The effect of voltage vs current intensity on human tissue

wildetudor

Hello everyone,

I was lately thinking about how exactly do these two main electrical measures - current intensity and voltage - differ in describing the effects of electricity on human tissue (skin/muscle/brain etc). For example, I know that most of the time there are warnings of high voltages that might harm you, whereas you rarely see a safety limit given in Amperes (so as current intensity), and I don't really understand why.

Isn't the flow of current what, after all, causes harm to tissue? And is that flow not best described by current intensity? Voltage being a difference in electrical potential of two spacially separated points, then, as I understand it, voltage has the potential (in the common sense of the word) to create a current, if the impedance between the two points is low enough - but again, isn't the current (the "effect") the one that has an impact on the tissue, rather than the voltage (which is, in a way, the "cause" of the current)?

Given that the electrical resistance of dry human skin is (probably) relatively constant, shouldn't, then, a particular safety limit be able to be expressed both as a voltage and as a current, given that the two are mathematically related via the skin's resistance?

If anyone could give a more informed opinion on this, or perhaps suggest where on PF this thread should be relocated, I would very much appreciate it - many thanks in advance.

SW VandeCarr

The basic equations are E=IR and EI=P=energy/time. E (electromotive force)is measured in volts, I (current intensity) is measured in amperes (1 coulomb of electric charge per second), R is resistance and P is power. Multiplying volts by amps gives power in watts. 1 watt delivers one joule of energy per second. So the total work done is a function of the total energy delivered, measured in watts x seconds, but damage done to tissue is mostly a function of the rate of energy delivery or power.

wildetudor

Hi, thanks for your reply. I'm still not clear, though, whether a safety limit should be expressed as a voltage or as a current, and how is a high voltage dangerous other than by creating the potential for a strong *current* to set in.

SW VandeCarr

Remember E=IR. Household voltage is typically 120 (North America) or 220 (Europe). Resistance of dry human skin is very high. The following link estimates it at 1 million ohms. So the experimental variable then becomes current. The predicted effects due to various levels of current are discussed here:

http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

It should be emphasized R can be very variable and is hard to measure or estimate. E and I however are easily measured and controlled so that one can be held constant while other is allowed to vary.

wildetudor

Thanks, that article was really useful! However, even after reading it whole, I still have one question. I'll have to expand a bit on the reasons that prompted me to start this thread in the first place.

I had in mind the brain stimulation technique called Transcranial Direct Current Stimulation (tDCS), which is used by neuroscientists to modulate the excitability of neurons in certain parts of the cerebral cortex. With tDCS, two electrodes are connected to a human participant's scalp, over parts of the brain relevant to the study. The electrodes are connected to a current source that delivers a constant current (usually ~1mA) for a certain amount of time (about 20 minutes). The electrodes are soaked in saline solution to bring the impedance of the total circuit to the order of a few tens of kOhms.

The tDCS stimulator (i.e. the current source) will not start delivering current if the impedance is above a certain threshold, which is by default 55 kOhms but that can be increased up to 100 kOhms. There is also an additional safety measure whereby the machine stops delivering current if the voltage required to produce the desired current exceeds a certain value. It's this bit that I don't understand -- since it's ultimately the *current* that we want not to exceed a certain limit, then why would it be dangerous to exceed a certain voltage, if there is enough resistance in the circuit to prevent too strong a current to flow? Obviously, high voltage means the potential for large amounts of current through the body, but even assuming the body resistance may sometimes drop unexpectedly, that still doesn’t mean that there will be a high current, because the stimulator (which, agan, is really just a source of constant current) instantly reduces the voltage, so as to keep the E/R ratio constant, i.e. to a safe ~1 mA.

I look forward to hearing your thoughts on this, thanks again!

SW VandeCarr

I'm posting a link so others will have some information on tDCS. With E=IR you can see that if you want constant I at 1mA, than the optimal voltage depends only on R. I'm not familiar with this kind of research but it looks like you answered your own question. If R drops unexpectedly, you need a corresponding drop in available voltage or you will get a current surge. This is mitigated by capping available voltage. That means that if R increased, you may not have enough voltage to maintain I=1mA. I would expect this is a necessary feature for the sake of safety.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2270099/

wildetudor

Did I? :-) My argument was that, assuming that there is enough voltage available to compensate for higher impedances, then, in case of a sudden drop of impedance (R), the voltage (E) can also drop at the same time, such that at no time does the current (E/R) exceed the set value. I still don't see why that isn't the case.

SW VandeCarr

OK, As I said, I'm not familiar with the details and the immediate adjustment of voltage in response to changes in impedance to maintain the current intensity would be desirable. One has to consider skin preparation materials, moisture and other factors that might cause short circuiting. I suppose it's unlikely, but for safety reasons you might want to put some reasonable upper limit on available voltage if for no other reason than not relying solely on the technology for safety.

wildetudor

Yup, that makes sense. Thanks again for your help!

Hrunting

No, and yes. (; Current doesn't flow.

As I recently discovered online, "current _is_ flow (of electrical charge)."

One doesn't say that the flow flows, does he? One says, "we appear to be seeing a small, but measurable current. Therefore, the electrons are playing musical atoms (not unlike musical chairs) in the direction of the positive terminal on the power supply." (Electrons try to go where they are wanted. There is, of course, a surfeit of electrons towards the negative electrode, and a deficit, towards the positive.)

You are correct, Wildetudor, that, when it comes to tissue being electrocuted - "It's the Volts that jolts, but it's the mills (as in milliamps) that kills."


High Voltage warning signs should read, High Power. But some people don't know that Power can be dangerous (in the wrong hands). So, "High Voltage" gets the message across.



To recap, the current intensity is the intensity of the flow of charge.
One wants to have a very low voltage for safety purposes because a low resistance path (which can be shorted accidentally) creates potentially dangerous current. Nevertheless, it is not the voltage that offends (much).


-Hrunting