Electrical QC Inspector Course & Interview Questions Part-2
31. What are Load Bank capacities?
Single Block – Entire capacity of load bank applied, on/off, in one step increment
Multiple Steps – Load bank capacity is divided into discrete increments for application in steps; defined in terms of load step resolution, which is the smallest step increment of load control. Steps can be coarse (50%, 33%, 25%) or fine (5KW, 1KW)
Single Voltage – Designed for a specific voltage, typical of exercise load banks to 15KV.
Multiple Voltage – Capable of dual or quad voltages. Typical of test systems. (Typical voltages 60HZ: 120/240, 208-240, 416-480; 50HZ: 190-220, 380-420; DC: 12-60, 125, 250) to 15KV.
Single Frequency – Designed for a specific frequency
Multiple Frequency – Capable of multiple frequency or AC/DC operation (Typical frequencies: 50-60-400HZ, DC)
32. What are some Load Bank Cooling systems?
Natural Convection Air – 20 KW Maximum
Integral Fan – Cooling fan included in load bank; up to 3000 KW.
Radiator Airflow – Load bank installed in radiator fan airflow; up to 1000 KW
Water – Cooling via heat transfer to water flow; water does not conduct electrical current. Open flow using commercial/natural water or closed loop using heat exchanger up to 1200 KW.
33. What are load bank enclosures?
Portable – For factory, shop, field use, self-contained
Stationary – For permanent installation at a specific site
Duct Mounting – For permanent installation in radiator airflow
Indoor – NEMA 1 enclosures
Outdoor – NEMA 3R enclosures
34. What are load bank controls?
Local – Control panel self-contained in load bank
Remote – Control panel for remote operation
Automatic – Wide variety of auto control available including auto on-off, auto load regulation, KW or reverse power response, computer control
35. What are load bank connections?
Cable Set – Connection cable included with load bank. Typical of portable units
Terminal Block – Power terminals for connection of building cables. Typical of stationary and duct mounted units.
Plugs – To match commercial wiring devices, aircraft plugs, MIL-SPEC plugs, cam-lock connections.
36. What are load bank features?
Short Circuit Protection – Load element branch circuit fuse or circuit breaker protection
System Protection – Protected against running failures (fan fail, overtemp, overvolt, etc.)
Control – Self-contained load control devices (contactors, relays, circuit breakers)
Handling – Forklift channels, lifting eyes, casters on portable units.
Instrumentation – Full instrumentation available: analog or digital; data acquisition systems
Options – Many options available
37. What are the purposes of AC High Potential Test Set?
This test set generates AC test voltages from 0-120,000 volts at currents between 0-5MA.
The test set is used to evaluate insulating materials, resistive and reactive properties.
The results are recorded as leakage current which is converted to resistance.
The resulting resistance can be used together with the DC hi-pot test readings to determine the AC resistance component for insulation evaluation.
38. The purposes of Digital Low Resistance Ohmmeter (Kelvin Bridge)?
The DLRO is a direct reading 100 Amp, 4 wire digital ohmmeter.
The DLRO is used to measure the DC resistance of contacts in a conducting path.
The DLRO is operated by applying a DC current through the desired conducting path.
The readings (DC micro-ohms) indicate the resistance at the contacts. This data is used to determine the amount of heating that can occur at the contacts as a result of the current flow through this connection.
39. The purposes of DC High Potential Test?
The test set generates DC voltages from 0-80,000 volts at currents between 0-10MA.
The testing potential is applied across the insulating medium in question, (bushing, standoff, barrier, buss bar, open air gaps, etc) raised to published voltage stress standards and maintained for one (1) minute and lasting for 15 Minutes.
The results are recorded as leakage current which is converted to resistance (typically megohms).
The resulting resistance readings are used in determining the insulation integrity in medium voltage air circuit breakers.
40. The High Potential Electrical Operation Test?
During this test the technician insures that all electrical functions of a circuit beaker or switchgear section(s) are operating properly and adhere to all customer and manufacturer’s specifications.
41. The Electrical Operation Such tests included?
Close and trip coil operate within published ranges
Mechanical Operation Test
Multi-Voltage Test Set
Overload Coordination Test Set
Three Phase Primary Injection Test
Optional Tests
42. Activities involves in close and trip coil operate within published ranges
Control relay sequence and timing
Auxiliary contact conductance
Proper charging motor operation
Meggar test insulation/components
43. Activities involves in Mechanical Operation Test
During this test the technician insures that all mechanical functions of a circuit breaker or switchgear section(s) are operating properly and adhere to all customer and manufacturer’s specifications.
44. Mechanical Operation Such test included in Prop Clearances:
Contact wipe
Gap measurements
Bolt torque test
Elevating mechanism operation
Interlock coordination
Complete circuit breaker operation
45. Mechanical Operation Such test included in Megohmmeters :
The megohmmeter (megger) is a high resistance ohmmeter used to measure insulating materials at test voltages between 0 and 5000 VDC.
The megger is operated in the same fashion as an ohmmeter in that the measuring leads are applied to the sample and results are recorded.
The recorded readings are direct readings in megohms and are used to determine the integrity of the insulation in low voltage (600 Volt) air circuit breakers or control wiring insulation.
46. Multi-Voltage Test Set:
The multi-voltage test set is an adjustable voltage source capable of generating AC/DC, single and three phase voltages between 0-600 volts.
The above test set allows operation and control testing of circuit breaker control circuits and switchgear control circuits, over the published ranges
47. Overload Coordination Test Set:
This test set is a low voltage, high current test device used to test the performance of electromechanical overloads and solid state trip devices in low voltage air circuit breakers. The test set is rated for operation at 20,000 amps continuous and 100,000 amps instantaneous.
The air circuit breaker is connected to the test set directly by use of the primary disconnects and current is raised to the proper levels to test all circuit breaker functions and characteristics that will ultimately trip the circuit breaker. All characteristics (long time, short time and instantaneous) are verified to fall within original manufacturer’s published curves.
The above tests the ability of the air circuit breaker to sustain high current, hot spots on the contacts and verifies that the overloads or solid state package operate properly within a given range of times.
48. Three Phase Primary Injection Test:
This test is designed to test the high current circuits in switchgear. Current transformer ratios, meter circuits and protective relay circuits are verified for proper operation by this test.
The current ranges are typically 50-5,000 AC amperes, depending on the ratings of the switchgear.
The test set is connected to the three primary conducting phases in the switchgear and the current levels are raised and monitored to check all applicable instruments in the primary current path.
The results will confirm and verify that all relays and metering systems are performing within the published times.
49. Speed and Motion Test (Optional)
A mechanical transducer is coupled externally to the operating rod of the air circuit breaker, which in turn provides the circuit breaker motion analyzer with electrical signals corresponding to the travel and velocity of the operating rod. A set of cables connect the motion analyzer to the circuit breaker contacts and the control circuit. Three phases can be monitored simultaneously.
The results are interpreted against published time standards which measures the electrical/mechanical performance of the circuit breaker.
50. Power Factor (Optional)
This test set generates AC voltages from 0-12,000 Volts and is used to measure the dielectric loss of insulating materials.
The capacitance of the specimen is being measured, from which the power factor can be computed.
This value can be used to chart the deterioration of the insulating specimen over time.
51. What is Absorption Current?
Current resulting form charge absorbed in the dielectric as a result of polarization.
52. What is Capacitance Current?
Current flow which charges the capacitor formed by the geometric capacitance of the cable under test
53. What is Conduction (Leakage) Current of the Cable Insulation?
Current flow resulting from conduction through the insulating medium or over surfaces (such as terminations). Corona discharge from energized elements will be indicated as conduction current, though technically it is not a conduction component.
54. What is Direct Current (DC)?
Unidirectional current, as used in this guide, denotes a practically nonpulsating current.