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Correct answer: turn off the power and remove the power plug

For safety when working on mains-powered equipment:

• always disconnect from the power source first
• ensure no live voltage is present

This prevents:

• electric shock

• accidental short circuits

• Shorting capacitors can be dangerous if done incorrectly.

• Checking components comes after power is removed.

Therefore, the first step is to turn off the power and remove the power plug.

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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.

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Correct answer: B — removes power to the circuit when the phase and neutral currents are not equal

A Residual Current Device (RCD) — sometimes called an earth leakage circuit breaker (ELCB) — continuously compares the current flowing in the phase (active) wire with the current returning via the neutral wire. Under normal conditions these currents are equal. If some current leaks to earth — for example, through a person receiving an electric shock — the two currents become unequal. The RCD detects this imbalance (the "residual" current) and trips the circuit open within milliseconds, greatly reducing the risk of electrocution.

• A is wrong — an RCD provides no filtering or suppression of electrical interference; that is the role of mains filters or ferrite chokes.
• C is wrong — the RCD monitors the difference between phase and neutral, not a comparison between phase and earth. Current in the earth wire is itself a symptom of a fault, but the RCD's sensing coil compares phase to neutral.
• D is wrong — limiting or regulating power is the function of a fuse, circuit breaker, or current limiter, not an RCD.

Therefore, an RCD protects against electrocution by tripping instantly whenever phase and neutral currents are unequal, indicating current is taking an unintended path to earth.

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Correct answer: does not develop a high voltage with respect to earth

An earth (protective ground) wire is connected to the metal chassis so that, in the event of a fault such as insulation failure, any live voltage that comes into contact with the chassis will be safely conducted to earth.

This ensures:

• the chassis remains at earth potential

• protective devices (such as fuses or circuit breakers) can operate

• It is not intended to maintain a specific voltage relative to the phase lead.

• Bleeding static charge is not its primary safety function.

• Lightning protection requires specialised grounding systems.

Therefore, the earth wire ensures the chassis does not develop a high voltage with respect to earth.

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Correct answer: B — prevent the chassis from becoming live in case of an internal short to the chassis

The three wires in a standard mains cord are Active (brown), Neutral (blue), and Earth (green/yellow). The Earth wire connects the metal chassis of the equipment directly to the ground potential of the building's wiring system. If an internal fault causes an Active wire to contact the chassis, fault current flows safely to earth, tripping the fuse or circuit breaker rather than allowing the chassis to sit at mains voltage — which would create a lethal shock hazard for anyone touching it.

• A — make it inconvenient to use: Incorrect. The three-wire system is a deliberate safety feature, not an inconvenience. Modern equipment is designed to use it as standard practice.
• C — prevent the plug from being reversed in the wall outlet: Incorrect. Polarisation (preventing reversal) is a function of the plug's physical shape, not the presence of a third Earth wire.
• D — prevent short circuits: Incorrect. The Earth wire does not prevent short circuits from occurring; it provides a safe fault-current return path when they do occur, causing protective devices (fuses/breakers) to operate.

Therefore, the Earth (third) wire protects users by ensuring any fault current from an internal short to the chassis is safely conducted to earth, tripping the overcurrent protection rather than energising the chassis.

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Correct answer: A — brown

In New Zealand (and consistent with the IEC 60446 standard adopted here), the standard colour coding for flexible mains cables is:

• Brown — Active (phase/live)
• Blue — Neutral
• Green/Yellow — Earth (protective conductor)

This replaced the older British colour scheme (red/black/green) and is now mandatory for flexible leads sold and used in New Zealand.

• B. Blue — Blue is the neutral conductor, not the active/phase wire.
• C. Yellow and green — The yellow and green striped conductor is the earth (protective ground), never the active phase wire.
• D. White — White is not a standard colour in the NZ/IEC flexible lead scheme; it was used in older North American wiring and does not apply here.

Therefore, in a New Zealand flexible mains lead, the phase (active/live) wire is always identified by its brown insulation.

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

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Correct answer: yellow and green

In modern mains wiring colour codes:

• brown = live (active)
• blue = neutral
• green/yellow = earth (protective ground)

The earth wire is identified by the green and yellow striped insulation.

Therefore, the correct colour coding is yellow and green.

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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.

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