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Subel. ZLB
10 questions
Subelement ZLB

Basic Electrical Theory

Section ZLB10

Safety

You can safely remove an unconscious person from contact with a high voltage source by

  • pulling an arm or a leg
  • wrapping the person in a blanket and pulling to a safe area
  • calling an electrician
  • Correct Answer
    turning off the high voltage and then removing the person
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For your safety, before checking a fault in a mains operated power supply unit, first

  • short the leads of the filter capacitor
  • Correct Answer
    turn off the power and remove the power plug
  • check the action of the capacitor bleeder resistance
  • remove and check the fuse in the power supply
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Wires carrying high voltages in a transmitter should be well insulated to avoid

  • Correct Answer
    short circuits
  • overheating
  • over modulation
  • SWR effects

Never work on any Mains appliance unless you are competent to do so. Before working on an appliance that uses mains supply, always turn the power off and remove the plug from the outlet. In a high power transmitter, high voltages are present. The wires are well insulated to avoid short circuits within the amplifier or transmitter.

Last edited by cmscouler. Register to edit

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A residual current device is recommended for protection in a mains power circuit because it

  • reduces electrical interference from the circuit
  • Correct Answer
    removes power to the circuit when the phase and neutral currents are not equal
  • removes power to the circuit when the current in the phase wire equals the current in the earth wire
  • limits the power provided to the circuit
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An earth wire should be connected to the metal chassis of a mains-operated power supply to ensure that if a fault develops, the chassis

  • Correct Answer
    does not develop a high voltage with respect to earth
  • does not develop a high voltage with respect to the phase lead
  • becomes a conductor to bleed away static charge
  • provides a path to ground in case of lightning strikes
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The purpose of using three wires in the mains power cord and plug on amateur radio equipment is to

  • make it inconvenient to use
  • Correct Answer
    prevent the chassis from becoming live in case of an internal short to the chassis
  • prevent the plug from being reversed in the wall outlet
  • prevent short circuits
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The correct colour coding for the phase wire in a flexible mains lead is

  • Correct Answer
    brown
  • blue
  • yellow and green
  • white
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The correct colour coding for the neutral wire in a flexible mains lead is

  • brown
  • Correct Answer
    blue
  • yellow and green
  • white

Wiring in a 230V appliance lead Phase or live terminal = Brown Neutral = Blue or Black Earth = Green or Green and Yellow

Last edited by cmscouler. Register to edit

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The correct colour coding for the earth wire in a flexible mains lead is

  • brown
  • blue
  • Correct Answer
    yellow and green
  • white
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An isolating transformer is used to

  • ensure that faulty equipment connected to it will blow a fuse in the distribution board
  • Correct Answer
    ensure that no voltage is developed between either output lead and ground
  • ensure that no voltage is developed between the output leads
  • step down the mains voltage to a safe value

Correct answer: ensure that no voltage is developed between either output lead and ground

An isolating transformer provides galvanic isolation between the mains supply and the output. The secondary winding is not connected to earth, so neither output lead has a defined voltage with respect to ground. This greatly reduces the risk of electric shock when working on live equipment, as touching one output conductor and ground does not complete a circuit.

  • Blowing a fuse in the distribution board depends on fault current and earthing, not isolation.
  • There is still a voltage between the two output leads, otherwise the transformer would supply no power.
  • Although some isolating transformers may also reduce voltage, voltage reduction is not their defining purpose; isolation can occur at 1:1 ratio.

Therefore, an isolating transformer is used to ensure neither output conductor is referenced to ground.

Last edited by jim.carroll. Register to edit

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