Tripping Out: A Field Guide to Circuit Protection

My introduction to circuit protection came at the tender age of eight. Being a curious lad with an inventive – and apparently self-destructive – bent, I decided to make my mother a lamp. I put a hose clamp around the base of a small light bulb, stripped the insulation off an old extension cord, and jammed both ends of the wires under the clamp. When I plugged my invention into an outlet in the den, I saw the insulation flash off the cord just before the whole house went dark. Somehow the circuit breaker on the branch circuit failed and I tripped the main breaker on a 200 amp panel. My mother has never been anywhere near as impressed with this feat as I was, especially now that I know a little bit more about how electricity works and how close to I came to being a Darwin Award laureate.

To help you avoid a similar fate, I’d like to take you on a trip (tee-hee!) through the typical household power panel and look at some of the devices that stand at the ready every day, waiting for a chance to save us from ourselves. As a North American, I’ll be focusing on the residential power system standards most common around here. And although there is a lot of technology that’s designed to keep you safe as a last resort, the electricity in your wall can still kill you. Don’t become casual with mains current!

Breaking It Off

What saved me that long-ago day was a circuit breaker. In its simplest form, a circuit breaker is just an electromechanical device designed to protect circuits by turning off the juice when the current flowing through it gets above the design point. Breakers need to sense the flow of current and turn it into mechanical action so that contacts can be physically separated quickly and safely. The most common mechanism are electromagnetic, where more overcurrent creates a magnetic field to pull apart contacts, and bimetallic, where an overload heats up a bimetal strip and bends it, activating the switch.

Residential breakers in North America come in a couple of different flavors. The branch circuit breakers are used to protect each branch circuit – the outlets, light fixtures, and appliances that are connected in parallel back to the main panel. Each branch circuit is typically rated for either 15 or 20 amps, and unless it’s running a large load like an electric dryer or a well pump, it’ll be a 120-volt circuit. In addition to the branch breakers, a panel will have a main breaker to catch any faults that a defective branch breaker misses, like what happened to me. It’s usually a 4-pole affair – one for each hot line, one for neutral, and one for the ground. A main breaker also acts as a switch to de-energize all the branch circuits, making it safer to work in the panel – but beware the lugs feeding the main breaker! Those are always hot, and the next breaker up the line is probably a big disconnector on a power pole.

Arrangement of bus bars in a distribution panel (L), and what can happen when a contractor really screws up (R). Source: Electrical

Main breaker panels around here have an interesting arrangement that allows for both 120- and 240-volt circuits. Power is distributed in a split-phase arrangement, where the transformer on the pole has a center-tapped 240-volt secondary winding. This results in two hot legs each at 120 volts relative to the neutral, or 240 volts across the two hots. Inside the distribution panel, the two hots are connected to bus bars with fingers that interlace. This allows installation of either single-pole breakers, which are connected to a single 120-volt hot leg, or double-pole breakers, which bridge the two hot legs for a 240-volt circuit.

If you ever come across a problem where every other single-pole circuit in your panel is working and none of the 240-volt circuits are working at all, you’ll know that one of the hot legs is not energized for some reason. You’ll probably want to call the power company in this case.

It’s the Current That Kills

On the first day of a college EE lab course, the instructor gave us a sobering safety lecture entitled, “It’s the Current That Kills.” The figures he quoted about how little current it actually takes to kill staggered me – how had I not gotten at least 30 heart-stopping milliamps through me at some point with the various line voltage shocks I had experienced? Turns out that I probably got way less current than that, and that it likely didn’t pass through my heart, but still, it’s nice to know that many circuits in a modern residential service panel are protected by ground-fault circuit interrupters. Known as GFCIs or GFIs in the States and residual-current devices (RCDs) in the UK, these devices are capable of cutting off the flow of current if as little as 5 mA leaks from the hot line. And it does so very quickly – about 25 to 40 milliseconds, which is less time than it takes a 30 mA current to put a human heart into ventricular fibrillation.

A GFCI built into a circuit breaker. Source: G. Fretwell

GFCIs work by sensing the current imbalances between two conductors using a current transformer. Both the hot and the neutral conductor pass through the current transformer’s toroid. Normally the currents cancel each other out, but if there’s an imbalance due to leakage, a current is induced in the transformer which can be used to trip an electromagnetic circuit breaker. That there’s a chip to take care of most of the sensing and tripping is not surprising; what is unexpected is the fascinating story of how the first GFCI chip, the LM1851, came into being in the mid-1970s. Spoiler alert: many graduate students were electrocuted in the making of the first GFCI.

In the US, GFCIs are required by code anywhere there’s the slightest possibility of water and electricity mixing. Originally intended for bathroom and kitchen outlets, GFCIs are now also found in basements, garages, and in any outdoor outlets, especially for pools and spas. While the most common form factor for GFCIs is built into a duplex outlet, some circuit breakers have GFCIs built in. And of course there’s the wall wart GFCIs that now grow out of the power cords of every hair dryer and curling iron manufactured.

Good Arc, Bad Arc

A more recent circuit protection modality is the arc-fault circuit interrupter (AFCI). Intended to prevent fires due to high-temperature arcs, AFCIs have been required by code since the early 2000s for branch circuits servicing bedrooms. Early AFCIs were subject to nuisance tripping from “normal” arcs, like the brushes in a vacuum cleaner motor. As the technology improved and nuisance tripping was reduced, code requirements were rewritten so that AFCIs are now required on all branch circuits, servicing just about every room in the house.

Typical voltage (black) and current (green) arc waveforms. Note the "shoulders" at zero-crossing on the current trace, and the way the resultant voltage waveform approaches a square wave. Source: Electrical Installation Wiki
Typical voltage (black) and current (green) arc waveforms. Note the “shoulders” at zero-crossing on the current trace, and the way the resultant voltage waveform approaches a square wave. Source: Electrical Installation Wiki

While AFCIs sense current anomalies like GFCIs do, the similarities pretty much end there. AFCIs also need to monitor voltage and to analyze the waveform of the circuit under monitoring for telltale signs that potentially destructive arcing is happening. In addition to recognizing arcs between the hot line and neutral or ground, AFCIs need to detect series arcs, which might happen when a hot wire wiggles free within a loose terminal. All these arcs have characteristic waveforms that a microcontroller analyzes to determine if a fault exists while ignoring arcs from equipment operating normally. It’s complicated stuff, and it’s no wonder it took a while for manufacturers to get it right. A good treatment of the specifics of the detection algorithms can be found in this paper (PDF link).

Of course there are plenty of other devices working to keep your electrical supply clean, like surge protectors, noise filters, and UPSs. But these are mainly for keeping your devices healthy and happy. Breakers, GFCIs, and AFCIs are are life safety equipment, whether by protecting the structure around you from catching fire, or by preventing you from getting bitten by many Angry Pixies when you drop your hairdryer in the toilet. But remember that they’re the last line of defense — at the end of the day it’s mostly up to you to make sure you don’t do something to as dumb as I did way back when.

62 thoughts on “Tripping Out: A Field Guide to Circuit Protection

      1. Funny how we see faces in those outlets. When they started coming through with polarized grounded outlets, I thought it looked like someone squinting.

        IIRC, the NEC states the preferred method to install a grounded outlet is with the ground circuit up. That’s to prevent a short circuit in the really unlikely event a plug is partially removed from an outlet and a conductor somehow falls along the wall and drapes over the hot and neutral. With the ground up, the offending conductor should contact only the ground. But I’ve only ever seen outlets mounted this way in hospitals, and it bugs the crap out of me. It’s supposed to be a face!

          1. My dad says he got into a debate with an electrical inspector about this once, back when he worked for the municipal offices where we lived. I don’t remember which side of the issue he claims to have been on, but the lunchroom consensus was that the situation was so fantastically rare as to make the argument all but academic.

            Later that very year, he and some uncles were installing aluminum siding on our house, and one of the guys noticed a “poke” while stepping off the ladder. Sure enough, a piece of siding had fallen lose and lodged against the hot pin of a partially-unplugged extension cord. The partially-complete siding along the whole side of the house was electrified! The ladder’s rubber feet were isolating it from the ground, leaving the hapless humans to bridge the connection.They stopped the job, killed the breaker, flipped the receptacles, and got back to work.

            Stuff’s no joke.

        1. Not actually that unlikely, especially in cases where outlets are used a lot (worn, so they don’t grip as well as they should) or have a heavy cable plugged in without retention (check the plug for the outlet strip behind your desk. Now push the plug all the way back in. You’re welcome), and especially if the cord is subject to pulling in use.

          The ground pin (stab) is longer than the current carrying stabs (in the US) and will not separate before the others do. Think of a plug partially falling out, like for your shopvac. The ground on top makes it a) less likely that the current carrying stabs will disconnect since, in the loose plug situation, the current carrying stabs will tend to stay in as the ground stab works out, and b) if, by chance one does, and it is the neutral (grounded) stab, BOTH stabs will be hot through the appliance. You go to check the plug. It is likely low. What are you going to hit first by accident? The one on top. The ground. Ditto if someone (a child, a common-sense challenged coworker) goes after the plug with a butter knife to get it out. Much more likely to go from the top. Yes, I have had coworkers use a knife to pull out a plug. Better than jerking the cord, I guess…

          Only possible safety advantage to ground on the bottom that I know of is the plug falling out situation when there is a fault in the appliance, as there is near zero chance the ground will separate first if it is on the bottom. Of course, the design is supposed to insure that a compliant ground lug can not separate fist no matter what angle the plug is coming out at, so this isn’t really a significant issue.

        2. Wow. In Australia our outlets are the other way. Our laws say the Earth pin needs to be down so if a plug is falling out, the Earth pin is the last thing disconnected. All new plugs now also require the last part of the active and neutral prongs closest to the plug base to be insulated for safety. I guess it would eliminate the situation described above.

          1. The world should just adopt the British plug. It’s got loads of safety features built-in, and an earth connection has been compulsory since it was introduced. Except for double-insulated appliances.

            The world should also call the thing in a socket that’s connected to the planet the “earth” pin. Ground is too easily confused with 0V. The British Standard chaps, back in the ’50s or whenever it was, pretty much got everything right first time.

            Except for what happens if you step on an upturned one with bare feet. Fortunately swearing is something the British are also accomplished at.

        3. In Australia, we have the active and neutral prongs on the plug insulated about halfway down their length so no metal is exposed until the electrical connection is broken even if the plug is slightly dislodged.

          I’ve seen many burnt plugs in workshops where metallic swarf works its way between the plug and socket and burns. The insulated type of plug fixes that along with slipping fingers and prying children.

      2. Because real outlets don’t have eyebrows, nor do they sweat. Useful trivia – ever wonder why all the outlets in hospitals are installed upside-down? Spoiler alert : they’re actually right side up. When the ground lug is up, it’s the first line exposed when a lug and cord starts to sag. That way it fails safe(r).

          1. I dont know about the exact lengths, but what you said doesn’t preclude what eriklscott said. The Ground can be the first to connect and last to disconnect, but that doesn’t mean that if the plug is out of the wall a bit, the hot/neutral wont still be in. It just means that the ground will definitely be in (which is the point). If the plug were the other way up, the ground being in wouldn’t make any difference as anything draped over the plug would connect only the hot and neutral.

  1. For those of you from North America, are these circuit ratings typical? How many sockets do you have on such a circuit? Total capacity (1.8 to 2.4kW as described) just seems very low from a UK perspective. All my socket circuits are 32A at 240V (7.6kW) and individual appliances can be up to 13A at 240V (3.1kW). Obviously lighting circuits are lighter duty so we can get away with thinner cabling but even my lighting circuits are rated at 6A, 240V

    1. UK wiring and plugs etc look “light industrial” compared to North American. Having dealt with UK electrics in my past, the cheapness of NA sockets and plugs is often mildly alarming. Wall sockets are more or less a consumable that you have to swap out every 5 years due to wear letting the plugs fall out, or arc.

      1. To answer the actual question, derp…

        More or less every 5 to 10 years the code is overhauled a bit so houses of different ages have different levels of service and/or safety provisions.

        Mine was last redone late 90s, I have two of 20A/115, one 15A/115 and one “stove” 30A/230 circuits in my kitchen, lighting are usually 600W circuits, typically you’d have one 15A circuit per bedroom, two in the living room, one for common areas, one outdoor 15A. “Service” is typically 100A from the “pole” since many areas have it all above ground on utility poles. You’d then have a “100A” panel with a main breaker, with breakers or fuses (older) for all circuits. Newly built houses may have “200A” service, that’s due to codes now specifying a large profusion of outlets, something like minimum 1 per wall and 1 every few feet, in every room, so if you’re going to have 10 double sockets in an averagely roomy living room, better make sure you can load them all up at once…

        1. Derp about the lighting, they’re 15A circuits but only 600W per fitting is usually provided for… I think… IDK, when I mess with anything I double check the specs, but in between times they evaporate.

        2. Interesting, thanks. It seems you have many more circuits than we do. We are unusual in having ring circuits in the UK (post WW2 copper rationing – they’re more efficient) and each one can cover 100m^2. Like you say code changes and we have all our circuits on RCD now so in my “typical” house with moderately up-to-date electrics I have:

          Main double pole breaker (100A)

          RCD1 covering:
          1x32A socket ring upstairs
          1x6A lighting circuit downstairs
          1x32A dedicated circuit for backup water heater

          RCD2 covering:
          1x32A socket ring downstairs
          1x6A lighting circuit upstairs
          1x32A socket ring for kitchen (not required but commonly done as the kitchen has a lot of high draw appliances)
          1x40A cooker circuit – amusingly I have a gas cooker and oven so the only thing I run on the fat cable is the tiny piezo-ignition circuit…

          You’d usually have separate circuits for outside etc but I don’t have any sockets there. I can also run specific circuits for appliances like the fridge/freezer if I wanted them so that they don’t defrost with a general fault. There are problems with having *all* circuits on RCD.

          1. Is the 100A main breaker combined rating or per phase?
            I was going to ask why so much, but then I remembered that most houses here that use electrical heating have 3*32A…

          2. To reply to AKA the A, it’s just a single phase supply with a live at 240V and a neutral at 0V. It’s possible to get multiple phase installations as they’re usually available on the street but at 415V phase to phase, they’re usually a little above what one needs in a house. Maybe for the welder ;-)

          3. just to be pedantic the double pole ‘breaker’ is not usually a ‘breaker’ at all in that it doesn’t provide overcurrent or short circuit protection, its simply a point of isolation for the whole installation as required by regulations.
            overcurrent protection for the installation is provided by a fuse before the meter which you aren’t supposed to touch because its owned by the network operator. typically the fuse is quoted at 100A but 60 and 80 are common.

    2. I’m not an NEC expert, just a homeowner (1960s home). 15A and 20A breakers are common for both lighting and outlets, depending on the age of the house. Number of outlets on a circuit is more of a rule of thumb if I’m not mistaken. High draw devices usually have their own circuits (such as 240v electric ovens), and my 120v fridge has it’s own 20A. NEC if I’m not mistaken requires bedrooms to be on individual circuits, same with bathrooms and kitchens. Since my house is old it’s rather loose on modern code. I’ve got garage outlets/lights, living room outlets/lights, and kitchen/dining-room lights all on one 15A 14AWG circuit. Currently working on splitting the garage to it’s own circuit.

      1. I had commented on that before but for some reason it is not showing… AFAIK, it is illegal to switch, breaker, or fuse a Neutral (allowed in UL508A for motor loads, though). And switching protective ground is never allowed. The 4-pole breaker is probably just two 2-pole 100A in parallel to allow for a 200A panel fed with smaller 100A rated wiring from the pole. I think that is still kinda sketchy but NEC allows it.

        1. A setup like this is really only done for two reasons:

          1: MWBC, a Multi-Wire Branch Circuit.
          –Uses a wide circuit breaker or “double breaker” that contains a cross bar so that both segments are turned on/off at the same time.

          Can be run from a service panel out to a junction box using THHN individual wires run through a conduit (can be metal or plastic [non-metallic,] rigid or flexible).

          At the junction box the wires can be separated out into separate circuits:
          Circuit 1 = Hot (black) + Neutral + Ground.
          Circuit 2 = Hot (red) + Neutral + Ground. The Neutrals would get wired-nutted together (a pressure wire connector.) The Earth’s would get connected together and if the box was metal, would get a pigtail ground to bond them to the electrical box.

          2: The load you are driving requires BOTH 120 and 240V. Example: a piece of equipment has a 120V light outlet socket and a 240V motor. The larger load, the motor is driven from the Red and black, no neutral connection is used. The motor’s body, if it is metal will be bonded to Earth ground at some point. The 120V light outlet would use either red or black + neutral.

      2. The hots are 115 each to neutral, but they’re different phases, so ~230 to each other. The neutral is the return, usually ground connected at the panel, however, you don’t want to be using that for a ground since normally returning current is going through it when an appliance on circuit is operating, so you also have a separate ground/earth.

      3. Only the two hot lines are protected. The neutral is bonded to the ground at the panel and the ground is connected to a grounding rod outside your house.

        Also the comment about one side of your panel not working is incorrect, too. It would be every other circuit not working, The phasing alternates down both sides of the panel, not across it.

        1. Thanks, you’re right. I had a pole fault that dropped one leg of my service in our first house, couldn’t remember the pattern of the breakers that didn’t work. I’ll fix the text.

          1. You’re killing me with the “phase” talk. It’s a single phase to any house.

            Need multiple phase circuitry then you’re using a rotary phase converter or variable frequency drive.

      4. As an EE from europe, i can tell that it is never allowed to switch protective ground.
        Neutral can be switched, depending on application, therefore you sometimes see a four pole switch for a three phase system, where the fourth pole is not always connected.

      5. Interesting guys thanks
        in the UK for a single phase installation a method of isolation typically a double pole (line and neutral) isolator is a requirement (537.1.4). isolation is required for every piece of equipment (537.2.1.2) which is usually a plug & socket and specifically motors too.
        Switching only neutral is specifically not allowed (530.3.2)
        Three phase systems under certain earthing arrangements can be isolated only on line/live and the neutral left connected but i don’t know the specific regulation.
        Three pole isolators are common for bathroom extractors but usually the third terminal is for a switched live facility, certainly not for the cpc!
        I don’t know if there are any specific regulations that precisely state that earth/ground must not be switched. considering the most common arrangement is to bond the neutral and earth at the cutout (around the incomer fuse) i suppose you could argue that a dual pole isolator in the consumer unit is the real oddity.

    3. Yes, those circuit ratings are the norm. As the author stated, there are higher current circuits for special purposes like electric water heaters, stoves, well pumps, etc.

      There can be anywhere from 1 to 13 (i think) receptacles on a circuit. We are supposed to have outlets everywhere so that an appliance cord need not be more than 6 feet long. The max allowed draw from an individual device is 1.5kW, or 12.5 A at 120V.

      Our electrical codes do specify a number of outlets that must be on individual circuits for purposes such as refrigerators, microwave ovens, laundry machines, etc.

    4. The apartment I live in was remodeled just before I moved in, so the electrical system is up to the current code. I have two bedrooms, one bath, living room, kitchen, and an entry way. In the breaker box, there are 14 breakers: 4x240V and 10x120V. Water heater gets 30 amps, air handler gets the same, HVAC gets 20 and the stove gets 40. Yes, the HVAC is not normal, it blends some of the heat pump function in with the hot water tank to store heat, or so I was told. For the 120V circuits, everything is 20A and each room’s outlets are ganged with the lights. The entryway and part of the kitchen (actually just the hallway) share one loop, then the kitchen gets a left side, right side, and dishwasher loop so the toaster and microwave don’t combine to need a larger breaker. Then one for each room’s lights and outlets, one ceiling lamp and around 5 outlet panels per room. Then the bathroom gets its two light circuit, and a separate electric heater and inlet fan circuit.

      Yeah, it is serious overkill, but it is nice to know that my experiments in the second bedroom/office won’t black out everything else. Kitchen and bath are GFCI, every outlet has small sliding panels that block the hot and neutral holes when there is nothing plugged in. They slide out of the way if you push on the center of the outlet “face”, or with a little extra force, or a ground pin will push them aside.

      Hope those details help.

    5. Has been 40 years since I owned a copy of the National Electric Code. At that time even a on bedroom apartment was required to have 100 A. service. Two 15 A. lighting circuits, 2 20 A. small appliance circuits. one 15 A. GFIC, one 15 A. laundry circuit. The actual size of the service had to be able to support an electric kitchen Range, electric cloths dryer. Central AC, and electric hot water heater if the home is to be equipped as such. The NEC book is very expensive to purchase and I don’t have modern cop since I no longer work in that field.

  2. Thanks for the tips about AFCI. Never heard of them. I wonder if they’ll trip when the next $5 tier-3 gizmo ripoff from overseas starts ignition?

    After working outside north america and being introduced to DIN rail in industrial electrical panels, I will always prefer that style of mounting for components in projects. I did learn a valuable lesson in never buying cheap knockoffs for switchgear and electrical safety. Yeah, those contactors rated for 10 amps? They melted *on* at 3 amps. Not the safest event in industry.

  3. In the GFCI, it is not the currents that cancel out; it is the magnetic fields these generate that do and therefore they do not induce an EMF on a secondary winding. When the two currents are different, there is a net varying magnetic field, which generate an EMF which is detected by the circuit.

  4. And then there are people like me who have a house that was built in 1954, and the people who built the house lived in it until they both passed away (Her in 2006, him in 2013) and has a lot of the original features, like all but two of the original Honeywell thermostats for the cable ceiling heat (Which all works, I just never use it), but since we’re on the subject of circuit protection, it still has the original Square D fuse panel with screw-in glass fuses! Some of them are modern fuses, but the cylindrical fuses for the main disconnect as well as for the water heater and the range look very old if not original, and a few of the thread in ones are the older style all-glass fuses. (Yes, I am replacing it with a modern breaker panel, but that’s gotta wait for tax time)

      1. That’s what I’ve heard, I’ve also heard that they are actually safer than a circuit breaker as they will trip faster… My mother in law’s house has a Federal Pacific load center, but she got all paranoid about our fuse box and insisted that we replace it before we moved in! (We didn’t, didn’t have the extra $ lol) Only “unsafe” part about the fuse box is the fact that every circuit in the box is double tapped, although the house is pretty devoid of outlets (2 or 3 in each room) and I’ve replaced every light fixture in the house with CFLs and LEDs (Primarily LEDs, one ceiling fan still has the 40W incandescent bulbs in it, but when we moved in all of the overhead lights were 4 and 5 bulb monstrosities with 120W bulbs… I can turn every light in the house on and it draws less than one fixture did before! 4 LED fixtures that are 18W each, ceiling fans have 3 or 4 5W bulbs (Two have 3, two have 4, one if the 4 bulb ones is still incandescent), rest are CFL or tube fluorescent… House is pretty well insulated too, my energy bill is usually around $60, will hopefully go down once I replace the fridge and the water heater…

    1. My parents survive with their Edison outlet style glass fuses. The kitchen one goes when they forget that they bought a 1500W microwave and try to use it with the toaster or electric grill. The rest, they must keep the electricity flowing.

  5. “It’s usually a 4-pole affair – one for each hot line, one for neutral, and one for the ground.”

    Breakers don’t cut off the neutral. And they certainly don’t cut off ground.

  6. Statement “current kills” is actually not true. I wonder what school you went to but that’s not the case. It takes both current and voltage to kill you. Without enough voltage to provide current the statement current is not accurate.

    1. That’s not actually true either. Voltage is (usually) necessary for current to flow through the body, but it’s the current that does the killing.

      Take a “nonlethal” voltage and then reduce the resistance involved. The voltage is the same, but the current is now deadly.

      Perhaps we could settle on “wattage kills”?

      1. I prefer to say power kills. That confuses the shit of those who recite it’s the amps that kill, not the volts mantra. ;) Generally it the same people who say 240 VAC is more efficient than 120 VAC.

  7. A couple of details. The wetness/dryness of your skin has an overwhelming impact on whether you get a lethal shock. (Wet is very, very bad. Dry skin has a lot of electrical resistance. A cut has very little resistance.)

    I was taught to keep my left hand in my pocket to force me to use my right hand to touch anything that might be even remotely live — that way the shock travels a little less through your heart on its way to your feet and the Earth.

    One slot on the outlet is larger because that’s the one that handles the neutral return wiring. That prevents you from pushing the large neutral plug on your lamp cord into the “hot” side of the line (which is too small to let it in). The neutral (large plug) should be connected to the outer casing on your lamp socket, NOT the small pin at the bottom of the socket. That makes it harmless if you accidentally brush your finger against the top edge of the socket/bulb when you’re searching for the lamp switch. Really old stuff has both pins the same size — replace the plug end or get rid of it.

    You can get a $5 tool that plugs into a socket and tells you if the wiring is correct. Two green lights is “good” and a red light tells you what’s faulty.

    Many U.S. homes built during the 1960’s used aluminum wiring (it’s cheap) and copper sockets. This causes a chemical reaction that gradually corrodes the connection. Eventually, you get arcs and a fire inside the wall. If a socket feels warm or acts weird you should replace it immediately or stop using it. Modern sockets are marked AL/CU because they can handle either aluminum or copper wiring. You’ll find the busiest fire departments in the country in neighborhoods built in the ’60’s.

    I installed a dimmer switch that controls the duplex wall socket and table lamp across the bedroom. That left one socket available, and the housekeeper kept plugging the vacuum cleaner into it and blowing up the dimmer switch. I finally blocked the open socket with a baby-proof plastic plug to remind her (and my wife) not to use that socket. I could have also broken off the small tab inside the socket that is meant to enable you to connect the second socket to a full-voltage connection.

    1. Depending on the particular scope of the product safety standard, the human body model for determination of shock hazard is about 1kohm to 1.5 ohm for a ‘normal’ healthy person. Body impedance tends to be a bit less for females, so can be more susceptible to shock hazards from similar touch and leakage currents. In any case, given this Z range, it becomes a matter of sufficient voltage at a low enough frequency(much touch/leakage current is in the kHz where reactance increases Z) to drive about 15mA to 25mA through the heart to cause defib.

      ITE and A/V safety standards, depending on type of equipment, typically limit this contact current to somewhere under 2.5mA to 10mA for normal operating conditions (more allowed during fault condition). The IEC committee (TC108) that writes safety standards for most computer and A/V equipment are very much frustrated with the medical community’s poor data on electrical models of the human body. Hopefully, the bioengineering community will fix this.

      The ‘seminal’ references for this:
      1. Effect of Wave Form on Let-Go Currents, Charles F Dalziel, AIEE Transactions
      in Electrical Engineering, Volume 62, December 1943.
      2. Hart, W. F., A Five-Part Resistor-Capacitor Network for Measurement of
      Voltage and Current Levels Related to Electric Shock and Burns, in J. E.
      Bridges, G. L. Ford, I. A. Sherman, and M. Vainberg (eds.) Electrical Shock
      Safety Criteria, 1985, Pergamon Press, New York, pp.183-192.

    2. “I was taught to keep my left hand in my pocket to force me to use my right hand to touch anything that might be even remotely live”

      As an Australian electrician, I was taught (and it was drilled into us) to ‘test before you touch’.
      Why even risk receiving a shock for a test when a multimeter will do it safely and accurately?

  8. “I decided to make my mother a lamp. I put a hose clamp around the base of a small light bulb, stripped the insulation off an old extension cord, and jammed both ends of the wires under the clamp. ”

    I’m just going to go ahead and evolve that design a little… hoseclamp, clothespeg, (clothespin), cord, screw, winebottle… clamp just ONE end of cord under bulb with hoseclamp, break off or cut a bit off one side of the clothespeg so it will hold on the screw housing of the clamp and touch the other jaw to the base of the bulb… now put a screw in that long side and tighten down the other end of the cord under it, so now when you clip that on, LIGHT!!…. now holding very carefully by the peg, so as not to aggravate the pixies, shove one side of the other end of the peg into the wine bottle to hold it…

    For extra extreme hacking sports points, do this in a bathtub full of water.

    Feel free to optimize with more or less parts to enhance operation or danger level.

  9. Worth mentioning that code enforcement in US can be pretty lax. A common very stupid thing I’ve found is that people “upgrade” the outlets to 3-prong type, however the ground pin is not connected to anything. Since the wiring in the walls is only 2 wire, some people are too lazy or cheap to replace it with 3 wire cables. So just because you have 3 pronged outlets DO NOT assume its actually grounded.

    1. This is how my house is, all the outlets are 3 prong but the ground isn’t connected. Some of the wiring was replaced, and it’s got three conductor wiring, but the ground isn’t connected to anything, so all my surge protectors complain about a ground fault.

  10. This reminds me of when I was building a transformerless power supply and the box of fuses, which I knew I had seen a few days earlier, was gone. Decided, against better judgment, that an M3x20 screw and nut would suffice (I did use an X2 cap). Anyway, plugged in the PCB and instead of the LED lighting up there was a very loud pop, followed by complete darkness and a toasted PCB. It turned out that in addition to the blown 10 A circuit fuse this incident had tripped the RCD as well, which was a surprise as it wasn’t earthed. I am still not sure how exactly that happened, but either the current had briefly exceed the 40 A it was rated at (40 A x 230 V = 9.2 kW!), before the ceramic 10 A fuse blew, or the capacitors on the board caused some strange current mismatch…

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