What kills a person high voltage or high amperage?
In general you can say that it's amps that injure or kill but volts that are dangerous. This is because the body and especially the (dry) skin has electrical reistance and a certain voltage is needed to drive a dangerous current through the body. SHOCK HAZARD: As defined in American National Standard, C39.5, Safety Requirements for Electrical & Electronic Measuring & Controlling Instrumentation: A shock hazard shall be considered to exist at any part involving a potential in excess of 30 volts RMS (sine wave) or 42.4 volts DC or peak and where a leakage current from that part to ground exceeds 0.5 milliampere, when measured with an appropriate measuring instrument defined in Section 11.6.1 of ANSI C39.5.
What happens if I touch a live electric wire?
If you touch a wire, what happens depends on whether or not current can flow. Current flow needs a path to complete a circuit (electrons flow around a circuit).What happens also depends on how much current flows. Refer to Ohm’s law where I=Voltage/Resistance-R (for DC) of Voltage/Impedance-Z (for AC). 12V is too low to generate noticeable current through your body, 120VAC is definitely enough and will burn if not electrocute you.As far as potential injury from AC v.s. DC there is not nearly as much significance as another factor which is whether that current flows through your chest and heart. Touching a hot wire while grounded by your other hand is much worse than, being grounded from standing on something. This because the current will go up through one arm, through your chest and out the other arm.On the issue of being able to let go, we have all seen in the movies you can't let go. Presumably with enough current in your body, it swamps any nerve signals your body might have. In effect, your muscles are full actuated.If you are insulated (e.g. standing on something that blocks all current paths back to the source) then no current will flow and nothing will happen. This is why birds on a high tension wires don’t get electrocuted.But nothing is a perfect insulator, so there is always a high enough voltage where potentially dangerous currents can flow. There is even the possibility of arcing across an air gap, but that is an extreme.
Why do signs read "Dager High Voltage" rather than "Danger High current"?
It's because of two factors: there is a voltage nearby that is significantly higher than the voltage one can usually assume is available (which has further implications a couple of which I'll mention later) AND that one must consider how one's exact situation may be vulnerable to that fact. You see, the person making the high voltage situation exist cannot know what your situation will be. He simply knows that he has created a situation that has a vastly greater chance of being a problem for others. And since that situation is one of higher than normal voltage, the sign must read "High Voltage." Then life happens. Someone coming near the area of potential danger may create a well-insulate barrier between himself and the ground/return used by the system, may remove paraphenalia from himself that could carelessly interact with the system, or interact in a markedly more "effective" manner (careless: loose clothing perhaps; markedly more effectively: a gold chain perhaps), may take steps to avoid wetness/moisture on body or equipment surfaces nearing or contacting the system, etc. The person may even have the ability/authority to de-energize the system before entering the (otherwise) dangerous zone. In other words, the person entering the danger zone may take many steps to avoid creating a circuit or to maximize resistance to current flow if one is created. Then any current flow may be so small as to represent no actual danger (say, less than 10 milliamps). So no "High Current" situation would exist, even though the "High Voltage" situation absolutely still would. You warn about the not-normal hazard that does exist in order to allow those who must risk nearing it to take any sensible precautions to avoid the danger that could otherwise result from that hazard. Yes, the actual danger is high current not high voltage, but since the person entering the area takes precautions, no high current may come to exist even though the high voltage may exist the entire time.
What is more dangerous to human, 20 volt & 5 Ampere or 5 volt & 20 Ampere circuit? I Think both have W = 100?
If you are considering a human body then the dangerous thing is voltage. If voltage is low, as it is the case in both of your examples, then the current that passes in a human body is insignificantly small and no danger is posed at all. It is the current that kills but current is dependant on voltage.
Why are welders not electrocuted when they touch the metals being welded?
Edit: as an astute observer pointed out, ‘electrocute' is terminology meaning a fatal shock. I have not been electrocuted but I have been electrically shocked... however the risk of electrocution remains if proper safety protocol is not followed.You CAN be electrocuted when welding, I’ve BEEN electrically shocked. Electricity always tries to flow toward the earth, if the electricity decides that you are an easier (less resistant) path to the ground you will be shocked. Thus, the fellow welding in the image is a prime candidate for electrocution (he doesn’t appear to be wearing insulated work boots or welding gloves). A key to being safe is to wear all appropriate welding apparel: Steel or composite toe and leather work boots, welding gloves (thick leather), leather or cotton clothing covering arms and legs fully up to the neck, and of course a full face welding mask.Don’t weld in wet conditions (rain, puddles, etc.) and if you have to, keep your boots and gloves completely dry. If you use a generator welding machine (engine drive) use an earth rod (conductive spike, longer the better) stuck into the earth and attached to the machine. Don’t open welding machines to try and repair them, they have large capacitors that could discharge into you if you don’t know what you’re doing (among other things). Also, always check that all cables and connections are properly installed and maintained, don’t use welding leads that have cracks or broken insulation, and same with the primary power cable.The reason welding works is that when welding, the current finds that the path through to the work is much less resistant than anything else, and it uses the energy to consume the welding electrode and work part at sufficient heat so as to join them together.
Which is more dangerous: AC or DC power?
DC can be defined as the ‘unidirectional’ flow of current. Current only flows in one direction. Voltage and current can vary over time so long as the direction of flow does not change.Lets assume that a certain battery provides 1.5 V, which can be described in graphical terms as:Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.*Plot of AC waveWhile considering which is more dangerous I hope you realize that 230 volts DC and 230 volts AC develop the same power in a resistor but they have different safety considerations. Its also known that 230 V AC is 325 volts peak, 650 peak to peak. So an AC supply can actually apply greater voltage than 230 volts DC.Plus the certain body facts like:-The human body works on electrical signals.-All the muscles are driven by generated signals.-When a source of sufficient voltage is applied the muscles work (contract).-Every articulation in the body is controlled by (at least) two opposing muscles. For example clenching you hands and opening them. In general one set is much stronger than the other (try it with clenching/un-clenching).-When a supply such as the mains is applied it is strong enough to drive pretty well all the body muscles.-So if you grab the mains by your hands both the clenching and un-clenching muscles will be driven and the clenching ones will win.That’s why it is said, "you cannot let go of DC, but you can (perhaps) let go of AC."This is the basis of the idea that DC is more dangerous than AC.ALSO:"Low-frequency (50–60 Hz) alternating currents can be more dangerous than similar levels of DC since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, a deadly heart rhythm that must be corrected immediately.[14] However, any practical distribution system will use voltage levels quite sufficient for a dangerous amount of current to flow, whether it uses alternating or direct current. As precautions against electrocution are similar for both AC and DC, the technical and economic advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard worldwide."http://en.wikipedia.org/wiki/War...PS Don’t try to check this doubt by yourself.. Stay safe :P