Insulation Testing Flashcards
Why is insulation testing important for underground cable systems?
It protects the high-cost investment by verifying installation integrity and tracking insulation degradation.
When is cable insulation typically tested?
Before energizing (acceptance) and periodically during its life (maintenance).
What are the three main types of insulation testing?
VLF (Very Low Frequency), Megohmmeter, and Hi-Pot testing
What OSHA regulation applies to cable testing safety?
OSHA 1910.269(o)
What safety practices are required during cable testing?
Safeguarding the test area, grounding, and safe use of control circuits.
Why must cable be properly grounded during testing?
High voltages may cause insulation failure; grounding reduces risk.
What do all cable insulation types have in common?
None offer infinite resistance; some leakage current is always present.
What does “good” insulation mean?
Insulation with relatively high resistance to current flow.
What factors degrade cable insulation?
Electrical stress, mechanical stress, chemical agents, and environmental elements.
Why is insulation testing included in maintenance programs?
To determine insulation health and schedule repairs or replacement.
What is an acceptance test?
A test performed after installation but before energizing, to detect defects.
What is a maintenance test?
A test during the life of a cable to detect deterioration and assess serviceability.
What is a “water tree”?
A degraded insulation area that resembles a tree and can evolve into an electrical tree.
What does VLF stand for?
Very Low Frequency.
What frequencies are used in VLF testing?
0.01 Hz to 1 Hz (most commonly 0.1 Hz).
What are the two types of VLF testing?
Withstand testing and diagnostic testing.
Why is VLF testing preferred over Hi-Pot?
It does not contribute to cable aging or failure.
What is the purpose of a VLF withstand test?
To confirm the cable can handle a specified voltage for a set time.
What is a drawback of diagnostic VLF testing?
It may cause failure in already degraded insulation.
What happens if a fault exists during VLF testing?
The test forces the insulation to break down, revealing the fault.
What test voltage is used for VLF acceptance tests?
2–3× the cable’s rated voltage.
What voltage is used for maintenance VLF tests?
80% of the acceptance test voltage.
How long should a VLF test last?
15 to 60 minutes; typically 30 minutes.
What standard governs VLF testing?
IEEE 400.
