11-CS-2 Engineering Communications · December 2019
Question 2 of 7: Electrical Safety
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams — December 2019 — 11-CS-2 Engineering in Society: Health and Safety. Closed book. Do any five of the seven questions (each 10 marks). Full essay-type answers are expected; sketches may support the explanations. Complete answers to all seven questions follow.
Note on the reference charts. Questions 2 and 7 depend on the two attachment pages: page 6 is the Brauer/Dalziel chart of the effects of 60 Hz AC current on a 150 lb (68 kg) man, and page 7 is the Fine risk-assessment nomograph. The nomograph prints only the qualitative descriptors (no numbers appear on its Likelihood, Exposure or Consequence axes), so the scale points used below (Likelihood 10 / 6 / 3 / 1 / 0.5 / 0.1, Exposure 10 / 6 / 3 / 2 / 1 / 0.5, Consequence 100 / 40 / 15 / 7 / 3 / 1) are the standard Fine–Kinney values that the chart's own axis spacing reproduces. Selecting a descriptor for each activity is a matter of judgement; the choices are stated explicitly below and reasonable justified alternatives are equally acceptable, as the paper's Note 8 permits.
(a) Current Levels and Effects (60 Hz AC, from the chart)
(i) Slight sensation / perception threshold — about 1 mA.
(ii) Painful shock — about 5–10 mA.
(iii) Severe shock with muscle contraction ("no-let-go" / muscular paralysis) — about 10–30 mA (the let-go threshold is ~10–16 mA; above it the victim cannot release the conductor).
(iv) Death (ventricular fibrillation) — about 100 mA (0.1 A) and above (roughly 50–200 mA through the heart).
(b) Women versus Men
Women (and smaller/lighter people, including children) suffer these effects at lower current levels than men. The physiological thresholds—let-go current and fibrillation current—scale roughly with body mass, and women on average have lower body mass, so a given current represents a greater hazard.
(c) Double Insulation
Double insulation provides two independent layers of insulation—basic insulation plus a second, supplementary (or reinforced) layer—between the live parts and the user, so that if one layer fails the other still protects. Because protection no longer relies on an earth connection, such tools carry the Class II "square-within-a-square" symbol. They are fitted with a two-pin plug (no earth pin): since the insulation, not a protective earth, provides the safety, an earthing pin is unnecessary—the tool's enclosure is typically non-conductive and cannot become live.
(d) Fuse versus Circuit Breaker
Fuse — a thin metal element that melts when current exceeds its rating (the physical process is I²R heating melting the element), opening the circuit. Advantages: simple, inexpensive, fast, reliable. Disadvantages: single-use (must be replaced), can be replaced with the wrong rating, and cannot serve as a switch.
Circuit breaker — an electromechanical switch that trips on overcurrent by a thermal mechanism (a bimetallic strip heats and bends on sustained overload) and/or a magnetic mechanism (an electromagnet snaps open on a large fault current). Advantages: resettable, can be used as a switch, and can incorporate ground-fault protection. Disadvantages: more expensive, has mechanical parts that can fail or stick.
Both protect against overcurrent; the fuse's operation is initiated by melting (heat), the breaker's by thermal deflection and/or magnetic force.
