LIBRARYEEG Safety & Isolation.

The one rule that keeps DIY EEG safe: never connect a person's electrodes to anything with a path to mains. Why, the real hazard, and how isolation works.

The one rule, read this first

NEVER connect a person's electrodes to any device that has, or could have, a conductive path to mains power. Power the subject-connected electronics from BATTERIES only. If any mains-connected device (a charger, a plugged-in laptop, a bench supply) is in the power or data chain, there must be a rated galvanic-isolation barrier between it and the electrodes, and never connect a person while the device is charging. This is not cautious advice; it is the rule.

EEG is uniquely exposed to electrical risk because of where and how it connects. Dry skin is a high resistance that normally limits current into the body, but EEG electrodes use gel or abrasion to make a low-impedance contact, deliberately bypassing that barrier, and they sit on the head, directly in a path toward the rest of the body. So if a fault ever puts mains voltage on those electrodes, the current that flows is far higher than a casual touch, and it is flowing through a person's head.

How much current is actually dangerous?

How much current is dangerous? At 50/60 Hz, close to the worst-case frequency for the human body, perception begins around 1 mA; the 'let-go' threshold is roughly 6–24 mA (above it, muscle tetany means you can't release your grip); and ventricular fibrillation onset is around 100 mA for surface contact, lower for prolonged exposure (IEC 60479-1). Those are macroshock numbers. Current delivered very near the heart (microshock) can fibrillate at just 50–100 µA, which is why medical engineers design to a ~10 µA safety limit. A microvolt-class EEG amplifier operates a thousandfold below even the perception threshold, but a fault in a mains-connected supply doesn't respect the amplifier's design. That is the whole point of isolating it.

The concrete failure mode

If a mains-referenced supply or a cheap charger reaches the electrodes through failed or insufficient isolation, line voltage (120/240 V) can appear on the electrodes and take the path of least resistance to ground, through the subject's scalp. Battery power removes this path entirely: there is no galvanic connection to mains for a fault to use.

Fig 1

How to isolate a DIY EEG safely

Approach	What it does	When you need it
Battery power	No conductive path to mains at all (the primary safeguard)	Always (the default for any subject-connected front-end)
Isolated DC-DC + digital isolators	Passes power/data across a high-voltage barrier no current can cross	When the design must bridge to mains-powered electronics
USB isolator	Galvanic barrier on USB power + data to a host PC	Any time electrodes are attached AND USB connects to a plugged-in PC
Keep mains gear away	Prevents a second shock path through a touched, earthed appliance	Always; isolation is undone if the subject can bridge to other gear

▸Deep dive· For context: how strict the medical bar is (IEC 60601-1)

Certified medical devices are held to patient-leakage-current limits under IEC 60601-1. For a Type BF applied part, that is about 100 µA in normal condition and 500 µA under a single fault; for the cardiac-contact Type CF, just 10 µA / 50 µA. The patient-isolation barrier is typically rated to withstand ~1500 VAC. The point of citing these is NOT that your DIY rig meets them; it almost certainly is not tested to any of this. It is to show how seriously the professionals treat current near a body, and why 'just use a battery and isolate' is the floor, not the ceiling.

Not a medical device

A DIY or hobbyist EEG is uncertified experimental hardware. It is not certified to IEC 60601-1 and must never be used for diagnosis, treatment, or any clinical or medical decision. Build it to learn, not to assess anyone's health.

The good news: doing this safely is cheap and simple. Battery-power the front-end, keep the subject clear of any mains-powered equipment, add a rated isolator if you ever bridge to a plugged-in computer, and never wire anyone up while charging. The board design that bakes isolation in from the start is exactly what the OTD Academy EEG front-end project walks through.

Checkpoint

Quick check

What is the one rule that keeps DIY EEG safe?