for Designing Safety
Note: Safety design is a highly regulated field. These notes are only intended to serve as a general introduction to the basic concepts.
Hazards, Risks and Safety
Hazard: environment
potential for human, property or
Risk: likelihood of a hazard materializing Safety: relative protection from exposure to
hazards. Safe means acceptable risk
Risk analysis well developed method of
Quotes from Dieter
assessing and comparing risks using stastics.
Acceptable Risk
- ability to reduce risk is always increasing - tolerance for risk is diminishing - increasing litigation
1
Acceptable Risk
Judgment of acceptable risk varies widely Voluntary vs. Involuntary risk
parachuting assembly line worker
Society vs. Individual risk.
plane crashes
The cost of saving lives.
Risk Reduction Alternatives 1. Improved x ray equipment 2. Improved highway maintenance 3. Screening for colon/rectal cancer 4. Road guardrail improvements 5. Screening for lung cancer 6. Driver Education 7. Smoke alarms in homes
Kumamoto, H, Henley, E., Probabilistic Risk Assessment and Management for Engineers and Scientists, 2nd ed., IEEE Press, New York, 1996.
Est. Cost* $3,600 $20,000 $30,000 $30,000 $70,000 $90,000 $240,000
* per life saved per year
Ranking Hazards: Risk Ranking Matrix
Wong, W., "How did that happen?", Professional Engineering Publishing Ltd. London, 2002
2
Risks are ranked based on the severity of a hazard and the probability that it will occur.
Wong, W., "How did that happen?", Professional Engineering Publishing Ltd. London, 2002
Design for Safety
design with no hazards\ project against remaining hazards warn against hazards that cannot be eliminated
In order of priority.
Design process
THINK ABOUT WHAT COULD GO WRONG: Serious accidents are not usually caused by designs operating in the customary way. They occur when by an unusual and unanticipated combination of circumstances:
- what happens if a child swallows the cap of a ball point pen -airbag + child
3
Possible Hazards to Consider
- acceleration - chemical contamination - electrical - explosion - fire - leak\spill - material faliure - pressure release - temperature - mechanical faliure - computer hacker - software hazards
Dieter 11.9.1
Techniques to minimize failure/maximize safety
Include a safety factor
- materials have less than 1/4 of the break pt - safety function depends on the consequence of failure and likelihood
Include redundancy in critical systems
- mars rover - hospital - aircraft - nuclear safety methodology developed for these
Designing for Safety
Design to codes and standards, if applicable. - stifle innovation push practicle Use conventional materials/methods. - stifles innovation Build and test prototypes.
- subject phototypes to service conditions
Benchmark competitive products and systems. - if widely used solution exists, you must not ignore
4
Designing for Safety
Design for inspection and maintenance make these easy Design against "human error" make it hard to do the wrong thing. easy to do the right Design for a specific service life aircraft engines are rated for - how long before service Design for non critical failure/damage tolerance leak before break
Design for Safety
Use redundant control systems that selfcheck to ensure that they are on. Guarding
- keep humans away from moving parts. - prevent intentional by of passing guards
Light Curtains Two handed controls
Designing for Safety
Use safety cutoff systems
- lock down operation with personal lock when servicing - interlocking guards
Failsafe design
- component fails in a predictable way - fails in a safe way
5
Warning Labels
Warn users how to avoid hazards that cannot be designed away or guarded against. "A manufacturer must warn the consumer of any dangerous potential of the product by appropriate labelling."1 Must anticipate misuse and possible abuse of the design. Must be expressed in clear, simple language. (<6th grade)
1. Marston, D.L, Law for Professional Engineers, 2nd ed., McGraw-Hill Ryserson, Toronto, 1985.
In-class exercise
Write the warning section of the manual for a powered mitre saw.
WARNINGS FOR MECHANICAL SAW: - keep away from children - blade is sharp - never apply lubricants when blade is running - wear eye protection - no loose clothing -do not remove guards - intentional - do not jam switch in on position - intentional - make sure blade is not cracked - inspect - do not cut metal or masonry - make sure workpiece is secure.
6
WHMIS Workplace Hazardous Materials Information System
Special labeling requirement for materials used in the workplace. Workers trained to interpret labels.
The role of codes
Building Code Electrical code OSHA WHMIS Pressure vessel code Consumer Product Safety Act Food Drug and Cosmetic Act
Workplace Risk
7
Pre-Start Health and Safety Review
Specific Workplace Hazards Requiring Pre-Start Approval
1 2 3 4 5 6 7 8 Storing or dispensing of flammable liquids A machine required to be provided with a guard Storage rack or stacking structure that is not designed to an applicable standard Process that may produce explosive gas, vapour, dust or fumes A dust collector collecting an easily ignitable (combustable) dust A factory producing aluminum or steel or that is a foundry that melts or handles molten material Constructing, adding, installing or modifying a lifting device, travelling crane or automobile hoist Process uses or produces a toxic substance that may result in exposure above the occupational exposure limits
http://www.gov.on.ca/LAB/english/hs/guidelines/prestart/gl_psr_app1.html
8
When hazards can't be mitigated?
CSA Workplace Standards
Table 3. CSA Standards
Standard Subject
CAN/CSA-Z142-M90 Code for Punch Press and Brake Press Operation: Health, Safety, and Guarding Requirements (under review) CAN/CSA-Z434-94 CAN/CSA-Z615-87 CAN3-Z180.1-00 CSA-B51-97 CSA-B52-99 CSA-W117.2-94 CSA-Z432-94 Industrial Robots and Robot Systems-General Safety (under review) Code for Hot Forging Producers, Health and Safety Requirements Compressed Breathing Air and Systems Boiler, Pressure Vessel, and Pressure Piping Code Mechanical Refrigeration Code Safety in Welding, Cutting and Allied Processes Safeguarding of Machinery (under review)
http://www.gov.on.ca/LAB/english/hs/guidelines/prestart/gl_psr_app2b.html
Machine Guarding - Gotcha Stick
Example of a "preengineered" safety related design to code. (Not official in the OSHA code, but nevertheless good guidline.)
http://www.motionshop.com/offers/STIgotchaform.shtml
9
Summary of good practice
Liability
Consequences of poor design Professional Competence and Licence Review Civil Liability Tort Law Criminal Liability
How good do you have to be?
10
Record Keeping
11