Voltage at Test Point for Deadbreak Connectors

[BigBreakfastJohn]

These two and a half questions came up following work with some dead break connectors. Scroll down to page 3 of the product link for a useful cut-away that shows the plug inserted.

We completed the work and I was left with one of the insulating plugs shown in this image. My understanding is that this acts as a capacitor, with the threads and stud being plates and the plug body the die-electric.

My fluke 376's capacity mode does not read anything across it. The lowest that this meter can read reliably is 1 µF according to the fluke datasheet. I assume the value would be lower than that, but I rarely if ever touch capacitors that are not on a board.

Question 1: What test equipment could I use to check the capacity?

Eaton states that the voltage on a capacitive test point is 10-12% of line-to-ground voltage. For a 7,976V-Ground system, that would be ~798 to 957V. I'm going to use 850V.

My thinking would be that all is needed is the current through the connection at the time of measurement and the capacitive reactance of the plug, assuming the capacitance can be found. But I clearly have a misconception:

V_drop= I * X_c ← V_drop across the insulating plug AKA the capacitor.
V_drop = I * (1/(2πfC)

Solving for C:
C=((V_drop/I)^-1)/2πf

Plugging in 850V for V_drop, 50A for current, and 60Hz for freq:

C=22.17 Farads, which I assume is incorrect.

Question 2: how would, or can one calculate an expected voltage from test point to ground if measured at the insulating plug test point?
Question 2a: how would, or can one calculate the capacity of the plug?


This is a theory only. I can't think of a reasonable situation where an electrician would put a multi-meter on a 25kV test point to measure the potential to ground.

 

[winnie]

The error in your calculation is the assumption of 50A current flow. The current you should use to calculate capacitance is the current through the capacitor, and there is no way you are going to get 50A through the tiny capacitance of this insulating cap.

The information that you need is the impedance of meter, and then you treat the meter and the insulating plug capacitor as a voltage divider. The impedance of the meter depends upon the specific design of the meter and can be quite low (Wiggy type solenoid tester) or quite high (lab grade electrometer), and this means that the voltage drop across the insulating plug will be quite variable.

It may be that the cap is more than a simple capacitor, and does something that limits the voltage at the test point. But that would require that the cap have a connection to ground.

 

[BigBreakfastJohn]

The error in your calculation is the assumption of 50A current flow. The current you should use to calculate capacitance is the current through the capacitor, and there is no way you are going to get 50A through the tiny capacitance of this insulating cap.

The information that you need is the impedance of meter, and then you treat the meter and the insulating plug capacitor as a voltage divider. The impedance of the meter depends upon the specific design of the meter and can be quite low (Wiggy type solenoid tester) or quite high (lab grade electrometer), and this means that the voltage drop across the insulating plug will be quite variable.

It may be that the cap is more than a simple capacitor, and does something that limits the voltage at the test point. But that would require that the cap have a connection to ground.

thank you Winnie. I forgot I'm talking about current across the capacitor, not the line current.

If I change the assumption to 1 microamp (not based on anything IRL), C is 3 pF, which seems far more reasonable.

There's not a direct connection to ground per-say, just a bleed ground placed lower on the elbow assembly, which I assume is for dissipating charge on the jacket to prevent tracking or something in that realm. I don't think it has much impact on the voltage seen at the insulating cap.