I am wondering how wristlet devices can safely measure a person's heartbeat without risk of electric shock. It seems everywhere I read, the threat of static discharge would be far greater with an electronic wristband. Isn't the "shortest route to earth" always through a person's body in this situation? The wristband and its internal battery ultimately are only connected by a person's body to real earth ground.

In the event of static discharge, thousands of volts are likely applied over some resistance in the watch - what guarantees, extra safety checks, and calculations are done to make sure this resistance is correct i.e. there isn't a short that goes through a person? Am I correct that the short case would ultimately go through whatever skin-contact electrode (lower resistance than the packaging) was being used to pick up the heartbeat signal from the wrist? Would these risks be mitigated using a high-resistance/high-impedance recording electrode?

For example, if discharge voltage were 1000V and deadly current were 1mA, a design with a factor of safety of only 1 would use recording electrodes with a resistance of 1 MOhm to avert deadly discharge. Real life you'd probably use 100 MOhm for safety?

  • \$\begingroup\$ Aren't we talking about a battery-operated device? \$\endgroup\$ – Janka Nov 6 '16 at 22:02
  • \$\begingroup\$ Yea. Isn't static discharge still a threat? \$\endgroup\$ – Kevin Nov 6 '16 at 22:03
  • \$\begingroup\$ Uh, no? This is the equivalent of a bird sitting on a high voltage power line. The device is the bird. It isn't connected to anything outside, so it can't "see" the voltage nor influence the current flow. The only exception would be a cable running from your hand to your foot. That one would actually help you defend twitches as it bypasses your body. \$\endgroup\$ – Janka Nov 6 '16 at 22:08
  • \$\begingroup\$ So do battery powered eegs even need protection circuits between the electrode and amplifier inputs? Are right leg drivers necessary for them? Or could you hook the third electrode up directly to the low impedance reference pin? This is purely from a safety perspective. See this question @Janka electronics.stackexchange.com/questions/267694/… \$\endgroup\$ – Kevin Nov 6 '16 at 22:59
  • \$\begingroup\$ Couldnt a discharge run from another object through the device? Dont we have to account for that? Like instead of touching fabric your wristlet touches it then touches you \$\endgroup\$ – Kevin Nov 6 '16 at 23:05

The sensors are low voltage isolated and often using IR optical methods to avoid galvanic noise from motion. Safety current thresholds are defined by class codes for products and depend on where user interface. Obviously 1mA only applies direct to heart, not thru skin etc.

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  • \$\begingroup\$ Let's assume you were trying to recreate a QRS complex using complex vessel wall fluctuations picked up from a skin contact electrode near the radial pulse. If the skin contact electrode were high resistance high impedance, they would scientifically obliterate the risk of shock right \$\endgroup\$ – Kevin Nov 6 '16 at 21:54
  • \$\begingroup\$ The risk of shock is only in the power supply kV Hipot leakage, BDV failure threshold and line impulse attenuation. Hi Z pads create poor attenuation of stray E fields \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 6 '16 at 21:57
  • \$\begingroup\$ Even still, 1 mA of current causes discomfort right? Can you recommend a reference \$\endgroup\$ – Kevin Nov 6 '16 at 21:57
  • \$\begingroup\$ yes CE/UL consumer/commercial class is 500uA for line filter cap leakage. which is sometimes exceeded by poor CM filtering in Laptop chargers from C coupling to case for a floating case via charger high f SMPS transformer. This failure is a weakness on test methods.. Try out a laptop with case to your knee or wrist and see if you feel it while touching earth ground. You might get a burning sensation from current density even though high R insulation and floating \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 6 '16 at 21:59

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