06/03/2025
Did you know 120V alternating current (AC) is much more dangerous to humans than 120V direct current (DC)?
This is due to AC's frequency, peak voltage, and effects on heart rhythm and muscle control.
AC repeatedly reverses direction 60 times per second (60 Hz in the US). This frequency interacts badly with the electrical signals that control your heart and nervous system.
The 60 Hz frequency is especially dangerous because it's close to the frequency range that can cause ventricular fibrillation (basically, fatal heart arrhythmias).
> In simple terms: 120V AC "jiggles" your muscles and heart rhythmically in sync with its cycle. Even relatively low currents can trigger fibrillation.
AC tends to make muscles contract uncontrollably and can cause "can’t let go" effect because the alternating current locks up your grip.
DC usually causes a single contraction — your muscles may clamp initially, but you can usually reflexively release.
120V AC is RMS (Root Mean Square) voltage. The actual peak voltage is:
120V \times \sqrt{2} \approx 170V
So, with 120V AC, you’re actually experiencing peaks of ~170V every cycle, while 120V DC stays constant at 120V.
> More peak voltage means higher instantaneous current through your body tissues, especially during skin breakdown or arcing.
Dry skin has high resistance, but AC's repetitive peaks and polarity changes can punch through that resistance faster than DC.
Once the skin is compromised, the internal tissue (which has much lower resistance) allows much more current to flow — that's when things get lethal.
In short:
120V AC:
Muscle effect Continuous spasms
Heart risk High (fibrillation)
Peak voltage is 170V
Skin resistance Breaks down faster
"Can't let go" phenomenon is common
While 120V DC:
Single shock
Lower (asystole risk at very high current)
Peak voltage is 120V
Breaks down skin resistance slower
"Can't let go" phenomenon is much less common
Learn more at:
Yellowpowersystems.com
