Before performing insulation testing, the circuit must be fully disconnected from the supply.

• A. True
• B. False
• C. Only on low voltage
• D. Depends on equipment

Answer: (A)

The key idea is safety and measurement accuracy when checking insulation with a tester. Insulation resistance testing puts a high voltage across the insulation to see how much current leaks through. If the circuit is still connected to a power source, that live supply can interact with the test voltage, giving false readings (paths through the supply can look like leakage), backfeed the source, or cause arcing and damage. Also, any stored energy in capacitors or other components must be discharged first, or it could discharge through the tester and harm equipment or personnel. Isolating the circuit from the supply ensures you’re measuring the insulation itself, without interference from live power. This approach applies regardless of voltage level; it’s standard safety practice to disconnect before insulation testing.