Electrical safety, with the numbers
What IEC 60601 actually limits, why a battery satisfies it and a wall supply must be designed for it, how isolation works, and what the 390 kΩ resistor is doing.
On this page12 min
The safety charter gave you a rule: batteries only. This page gives you the numbers behind it, so that when you design something that cannot run on batteries, you know what it has to meet. The standard is IEC 60601-1IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry, and the number at its centre is a few tens of microamps.
What current does to a body
Current through the body, not voltage, is what hurts. Around 1 mA at 60 Hz through the skin is the threshold of sensation. Around 10 to 20 mA, muscles contract and you cannot let go. Around 100 mA across the chest for a second or two, the heart fibrillates. Those are figures for current entering through dry skin at a hand; an electrodeElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry with conductive paste has a far lower contact resistance, and a wire in the heart (a pacing lead) needs only tens of microamps to fibrillate, which is why the strictest limits are for equipment that can touch the heart.
The body’s resistance from hand to hand is typically 1 to 2 kΩ. Ohm’s law says 120 volts across that is 60 to 120 mA. That is the whole danger in one line.
What the standard limits
IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps). Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body. , Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient. . Glossary entry-1 classifies patient-connected parts. For a part that touches the skin (type BF, “body floating”), the allowed leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body. Related: IEC 60601IEC 60601 The safety standard for medical electrical equipment, including how much current may leak into a patient (microamps)., Electrical isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient.. Glossary entry through the patient is 100 µA in normal conditions and 500 µA with a single fault. For a part that can touch the heart (type CF), it is 10 µA normal and 50 µA fault. “Single fault” means one thing has gone wrong: a ground wire broken, an insulation barrier failed, a supply shorted.
Those numbers are why the 390 kΩ resistor exists. The bias drive output is a low-impedance amplifier that could, if the amplifier failed, sit at a supply rail. With ±9 V rails and 390 kΩ in series, the worst-case current into the body is 9 / 390,000 ≈ 23 µA. Under the fault limit even for a cardiac-class device. The AD8232 datasheetDatasheet The manufacturer's document for a chip: pinout, limits, typical performance, and the reference circuit you should start from. Glossary entry, the ADS1299ADS1299 Texas Instruments' 8-channel, 24-bit biopotential front-end chip used in OpenBCI and many research amplifiers. Related: Delta-sigma ADCDelta-sigma ADC A converter that samples very fast with a crude 1-bit decision and averages, achieving 24-bit resolution at modest rates; what the ADS1299 uses. , OpenBCIOpenBCI The open-source EEG hardware company whose Cyton (ADS1299) and Ganglion boards are the default hobby and research-lite amplifiers. . Glossary entry reference design, and every clinical amplifier have this resistor for this reason.
Why a battery passes and a wall supply must be designed
A battery has no connection to anything else. The only path for current from the battery through the patient is through the amplifier’s own circuits, which the series resistors limit. A single fault (say, a shorted input protection resistor) still leaves the other resistors and the battery’s low voltage; there is simply no 120 V anywhere in the system.
A wall supply has 120 V on one side of a transformer and your circuit on the other. The transformer is the isolationElectrical isolation A barrier, such as a battery, an optocoupler, or an isolated USB link, with no conductive path from the mains to the patient. Related: Leakage currentLeakage current Current that flows into the patient through unintended paths; limited to tens of microamps for anything touching the body.. Glossary entry barrier. Its windings have a small capacitance between them, through which a tiny 60 Hz current flows to the secondary and then through anything connected to it, including a patient. Medical-grade supplies are built with reinforced insulation and low inter-winding capacitance so that this leakage stays under 100 µA even with one fault. A phone charger is not built that way, and its leakage is often hundreds of microamps in normal operation. That is the difference between “medical” and “consumer” power supplies, and it costs about ten times as much.
Your amplifier runs from batteries. You connect its output to a laptop's USB port, and the laptop is on its charger. Is the patient connection still isolated?
No. The USB cable’s ground pin ties your amplifier’s ground, and therefore its reference and bias electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry, to the laptop’s ground, which the charger ties to the wall. Any leakage in the charger, or any fault in the laptop, now has a path through the electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry. This is the most common way the battery rule gets broken, and it is why the plotter page says to unplug the laptop or put a USB isolator in the line.
Isolation devices
A USB isolator puts a digital isolation chip (capacitive or magnetic coupling) and an isolated DC-DC converter in the cable, so data and power cross a barrier with no conductive path. Good ones are rated for a few kilovolts. Optical links and wireless (the ESP32ESP32 A cheap microcontroller with Wi-Fi and Bluetooth, good for streaming data from a sensor board to a laptop. Related: RP2040RP2040 The Raspberry Pi microcontroller, cheap and well documented, with fast SPI and USB. , FirmwareFirmware The program running on the microcontroller, which configures the chip, reads samples on time, and ships them to the computer. . Glossary entry’s Wi-Fi or Bluetooth) are isolation by default; there is no wire to carry a fault. Isolation amplifiers do the same for analog signals. Every clinical EEGElectroencephalography (EEG) Recording the brain's electrical activity from electrodes on the scalp, a few tens of microvolts of blurred, summed cortical activity. Related: Electrocorticography (ECoG)Electrocorticography (ECoG) Recording from electrodes placed on the surface of the brain under the skull, usually during epilepsy surgery evaluation; cleaner and sharper than EEG. , Magnetoencephalography (MEG)Magnetoencephalography (MEG) Recording the tiny magnetic fields produced by cortical currents with superconducting sensors in a shielded room; less blurred by the skull than EEG, and far more expensive. , Volume conductionVolume conduction The spreading of currents from a source through the conductive tissues of the head, which is why one source shows up on many electrodes and why scalp EEG is blurry. , Microvolt (µV)Microvolt (µV) One millionth of a volt. Scalp EEG is tens of microvolts; a AA battery is 1.5 million of them. . Glossary entry system has one of these between the patient-connected front end and the computer, and the ADS1299ADS1299 Texas Instruments' 8-channel, 24-bit biopotential front-end chip used in OpenBCI and many research amplifiers. Related: Delta-sigma ADCDelta-sigma ADC A converter that samples very fast with a crude 1-bit decision and averages, achieving 24-bit resolution at modest rates; what the ADS1299 uses. , OpenBCIOpenBCI The open-source EEG hardware company whose Cyton (ADS1299) and Ganglion boards are the default hobby and research-lite amplifiers. . Glossary entry board design put one in from the start.
The other half: single-fault thinking
The standard’s habit of asking “what if one thing breaks” is worth adopting for everything. What if the bias amplifier saturates? The resistor limits the current. What if an input protection resistor shorts? The other one still limits. What if the battery is reversed? Add a diode. What if the isolator fails shorted? Use one rated for the voltage with margin, and consider a second barrier. This is the reasoning that FMEAFMEA Failure mode and effects analysis: a table of every way the device could fail, how bad and how likely each is, and what you did about it. Related: ISO 14971ISO 14971 The international standard for medical device risk management that every company follows.. Glossary entry in Phase 5 formalizes into a table.
Deep dive Why frequency matters 2 min
The body is most sensitive to current at mains frequencies, 50 to 60 Hz, which is unlucky. At DC and at frequencies above a few kilohertz, the thresholds for sensation and fibrillation rise, which is why the standard’s limits are frequency-weighted and why some stimulation and impedance-measurement systems deliberately use kilohertz currents. The ADS1299ADS1299 Texas Instruments' 8-channel, 24-bit biopotential front-end chip used in OpenBCI and many research amplifiers. Related: Delta-sigma ADCDelta-sigma ADC A converter that samples very fast with a crude 1-bit decision and averages, achieving 24-bit resolution at modest rates; what the ADS1299 uses. , OpenBCIOpenBCI The open-source EEG hardware company whose Cyton (ADS1299) and Ganglion boards are the default hobby and research-lite amplifiers. . Glossary entry’s lead-off detection injects nanoamps at 31 Hz, far below any limit, and its impedance measurement is safe for that reason.
Deep dive Defibrillator protection 2 min
Clinical amplifiers must survive 5 kV from a defibrillator applied to the patient they are connected to. That is what the large series resistors, gas discharge tubes, and clamp diodes on clinical front ends are for. Your board does not need this, but if you ever wonder why a clinical ECGElectrocardiography (ECG) Recording the heart's electrical activity, about a millivolt on the chest; it leaks into EEG as a small regular spike. Related: ArtifactArtifact Anything in the recording that did not come from the brain: blinks, muscle, heartbeat, movement, sweat, mains hum, electrode pops. . Glossary entry front end has parts your design lacks, this is usually the answer.
Explain what this page was about to your roommate in three sentences. No jargon they would not know.
We keep patient current under 100 microamps normally and under 500 microamps with one fault for skin contact, with stricter 10 and 50 microamp limits when a path could reach the heart. We get that protection from a 390 kiloohm resistor in the bias path, which holds a 9 volt fault to about 23 microamps. We stay isolated by running on batteries and keeping wall connected chargers and grounded laptops out of the patient path, since body resistance near 1 to 2 kiloohms turns 120 volts into 60 to 120 milliamps.
Yours does not have to match. Check that the three ideas are there.