Electronics notes/Electricity and humans

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⚠ This is for beginners and very much by a beginner / hobbyist

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes

Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes

Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · power supply considerations · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.


Galvanic skin response (GSR)
This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Galvanic skin response (GSR), Electrodermal activity (EDA), electrodermal response (EDR), psychogalvanic reflex (PGR), skin conductance response (SCR), sympathetic skin response (SSR), and skin conductance level (SCL).

(Because of the varied names accumulated over time, EDA is intended as a new standard name, but some fields haven't moved yet, so many existing documents will use old terms.)

All are fancy names for 'how much electricity does your skin conduct'.

Some check just that.

Some suggest indirect measurement of mental state.

Sweating is a response controlled by the sympathetic nervous system, to things like

basic physics - warming up due to exertion, or environmental temperature, that mean you have to thermoregulate
health - hydration level, fever, ingestion of irritants, contact with toxins, menopause, other hormonal changes, like change in medicine, hyperthyroidism, reaction to caffeine, morphine, alcohol and other drugs, insulin levels, differences in ability to thermoregulate due to health, differences in ability to thermoregulate due to humidity
mental origins - strong forms of anger, fear, anxiety, arousal

...mental state?

The idea is that

if you can control for everything else on that last list - physics of the moment, and health, and further calibration,
then sweating could indicate the last few on that list, making skin conductance one measure of emotional and sympathetic responses.

Criticism includes:

  • that's a lot of ifs
  • hydration level affects the response regardless
  • response varies by location, being driven by somewhat different responses
  • sweat response has delay, seconds to longer depending on the type of response. It becomes hard to tell what it's a reponse to.

It's impossible to avoid mentioning the polygraph-style lie detector, which measures not only GSR, but also heart rate, breathing rate, and blood pressure.

The idea idea is that these are all controlled by the sympathetic nervous system rather than consciously, so you can't voluntarily change them, which is the entire reason they're sold as lie detectors.

But there are reasons that they are no longer admissible in most courts, for many detailed reasons - that all amount to that they are unreliable at best.

Even its inventor has spoken out their frustration by the the ways is regularly misused.

Problems include that

  • it cannot distinguish between different forms of arousal - consider
    • anger (because of strong personal ethics)
    • worry - that this will be treated as proof, thinking of social implications
    • fear - including the one where it it just sunk in what the implications might be if this pseudoscience is taken seriously
    • startle response -
    • sexual arousal - maybe you just have a thing for authority figures
none of which have anything to do with lies, let alone guilt
  • it cannot distinguish between such responses being involuntary, and such responses being triggered intentionally
remember, the indicators are very basic and low resolution. All they really measure is 'body is a little different', not how or what
  • you can intentionally mess with the calibration, e.g. by thinking of some anguish at the right time
  • you can intentionally to suppress these responses by mentally distracting yourself
Also means ADHD people are just less guilty overall, huh?
more to the point, this can be trained
  • it does not consider that sociopaths won't have some of these responses to start with, by definition

See also:

electrocardiography (ECG)

Measuring the heart muscle (-cardio-), which at skin surface is seen on the scale of a millivolt or so.

A little harder to DIY, because that's weak there is also typically coupling of electrical fields (and easily AC line noise). For a start you want a differential amplifiers (probably an inamp), and probably some shielded or twisted wire to minimize induction, think about positioning (or you may get common mode voltage via impedance mismatch(verify)), maybe a passband filter.

And right-leg drive (probably here called something else) can't hurt.


Electromyography (EMG)

Measuring skeletal muscles (-myo-), which seems to be one the scale of dozens of millivolts (verify)}

See http://en.wikipedia.org/wiki/Electromyography


Electroencephalography (EEG)
This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Measuring activity in the brain (-encephalo-) - which (outside) is on the scale of dozen microvolts (to at most millivolts (verify))

At a low level, you measure a tiny fluctuating voltage change at a spot on the surface of the scalp, comparing that voltage to other such spots for reference, using metal electrodes and conductive paste for better contact.

Muscles nearer the surface are much stronger signal - a clenched jaw or blinking your eye is very visible in data (in very different ways - jaw is high frequency, blink low). In fact some of the electrodes are only there to measure those in isolation, so that we have a chance to remove them later

Comparing the voltage to other such voltages can (only) tell you things like

  • that an area broadly became more or less active
  • when you started reacting
  • differences between things like "makes sense" and "that's weird and I'm processing why".
  • broadly, which areas of the brain engaged more than others

Electrocorticography (ECoG)

Electrocorticography (ECoG), or intracranial electroencephalography (iEEG), is like EEG but sensing directly on the brain itself, rather than from outside the skull and skin.

Is surgically invasive, so generally only seen around things like epilepsy treatment.


Magnetic resonance imaging (MRI)
Magnetoencephalography (MEG)
Functional Near-Infrared Spectroscopy (fNIRS)

Functional near-infrared spectroscopy (sometimes Optical Topography (OT)) uses near-infrared spectroscopy to estimates the concentration of hemoglobin to estimate activity, so is useful for basic functional neuroimaging.

It is noninvasive and has high temporal resolution, but low spatial resolution and only works near the surface.

(it's not electronic and doesn't really belong on this page, but it mostly does belong in this list of measuring humans)

Electrooculography (EOG)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Electrooculography (EOG), a.k.a. retinal Electrooculography, basically helps record eye movements.

EOG attaches electrodes above and below, or left and right, of the eyes.

It measures the electrical potential between the front and back of the eye, which (due to where that potential actually comes from) is an indirect measure of a few different things.

  • dark adaptation and light adaptation
  • Under certain conditions, it is also a decent measure of eye position.
  • EEG recordings will often spend a few channels on EOG,
...because this varying potential, as well as the muscle activity of blinks, will make it onto nearby EEG channels, and recording this makes it easier to later analyse and mostly-remove this influence
  • some diagnoses, related to light response or muscle atrophy (verify)

See also:

Electroglottography (EGG)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Measures the how closed the vocal folds are, via the resistance of two electrodes around the neck.

Electronics side

Electropalatography (EPG)


Electromagnetic articulography (EMA)

a.k.a. Electromagnetic Midsagittal Articulography (EMMA)

Positioning the tongue, using sensor coils in EM field, receiving signals from multiple fixed transmitters (chunky thing above head).


Not actually electrical

...but nice for this list if you were looking for a "how to measure humans" list.

Photoplethysmography (PPG)


This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.



TENS, Transcutaneous Electrical Nerve Stimulation

Aims to desensitive nerves, so is used for short-term pain relief.

Higher frequency (and less strength?) than EMS, only enough to feel.

Electric massage is closer to TENS than EMS(verify)

See also:

EMS, Electrical Muscle Stimulation

Also known as NMES (neuromuscular electrical stimulation) and electromyostimulation.

Used to help muscle training, such as that to avoid atrophy.

It uses pulses that are strong and long enough to trigger muscle contraction, and helps engage more of an muscle that you consciously engage. It's not that effective without your help, though.

See also:


High frequency

Purple wand

Static electricity, ESD, and humans

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

ESD and electronics

Avoiding ESD

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.



On (not) causing tingling

Laptop tingle / zap

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.


  • continuous tingling on exposed metal (sockets, screws, metal bodies, e.g. in your hands, on your lap)
  • felt only when the adapter is connected
  • Felt more clearly when
the contact area is small
you have good contact with the device (e.g. sweaty hands/legs)
you have good contact with the floor (e.g. no shoes or socks, moist concrete)
When measured, this is AC, and up to a few dozen volts, and usually not more than a milliAmp or two

Phone zap / tingle

Mostly similar to the laptop example above.

Many of these chargers are ungrounded wallwart-style, making this a likely thing.

One difference to the laptop case is that there are a lot more cheap-and-particularly-crappy phone chargers out there (than there are low-grade laptop adapters).

Some of them just low quality output, a few actually unsafe.

See e.g. [1] [2]

It the touchscreen doesn't work as well, or at all (interferes with the way capacitive touch works) whenever it's on a charge cable, it's probably just a low-quality charger and you can fix that by buying a better one. It can be hard to tell quality from looks. Reputable brands are a decent though not sure-fire way, and there are some decent cheaper-brand ones.

Less intentional / on not killing people

Electricity through humans

Human resistance ratings
Current through humans - where does the risk start?
More context
"It isn't the volts that kill you, it's the amps"
On current capacity

Electricity and heat

Resistive heat
On circuit breakers
On extension cords
Arcs and arc faults


Safety earth


Earth wiring within a house is there for safety.

Short version: it is a return connection that is much lower impedance than you are - and passes enough current enough that a fuse or circuit breaker would soon disconnect the live wire.

One of the main uses is earthing a conductor that should never be live - but might become so when things fail. For example, the chassis of a toaster or washing machine or such.

If they eventually break so that wiring comes loose and touches the outside, and you want to design for that possibility.

With safety earth on that chassis, that's a short across your house's breaker, which will probably trip, meaning the dangerous voltage on that chassis is on there for a fraction of a second.

(Also it's across two chunky wires that won't have any issue with that current for factors longer than that)

Even if you touch it within this time, the earth wire is much lower resistance to earth and (since this is a voltage divider) you shouldn't feel much.

Without safety earth, the live wire touching the chassis mean nothing other than that it is now at mains potential.

This has few immediate implications, but you will notice this when you next touch this and you are even vaguely earthed.

The current through you is also much lower than any real use, so there is no reason for a breaker to trip.

(Which is why RCDs exist, and complement breakers)


Earth (or earth ground) refers to a physical pole hammered into the earth somewhere nearby.

Earth wiring is connected to that pole.

Typically this is also bonded to things like radiator pipes, water pipes, steel structures. So generally all these things are at most a few ohms to this earth.

Devices like think ovens, washing machine, etc. are typically designed with safety earth, and using them safely will require that connection to earth - which, due to electrical code, usually just means 'socket with earth pin'.

Some (big) devices will even test the grounding and refuse to work if not present. Yet most will assume that your electrical wiring is not violating electric code. Just at all, or because they have an earthed plug and it's technically a code violation to have an earthed socket that isn't actually earthed.

(Note that powering things on generators is a little more interesting, ground-wise)

Earth versus ground

Residual-current devices

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Known under varied terms, including:

  • Residual-current devices (RCD)
  • Ground fault circuit interrupter (GFCI)
  • Ground fault interrupter (GFI)
  • residual-current circuit breaker (RCCB)
(Note: an RCBO is a residual-current protection and an overcurrent protection in one)
  • Earth leakage circuit breaker (ELCB)
  • Appliance leakage current interrupter (ALCI)
  • Leakage Current Detection Interrupter (LCDI)
  • FI - Fehler and I indicating current (verify)
  • Differential circuit breaker

There are apparently a few different designs. The common variant measures the current difference between the current going through the live and neutral wire, because those two currents should be exactly the same. Conceptually / component-wise, you can think of it as a small sensing transformer coil around both live and neutral wire. When current in is equal to current out, that sensing transformer outputs nothing.)

And if there is a difference, we assume that the extra current path is via ground (either the socket's safety earth, or the literal ground), and for safety we assume this might be going through a human, so it trips to cut power.

Electrical code varies when it comes to RCDs. Building styles also vary, with country and with time.

Sometimes you'll see a chunky RCD on each leg/phase at the power board - only.
Others add them per socket, and then often only in the kitchen and shower.

Others protect other areas, double them up to project everything but trip smaller zones, and other variations.

Building sites may put a series of larger-and-smaller RCDs on different areas because it interrupts fewer people from working, and makes it easier to diagnose who's tripping it.

You can also get RCD extension cords, which can e.g. make sense when you do electronics repair work, possibly for rooms that are not protected (if that applies to your house).

While a lot of them are standard-valued, you can sometimes get ones that responds faster and for lower current, which can be marginally safer, and sometimes more importantly, means the one at your desk trips, not the house one, saving you and possible housemates a lot of annoyance and waling.

You'll probably accept just your DIY setup being overly triggery, (and/or better isolate the causes).

Certain situations (e.g. datacenters) may use higher values, because false trips are much more costly, there can be some protective equipment that would trip RCDs more easily(verify), and technicians can be instructed better than random people in random houses.

RCD aim to trip at the order of a dozen milli-amps for a dozen milliseconds or so, as a balance between 'most humans will survive that' and making it less sensitive to stray currents, and making nuisance trips unlikely.

The typical trip currents vary, e.g. For

home installations in 230V countries, typically ~30mA for ~20 milliseconds
home installations in 120V countries, often quoted as just "5mA".
I've not been able to find the time that belongs to. A graph from UL943 suggests it's allowed to be much longer

See also:

Arc fault protectors

An arc fault is an unintended discharge between conductors, from things like

loose wiring connections
overheating wiring that broke down insulation over time (note that breakers should make that impossible)
otherwise damages wire insulation

The sparks and heat generated can further damage the wire, and lead to fire.

An arc-fault circuit interrupter (AFCI) measures

fast switching (order of 100 kHz)
of non-negligible current
sustained for a few seconds.

They seem to often be digital(verify)

They seem to often also integrate ground fault sensing.

Design safety, and certification

Double insulation

Galvanic isolation, floating outputs, floating ground

Isolation transformers

Is 120V safer than 230V?