Electrical Materials, Testing, and Commissioning
What this domain tests
Electrical Materials, Testing, and Commissioning tests cable and busbar selection, insulation testing, continuity and polarity, protective-device testing, transformer and CT checks, and the sequence from factory acceptance to energization. The exam expects practical values and a logical commissioning order, not just a list of instruments.
Core rules and concepts
- Insulation resistance is measured with a DC tester at the appropriate voltage: commonly 500 V DC for 400 V circuits and 1,000 V or higher for MV equipment. A common minimum acceptance value for LV circuits is 1 MΩ, but project specifications may require much higher.
- Continuity and polarity must be proved before energization. Protective conductors, bonding, and switchgear earth connections are tested for low resistance.
- Earth-fault loop impedance and RCD trip tests prove disconnection. A 30 mA RCD should not trip at 15 mA (50% test) but should trip at 30 mA within 300 ms and at 150 mA within 40 ms under standard IEC testing.
- CT ratio and polarity tests are performed before protection commissioning. A 200/5 CT should give 5 A secondary at 200 A primary; ratio error and phase displacement must meet the metering or protection class.
- Commissioning sequence: visual inspection, insulation resistance, continuity and polarity, protective-device tests, functional tests, then load tests and documentation.
Worked example
A 200/5 CT is tested by primary injection at 100 A. Expected secondary current = 100 × 5/200 = 2.5 A. The measured value is 2.45 A. Ratio error = (2.45 − 2.5)/2.5 × 100 = −2.0%. For a metering CT of class 0.5, this error at 50% rated current would be excessive and the CT or test setup should be investigated. For a protection CT, the class and accuracy-limit factor determine whether the result is acceptable; do not apply metering limits to protection cores.
Traps that cost marks
Using a low-voltage megger on MV cable; accepting a 0.5 MΩ LV reading because “it works”; testing an RCD only at rated current; confusing CT ratio with burden; and energizing before polarity and phase-sequence checks. Also, a factory test certificate does not replace site acceptance testing after transport and installation.
Exam-focused summary
State the test voltage, acceptance limit, measured value, and pass/fail conclusion, then record it in the commissioning dossier.