Sunday, June 7, 2009

Technical Explanation

Image via Wikipedia





Electric Shock

An illustration of how an electric or electronic product could pose a potential electric shock
The conductive chassis of these products are so soft that with your bare hands you can bend after applying some force , or create a dent or curve with a small object .

prepared on 1 June 2009


Most of the audio and video processing equipment in the North American market implement two-prong ac plug equipment , instead of three-prong .


In spite of three prong plugs are the ones that help guard against electric shock .Three-prong equipment have the third rod ,which is connected to the ground from the circuit breaker is also bound to the conductive enclosing cabinet of the product ,or to the exposed metal conductors for protection against electric shock .

This will detect and shut off the AC power if there was any defects inside the faulty product (see the first diagram below).
(see both following diagrams below,both zoom enabled when clicked on them  , and see the red arrow touching the  deformed panels inside the unit illustrating short between the metal chassis and the live terminal )
The most prominent reason for not implementing three-prong method is because the ground terminal on the three-prong gives a path for the infamous 60 hertz noise to penetrate into the equipment's circuitry and result with poor audio and video processing performance.
But the manufacturer , when designing these equipment , should implement utmost care to insulate the AC inlet terminals ,and provide total isolation.Especially from the conductive metal enclosing the circuitry and allow good clearance between them .And the enclosing conductive material should be built from solid ,rigid and robust material -not easily bendable -to reduce the risk of bending ,and hence from touching to the non-insulated live terminal .This could happen due to deformation after a damage impact or applied accidental pressure on the corners of the conductive panels , covers ,metal rear panels or top covers .
Without these measures taken ,the possibility of occuring 120 volts electric shock will increase much further . 120 volts electric shock could cause injury or even death depending on many factors such as wetness of the skin, area of contact, duration of contact,age,health condition and so many SEE FURTHER DETAILS .
Here is a graph of two-prong product with a fault of short circuit between not-bound-to-ground metal chassis and uninsulated live terminal , and it is undetected by the circuit breaker . Meanwhile the product is up and running ,the product will still work ! But once the unit is touched by the user and if the user is standing in a conductive position electric shock will definitely occur .




If any such an incident occurs , even if it was an electric shock that did not cause any injuries please report immediately to the Electrical Safety Authority (ESA)

see also photographic illustration
Matthew Bazajian

©2009 TechSonAr see http://www.techsonar.com . All rights reserved



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Tuesday, May 19, 2009

This is the power supply side of a DVD-Recorder










This is an example of an imported DVD-Recorder purchased in the Canadian Market with Certified (UL) marked on it .

Underwriters LaboratoriesImage via Wikipedia



Observe gray conductive chassis underneath the two-pronged AC receptacle , the clearance is less than half an inch and there is no insulation with an enclosing chassis that is soft and easily bendable .


After removing the board there is no layer of polycarbonate or any other insulating sheet for isolation .















And below here where the Q-Tip is pointing is the conductive terminals that could touch the conductive chassis underneath or see other marked picture with red circles .














for further technical explanation click here






According to my Engineering experience I believe we could request for better standards than this from the manufacturers especially from the  manufacturers of the imported products .



Matthew Bazajian
a TechSonAr engineer


Please note that in such products the hazard is not imminent but it is very possible !!!

©2009 Servo Electronics(TechSonAr) see http://www.techsonar.com/safety_issues . All rights reserved


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