The guarded step voltage test is a method used to assess the insulation resistance of electrical cables. It involves applying a series of voltage steps to the cable and measuring the insulation resistance at each step. The test is called "guarded" because it helps minimize the surface leakage current that could otherwise affect the accuracy of the measurement.
Here are the general steps to perform the guarded step voltage test on electrical cables:
Equipment Preparation:
You will need a high-voltage DC power supply or a VLF (Very Low Frequency) AC test set capable of providing a range of voltages.
Ensure that the power supply or test set is properly calibrated and in good working condition.
Set up the test equipment in a safe and controlled environment, adhering to all safety regulations and guidelines.
Cable Preparation:
Make sure the cable to be tested is disconnected from any electrical equipment or system.
Clean the cable ends to ensure good contact and reduce surface leakage.
Setup the Guarding System:
The guarding system is designed to reduce surface leakage currents and improve the accuracy of the insulation resistance measurement.
Use guard rings or screens around the cable ends to create a low-leakage path for surface currents.
Connect the guard system to the ground potential.
Voltage Steps Application:
Start with the lowest voltage step. For example, if using a VLF AC test set, the lowest step could be 1 kV or 2 kV.
Apply the voltage step to the cable and allow sufficient time for the cable's insulation to stabilize. Typically, you'll wait for several minutes to allow the insulation to charge.
Measure the insulation resistance (R) at this voltage step using an insulation resistance tester (megger). The value of R will be in ohms (Ω).
Incremental Steps:
Increase the voltage to the next step. For example, you can raise the voltage by another 1 kV or 2 kV.
Again, allow time for the insulation to stabilize and then measure the insulation resistance (R) at this new voltage level.
Continue Steps:
Repeat the incremental voltage steps until you reach the desired test voltage or until you see a steady trend in the insulation resistance values.
Be cautious not to exceed the maximum allowable test voltage for the cable's insulation, as this could lead to damage.
Recording and Evaluation:
Record the insulation resistance (R) values at each voltage step.
Plot the results on a graph with voltage (V) on the x-axis and insulation resistance (R) on the y-axis.
Evaluate the insulation condition based on the trend of the graph. A stable and increasing insulation resistance indicates a healthy cable insulation.
Interpretation:
Compare the obtained results with the manufacturer's specifications or industry standards to assess the cable's insulation condition.
If the insulation resistance is below the acceptable level, the cable may have insulation issues and requires further investigation and possible replacement.
Remember that the guarded step voltage test is a potentially hazardous test, involving high voltages. Always follow safety procedures and use appropriate personal protective equipment (PPE) when performing this test. If you are unsure about the testing procedure or lack the necessary experience, it's best to seek assistance from a qualified electrical engineer or professional technician.