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11-CS-2 Engineering Communications · December 2019

Question 6 of 7: Machine Protection — Paper Guillotine

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.

Question 6: Machine Protection — Paper Guillotine (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The danger zone is the blade path during its descent. The operator's hands are legitimately in that zone while loading and re-aligning the stack, so the protection must guarantee the hands are clear at the instant of the cut. Two (plus a third) complementary methods:

Method 1 — Two-Hand Control (with anti-tie-down/anti-repeat)

   [ BLADE ]         Operator must press BOTH buttons,
     ||              spaced apart, SIMULTANEOUSLY and
  ===table===        HOLD them for the whole descent.
  (O)        (O)     Releasing either stops the blade.
  left       right
  button     button

Two palm buttons are mounted far enough apart (and away from the blade) that both hands are occupied on the buttons, well clear of the blade, throughout the cut. The control requires concurrent actuation (within a fraction of a second) and continuous holding, with anti-tie-down logic (a button held down is detected and the machine will not cycle) and anti-repeat (must release and re-press for each cut). Philosophy: the operator physically cannot have a hand at the blade while initiating the cut.

Method 2 — Presence-Sensing Light Curtain

   [ BLADE ]
     ||          Infrared beams across the blade opening.
 |: : : : :|     If any beam is broken (a hand enters),
 |: : : : :|     the blade is stopped / prevented from
  ===table===    descending.  Mounted at safe distance S.

A light curtain guards the blade opening; if a hand interrupts a beam, the machine stops or will not initiate. It must be mounted at the calculated minimum safety distance (so the blade stops before a hand can reach it) and used only if the blade motion can be stopped in time; otherwise it prevents initiation. Philosophy: detect intrusion and remove the hazard before contact.

Method 3 (supplementary) — Interlocked Hold-Down / Fixed Guard

A clamp/guard bar comes down to hold the stack and cover the blade line, interlocked so the blade cannot descend unless the guard/clamp is in place and hands are outside. This also improves cut quality by holding the stack.

For this machine, the best practice combines a two-hand control (guarantees hands are clear at initiation) with a light curtain or interlocked guard (protects against a second person or a hand re-entering)—defence in depth. All controls must be fail-safe (a fault stops the blade).