What is the safe limit of DC voltage for humans to touch?
The safe limit of DC voltage for humans to touch depends on several factors, such as the duration of exposure, the resistance of the skin, the health condition of the person, and the path of the current through the body. There is no definitive answer to this question, but some general guidelines can be given based on scientific research and safety standards.
According to one source¹, the typical current to kill a healthy person is many mA (milliamperes), and the minimum amount that could be harmful for a person not in perfect health might be a lot less. The same source also states that for most purposes, 24VDC or lower will be considered safe enough, as it is unlikely to cause enough current to kill you if applied on the skin surface. However, it also warns that higher voltages can be lethal if applied below the skin surface or directly to the heart.
Another source² suggests that a touch voltage of 50 V DC for long shock duration (> 3 s) should not be exceeded in healthy adults, otherwise a life-threatening condition may occur. For children and livestock, the touch voltage is limited to 25 V DC. This source also provides a table of different effects of electric current on the human body, ranging from perception threshold (0.5-1 mA) to ventricular fibrillation (100-300 mA).
A third source³ states that limiting current flow in the human body to safe levels is entirely dependent on the resistance of the short-circuiting jumper. To achieve this safe current level, the voltage across the human body must not exceed 100 volts.
A fourth source⁴ explains that small contact voltage exposures (less than 50 V) are not lethal to humans, as they are not capable of delivering more than 13.5 W-sec (watt-seconds) in less than 3 sec, which is the estimated threshold for cardiac arrest.
As you can see, there is some variation in the safe limit of DC voltage for humans to touch, depending on how you define safety and what factors you consider. A general rule of thumb is to avoid touching any voltage higher than 50 V DC, and preferably lower than 24 V DC, especially if you are wet or have a medical condition. However, it is always better to be cautious and avoid contact with any live wires or electrical devices that may pose a risk of electric shock.