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Circuit Protection Flashcards

6 cards from real PART P practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 Circuit Protection flashcards as text

  1. What is the purpose of a surge protection device (SPD) Type 2 in a domestic consumer unit?

    Answer: To limit transient overvoltages (lightning surges and switching surges on the distribution network) entering the installation from the incoming supply

    A Type 2 SPD installed at the consumer unit clamps transient overvoltages conducted in via the supply cables (from lightning or network switching operations), protecting equipment connected throughout the installation.

  2. What does 'prospective short circuit current' (PSCC) mean?

    Answer: The maximum short circuit current that could flow between phase conductors at a given point in the installation

    PSCC is the maximum current that would flow if a direct short circuit occurred between phase conductors at a specific point; it must be within the breaking capacity of the protective device at that point.

  3. What does a 'BS 88-3' (formerly BS 1361) fuse protect in a domestic installation?

    Answer: It is the consumer unit main fuse or service fuse protecting the consumer unit and installation from overcurrent, and is controlled by the electricity supplier

    The BS 88-3 (or BS 1361) fuse is the service fuse at the head of the supply, owned and sealed by the electricity supplier; it protects the consumer's installation from severe overcurrent and distinguishes the consumer's installation from the network.

  4. What is the purpose of a current transformer (CT) clamp in electrical testing?

    Answer: To measure current in a live conductor safely without breaking the circuit, by induction

    A clamp meter or CT clamp measures current in a conductor by inductive coupling (Faraday's law) without requiring the circuit to be broken or any direct connection to live conductors.

  5. What is the purpose of a 'test button' on an RCD or RCBO?

    Answer: To test that the mechanical trip mechanism operates correctly — it does not test the RCD's sensitivity

    The test button creates an artificial test circuit through a resistor, operating the mechanical trip to confirm the trip linkage works; it does not verify that the RCD will trip at 30 mA from a real fault — proper testing requires a loop impedance tester or RCD tester.

  6. What is an arc fault detection device (AFDD) and what protection does it provide?

    Answer: A device that detects the electrical signatures of series and parallel arc faults in a circuit and disconnects it, providing fire protection beyond conventional MCBs

    AFDDs use digital signal processing to detect the irregular current waveforms characteristic of dangerous arc faults (loose connections, damaged insulation) that MCBs and RCDs cannot detect, disconnecting the circuit to prevent fire.