Hot answers tagged

91

This is a bit complex. Basically, there are a number of limiting factors: The IO lines from the microcontroller (i.e. the analog and digital pins) have both an aggregate (e.g. total) current limit, and an per-pin limit: From the ATmega328P datasheet. However, depending on how you define the Arduino "Pins", this is not the entire story. The 5V pin of the ...


86

Without a transformer the live wire is live relative to ground. If you are at "ground" potential then touching the live wire makes you part of the return path. {This image taken from an excellent discussion here With a transformer the output voltage is not referenced to ground - see diagram (a) below. There is no "return path" so you could (stupidly) ...


73

Assuming the bird still is at earth potential when entering in contact with the wire (say, it jumped right on it from the pole). There are lots of unknowns in this problem but let's try to fill some gaps with data we kind of know in humans. So until an EE stackexchanger who is an ornithologist shows up with interesting data, let's assume humans can fly and ...


70

frankly the difference between 48V and 120V doesn't seem to be that significant. 120V is 2.4 times higher than 48V - hardly what I would call 'not that significant'. 120VAC is even worse, for two reasons:- 120VAC has a peak voltage of 170V, 3.5 times higher than 48VDC. The 'electric shock' feeling occurs on every peak of the AC waveform, whereas with DC ...


60

How much voltage is dangerous is not really a static number as it depends on your body resistance, time of exposure and source "stiffness" (i.e. how much current it can supply). You get figures like 60V (or as low as 30V) which are an attempt at an average figure above which "caution should be taken". However, depending on how "conductive" you are at any one ...


58

The Edison lamp base design dates predates all twentieth-century safety regulations (because it predates the twentieth century altogether). Having light bulbs screw in and out is not great in environments where vibration is a problem, and replacing a bulb which has just burned out while in use may require use of a cloth to hold it, but it is generally ...


57

Oh yes. The phenomenon is called "Electrical Drowning". In this tragic case, a girl decided to dance in a fountain, unawares that the underwater lights had a ground fault. Her muscles contracted and she fell down. One friend went in to try to grab her, and she too lost control of her leg muscles and fell down. Her two other friends tried to rescue the ...


47

Here's your opportunity. The market's looking for that right now so build a better mousetrap. The USDOE and California CEC want to murder the Edison base to finally stop people from using incandescent bulbs, and enable fixture designs that don't have to worry so much about dissipating heat. They mandated GU24 in 2008, which solves some of your concerns. ...


43

Lead batteries, used in a UPS, can explode violently due to hydrogen gas buildup. Mechanical damage will be evident, since the battery encapsulation will have failed. I put my money on this, if it can be heard from rooms away. Diodes and traces can explode without much mechanical damage or residue. Yet they can sound like a small lightning bolt depending on ...


42

The Fire Dept is wrong - it is perfectly normal to plug a device with a 2-pin plug into a 3-hole socket. Breaking the ground pin off a 3-pin plug, then plugging that into a 2-hole or 3-hole socket may produce an electrical hazard - possibility of a shock. If a high-current load, like an electric heater, was plugged into that burned outlet, and the contacts ...


41

Skin Depth The human body does have a "skin effect" but it's not as thin as you might think. Electric currents are confined to the outside of a conducting body, but humans are not very conductive, so the fields penetrate quite deep. The best example that comes to mind is 2.45 GHz - we all know that a microwave oven cooks about 2 or 3 cm into a piece of ...


40

Wingspan. Condors are large birds and can easily bridge the gap between the divided power lines. Pigeons, Crows, etc, are tiny. The only way to get shocked is by completing a circuit after all. Which is why that article has a spread its wings pun in the title California Condor Recovery Program Spreads Its Wings. From another article: The California ...


37

For a good connection that can withstand lots of movement/bending, use: Solder Crimp terminals (either permanent "butt" connectors, or male and female terminals if you want to connect and disconnect) Screw terminals ("terminal block") (Image from pixabay.com) (Image from wikipedia.org) For a more temporary connection that is quick to add/remove: Use ...


36

That's a cement block style, wirewound power resistor, sometimes called a square carbon power resistor. They look like this when new: The one in your circuit board has been mounted upside-down from usual, so you can see the guts of it through the opening slot in the bottom, but that's no big deal. Like all resistors, this component contains no fluid, so ...


34

I wouldn't get into that pool, but this isn't as bad as it might first appear: If the rubber is intact, then there is no path to ground. Electricity can only flow between the two conductors. There would be a strong field between them, but that would diminish rapidly with distance. The remaining currents thru the water (and you) a foot away from two ...


34

Ok, let's first get some things out of the way that may have to do with misapplication of a multimeter... Depending on the exact type of multimeter you use, your mileage may vary, but here's my guess about what happened, assuming your multimeter has separate inputs for current and voltage measurements, often labeled "[mA] [A] [COM] [V,Ω]...


33

Safety capacitors are classified by X and Y ratings. Let's properly define everything, and then it should become clear how those capacitors can be rated for both X and Y at the same time. Class X Capacitors: These are capacitors are only for use in situations where their failure would not present an electric shock risk, but could result in a fire. That ...


32

You got lucky. Electrocution isn't an exact science. There are multiple things which make a difference: The current which flows through a person depends on the resistance. That in turn depends on whether the skin is wet or dry, the area of skin in contact with the wire, and a lot of other biology things. The resistance of the things completing the circuit -...


31

The best way is to use an ESD strap, that is connected to earth via a 1MOhm resistor. This slowly but continuously discharges you without creating high currents. This way it does not hurt or damage anything. But I also want to mention one "trick", when you dont want to use special equipment like an ESD strap. Discharging at the earth connection of mains ...


30

Judging by the sheer volume, my bet is on an electrolytic capacitor. Those can build up a lot of pressure and violently burst under the right (wrong) conditions. I know you said the caps "look good", though electrolytic failures are not always obvious upon first glance. Sometimes they vent on top, with just a small slit. Sometimes they vent from below (...


30

First you should read up on the different ways of mitigating the risk. There's a few different, with different implications: Ex d Ex e Ex i Ex m Ex n Those are the main. Often a product will use many of the protection methods. Ex d is an approved enclosure, manufactured to withstand an internal explosion. That means that you can use normal equipment ...


29

FACT: 12 VDC CAN kill and has killed people. While 12V is almost always safe, worst case situations can and have lead to death. Mechanism may be ventricular fibrillation BUT paralysis of the respiratory muscles occurs at about 20% of the current needed to introduce fibrillation. See discussion and references at the end of this answer. 12 VDC applied ...


29

Thou shall NOT open-circuit a charged inductor. Thou shall NOT short-circuit a charged capacitor. If you think about it from their fundamental equations: \$V = L\dfrac{di}{dt}\$ - a sudden change in current (i.e. forced open circuit) will result in infinite voltage. \$I = C\dfrac{dv}{dt}\$ - sudden change in voltage (i.e. short circuit) will result in an ...


28

Yes it's toxic; No it's not mercury; Yes you'll live :) If it was a "wet" capacitor type, then most likely that was sulfuric acid or some organic or inorganic solvent. If it was a solid, then perhaps manganese dioxide. Whatever it was it isn't good for you so don't breath it, take a bath in it, or move to a planet full of it. But... one capacitor one time ...


28

48V is the practical and LEGAL definition of the maximum voltage to be considered "low voltage" and intrinsically "safe". Certainly 48V delivered UNDER your relatively insulating skin surface could kill you if delivered in the "right" place. But we are assuming people aren't walking around with subcutaneous electrodes exposed to accidental contact with "LV" ...


28

In something like water electricity does not "flow to ground" in a neat straight line. There is a potential difference between sections of water radiating out from the HV contact point. That might also mean that your feet are at different potentials, and there will be current flow which could be fatal. This is one reason why cows in fields can be ...


26

Circuit breakers are not enough to protect life. Circuit breakers are there to stop the cable in the walls of your house melting and possibly catching fire – circuit breakers and fuses perform the function of stopping a fire (which of course is also very dangerous to life). For direct contact with a live AC part, in the UK we have residual current devices (...


25

Engineers with a conscience don't let others use breadboards. Oh, I know, it starts out easy, like any other drug. You're uncertain of yourself and instead of doing calculations, or even back of the envelope calculations you say to yourself " I'll do the easy thing, I'll breadboard it- I don't have to think too much and then I can move on". But that is ...


25

It's not advisable unless the entire project is low voltage and battery operated. Anything mains operated could be dangerous even if transformer isolated. There are specific safety requirements for mains operated patient attached equipment to protect against excessive leakage currents and potential safety hazards due to equipment or component failures. ...


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