NETA Cable Testing and Diagnostics

Question 1

A DC hi-pot test on a shielded power cable measures:

Accepted Answer

The cable's insulation integrity by applying DC overvoltage and monitoring leakage current

Answer

DC hi-pot tests apply a DC overvoltage (per IEEE 400 or cable manufacturer specifications) to cable insulation and monitor leakage current. Excessive current, current instability, or sudden increases indicate insulation defects.

Question 2

The VLF (Very Low Frequency) test for medium-voltage cables uses a test frequency of approximately:

Accepted Answer

0.1 Hz, which reduces the charging current requirement by a factor of 600 compared to 60 Hz testing

Answer

VLF tests at 0.1 Hz (one-tenth of a cycle per second), which reduces the capacitive charging current by a factor of 600 compared to 60 Hz (since Ic = 2*pi*f*C*V). This makes portable test equipment practical for field cable testing.

Question 3

Time Domain Reflectometry (TDR) is used on power cables to:

Accepted Answer

Locate the position of faults, splices, and discontinuities by measuring the travel time of reflected pulses

Answer

TDR sends a fast electrical pulse down the cable and measures the time for reflections to return from impedance discontinuities (faults, splices, terminations). Using the cable's propagation velocity, the distance to each discontinuity is calculated.

Question 4

The dielectric loss tangent (tan delta) test on cables measures:

Accepted Answer

The ratio of resistive (loss) current to capacitive current, indicating insulation quality and aging

Answer

Tan delta (dissipation factor) measures the ratio of power loss (resistive current) to reactive power (capacitive current) in the insulation. Values increase with moisture contamination, chemical degradation, or water treeing.

Question 5

Cable jacket testing (sheath integrity testing) using DC current verifies:

Accepted Answer

That the outer jacket/sheath has no breaches allowing moisture to reach the metallic shield

Answer

DC sheath testing applies low DC voltage (typically 1 to 5 kV) between the metallic shield and the outer jacket, measuring insulation resistance to verify no holes, cuts, or abraded areas allow moisture ingress that could cause shield corrosion and insulation moisture treeing.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Cable Testing and Diagnostics