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Correctly designed electrical products make proper use of the earth connection to help prevent death or serious injury. Some cheap goods from China do not implement this standard safety feature, yet still offer their products for sale. The purchasers of the product usually does not realise the terrible
risk they are taking in using the product.
"Aha" you say? Sockets mechanically refuse plugs without earth pins already? Ah yes, you are right. However, some cunning sub-standard manufacturers (the manufacturer of the power block for my Dell laptop included) have resorted to providing plastic earth pins to trick the socket.
The safety features on a socket are there for a good reason! It is highly irresponsible for manufacturers to put plastic pins in place!
Therefore my idea is this: a simple circuit (perhaps based on a relay) which tests conductivity of the earth pin (e.g. between two contacts within the socket) and only switches on the live wire if the pin proves to be made from something conductive.
Admittedly, this does not deal with the manufacturers who use metal pins and just do not connect them, but it is better than nothing. I am working on a phase response system to detect a pin with nothing connected to it (suggestions here welcome).
What things should be earthed:
http://en.wikipedia...i/Appliance_classes
[wagster, Oct 30 2009]
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These products are double-insulated or Class2 electrical and have been designed in such a way that it is extremely unlikely they will ever expose live parts to the consumer. They are still correctly designed and have passed the Class2 safety tests and should be marked as such. If you attempt to disassemble your laptop PSU you will find out just how hard it is to get at the wiring. The laptop itself is Class3 and is too low-voltage to pose a threat. Your kettle on the other hand is Class1 and must therefore be earthed. |
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The more annoying case is when one of said plastic-enrobed power bricks has said plastic earth pin snapped off and left in the socket by some idiot... |
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//The more annoying case is when one of said plastic-enrobed power bricks has said plastic earth pin snapped off and left in the socket by some idiot...//
If you're lucky, you can get a bent wire coat-hanger into one of the lower apertures in the socket (which will handily have the pesky shutters retracted), and bend it upwards to release the blockage. |
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Only stick metal objects into the right hand hole though. Or is it the left? Whatever... |
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Personally I don't buy it. It is always going to be safer to have an earth circuit within the device, even if it is under two layers of plastic insulation. This isn't about what appears good enough, this is life and death. Who's to say that the PSU won't fuse and feed mains voltage into the laptop, onto which I recently spilt coffee? |
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Your low-voltage laptop would act as a fuse and melt before the mains voltage reached you. |
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Its much more likely for a transformer to develop a short in the coils of one side than across to the other coil. |
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// onto which I recently spilt coffee // |
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// suggestions here welcome // |
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We suggest you go and read some of the standards documents on the IEC and ISO websites. |
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3rd pin is a chassis ground; no chassis: no ground needed. [-] |
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I think the problem here is with annoying and complicated safety devices, which only encourage abuse just to get things to work (such as the plastic earth pin, and (so I've heard) the practice of cutting off the earth pin to use a 3-pin plug in a 2-pin socket in the US). |
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At least in this country, legally imported electrical goods have to meet our standards, regardless of country of origin. As others have said, if they are designed to operate without an earth connection, that's how they should be operated. |
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A further problem is that your device will give the thumbs up if there is any conductivity via the earth pin, and will not guarantee that the earthing is good enough to carry the full 15 amps (or whatever) required to trip a circuit breaker / fuse. Also you will need to send some voltage via the earth wire for testing, which detracts from the deliberate redundancy which is the purpose of the earth connection and would seem to increase the likelyhood of the chassis' becoming dangerously live. To borrow your words, Who's to say that the Earth Conductivity Switch won't fuse and feed mains voltage into the toaster chassis? |
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Perhaps if you want more intelligent methods to limit the supply of current in a potentially unsafe situation the best approach would be to rethink the whole system, rather than apply an electronic patch to a system which is inherently simple and physical. |
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the author doesn't seem to understand why and where an earth is needed or even what it does. |
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my cat can fall faster than your cat. |
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//otherwise the cat will continue to fall infinitely// How exactly is that a problem? |
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//where an earth is needed// That one is easier to answer - beneath us. |
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Said Dell laptop has started electrocution of owner from just below the cursor-down key. Mains has not yet tripped despite a number of jolting shocks to my arm. This is proof positive that this idea is both needed and good! |
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[-] Sometimes third prong grounding/earthing is not only impractical but in some cases, counter-productive. Like in the usage of professional audio/visual equipment and musical instruments, some devices are purposely designed with 2-prong plugs to eliminate the possibility of ground looping issues. |
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// a number of jolting shocks to my arm. // |
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We're so glad to hear that. |
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@vince3: your laptop is fed by a power supply that provides low voltage dc to the computer. If line voltage was reaching any part of your laptop then your power supply is critically shorted. Needless to say this is very unlikely and possibly quite dangerous. It could be a static discharge. It could also be that your finger is completing a DC short to the case grounds to the battery/power supply. You should unplug the power supply and see if the computer will shock you on battery power. |
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Laptops (except recent ones with LED backlights) generate high voltages to power the cold cathode flourescent backlight. This is a much more likely source of shocks than leakage of mains voltage. |
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