Electrical Interlocks

Electrical Interlocks PowerPoint PPT Presentation

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Principles of Interlocking. Prevents movement of dangerous parts when guard is openPrevents access to danger area until hazard has been made safe (I.e. power is off)If failure occurs, it should fail-to-safety. Limit Switches. Normally Open Limit Switch (Negative)Spring keeps contacts open until p

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Electrical Interlocks

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1. Electrical Interlocks Principles Limit switches Failure modes

2. Principles of Interlocking Prevents movement of dangerous parts when guard is open Prevents access to danger area until hazard has been made safe (I.e. power is off) If failure occurs, it should fail-to-safety

3. Limit Switches Normally Open Limit Switch (Negative) Spring keeps contacts open until pressure is applied to roller Serious defects: Easy to defeat by holding down with hand or using adhesive tape If spring breaks, switch is left on If roller arm becomes bent, switch could be jammed in on position Shouldn’t be used in isolation

4. Normally Open Limit Switch

5. Limit Switches Normally Closed Limit Switch (Positive) Spring closes contacts when there is no pressure on roller Safer than normally open: Difficult to defeat If spring breaks, fail to safety Defects: Wear, misalignment, or overrun of linear cam on guard will result in fail to danger If guard is removed, switch will become activated, creating fail to danger

6. Normally Closed Limit Switch

7. Limit Switches Positive and Negative in Series Extremely difficult to defeat Rigorous maintenance required Switches can be monitored by incorporating warning lights for each switch, using spare contacts - no light means switch is not returning to open circuit (or bulb has gone)

8. Positive and Negative in Series

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