10 Questions
What is the purpose of shorting out windings during transformer testing?
To eliminate inductance and hazards
What is the purpose of the Step Voltage Test mentioned in the text?
To increase the voltage in equal steps
How can low quality leads affect insulation resistance measurements?
They contribute leakage current, causing measurement errors
What is the purpose of measuring parallel resistances when testing busway systems?
To get the correct final reading
How can electric shock hazards be mitigated according to the text?
By creating safe test areas for insulation testing
What is the primary requirement that insulators must meet?
Electrical resistance
What is the measure of electric stress required to move charge through a dielectric?
Dielectric strength
What type of insulation is commonly used in cables?
Flexible insulation
What is a measure of how much current flows through the insulation?
Insulation resistance
What is the steady state current that flows through the insulation during insulation resistance testing?
Leakage current
Study Notes
- Electrical insulation is a non-conducting material used to provide electrical isolation between two parts at different potentials.
- Insulators must meet primary requirements of electrical resistance and dielectric strength, as well as secondary requirements related to thermal and mechanical properties.
- Dielectric strength is a measure of electric stress required to move charge through a dielectric; it deteriorates with water ingress and high temperatures.
- Solid insulation includes flexible (thermoplastic) and rigid types. Flexible insulation is commonly used in cables, while rigid insulation is used in equipment terminations and support insulators.
- Liquid gases and vacuum are also used as fluid insulation.
- Dielectrics contain weak bonds between atoms, while insulators contain strong covalent bonds.
- Insulation resistance is a measure of how much current flows through the insulation.
- Insulation resistance is measured using an insulation resistance tester or megometer.
- Insulation resistance testing involves applying a known voltage and measuring the resulting current.
- Insulation resistance is affected by capacitive charging current and absorption or polarization current.
- Conduction or leakage current is the steady state current that flows through the insulation and is measured during insulation resistance testing.
- Different connection configurations can be used when testing insulation resistance in multi-conductor cables.
- Insulation resistance testing of transformers involves measuring the leakage current between high voltage windings and ground or between high and low voltage windings.
- Safety precautions include shorting out windings during testing to eliminate inductance and hazards.- Transformer testing involves measuring leakage current between windings and to ground in different configurations for safety reasons.
- For high voltage winding to low voltage winding and ground measurement, X windings are tied to the chassis, and the megometer positive lead is connected to the chassis ground, negative lead to H windings.
- To measure low voltage winding to ground and high voltage winding, windings are shorted out, and the positive lead is connected to the chassis ground, negative lead to low voltage windings, and the guard circuit to high voltage windings.
- When testing AC Motors, each phase should be isolated and tested separately for comparison and accurate results.
- When testing all phases simultaneously, only insulation to ground is measured, and no comparison is made between phases.
- It is recommended to disconnect and ground all external equipment for accurate insulation resistance readings.
- A common ground should be used to avoid stray losses in the ground circuit.
- Breaker testing involves measurements with the breaker in open and closed positions for line to ground, load to ground, and insulation of arc chute or vacuum bottle.
- The Polarization Index or Dielectric Absorption test requires two readings and a simple division to evaluate insulation quality based on relative readings.
- The Step Voltage Test increases the voltage in equal steps to detect defective insulation.
- Low quality leads with voltage rating less than test voltage can cause measurement errors by contributing leakage current.
- In series circuits, resistances add, but in parallel circuits, resistances add up as reciprocal values.
- When testing busway systems, it is essential to measure parallel resistances to get the correct final reading.
- Electrical shock hazards can be mitigated by establishing safe work practices, creating safe test areas, and grounding non-active cables or bus bars.
- Electric shock hazards can cause injury or damage to health through current passing through the body, increasing with lower body resistance and higher contact voltages.
- Research on electric shock effects on the human body indicates maximum safe body current is an inverse function of time.
- At five milliamps, a slight shock is felt, but strong involuntary reactions may lead to injuries.
- High potential test sets output currents of 100 to 300 milliamps, which can cause ventricular fibrillation, muscular contraction, nerve damage, and death.
- Energized circuits or circuit parts put out electrostatic fields, which can lead to electric field induction or capacitive coupling, posing electric shock hazards.
- Approach distances in safety standards establish safe distances from energized circuit parts based on voltage levels.- "D" is known as The Limited Approach and is a shock awareness boundary in electrical work.
- The Limited Approach boundary is an approach limit at a distance from an exposed energized electrical conductor or circuit part, within which a shock hazard exists.
- The boundary is determined based on the nominal phase-to-phase voltage of the energized conductor or circuit part.
- NFPA 70E also introduces the concept of a Restricted Approach boundary for shock awareness.
- The Restricted Approach boundary is an approach limit at a distance from an exposed energized electrical conductor or circuit part, within which there is an increased likelihood of electric shock due to electric arc over and inadvertent movement.
- Safety approach boundaries for different voltage levels are provided in NFPA 70E tables.
- For AC applications, the Limited approach boundary for 10,000 volts would be 5 feet, and the restricted boundary would be 2 feet 2 inches.
- For DC applications, the Limited approach for 5,000 volts would be 5 feet, and the restricted approach would be 1 foot 5 inches.
- Safe work practices when applying any type of test voltage include obtaining and reviewing up-to-date drawings and prints, tracing out and documenting circuits, verifying circuit paths, barricading access points, and treating all parts as energized.
- Megometers, which use DC, present relatively low hazards but dangers can come from the working environment if the connected load is very capacitive in nature.
- The circuit under test must be switched off, de-energized, and isolated before insulation tests are made.
- Capacitive currents must be completely discharged before disconnecting test leads or handling the circuit.
- The webinar covers safety procedures for testing electrical circuits and introduces the Limited and Restricted Approach boundaries.
Test your knowledge of electrical insulation and safety procedures with this quiz covering topics such as insulation resistance, dielectric strength, insulation testing for transformers and AC motors, safety approach boundaries, and mitigation of electric shock hazards. Learn about the primary requirements of insulators, different types of insulation, and the safety precautions involved in testing electrical circuits.
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