test, then the 55kOhm reading is really telling you something about a load, resistance, or leakage between the two wires.
But if it applies AC or pulsed DC during the test, the very large capacitance between the two wires could easily draw a current comparable in magnitude to a 55k resistance.

An impedance bridge which can measure both the resistive and reactive components of the line-to-line impedance will confirm or rule out this problem.

However, it the capacitance between wires is that large, I would expect the capacitance to the raceway, earth, or whatever surrounds the two wires to be in the same ballpark. If the wires are overhead or inside PVC, then the capacitance to ground would be reduced. And if the wires are closely lashed or twisted it will raise the capacitance between them.