Safe To Touch Tests 1

[Cam83]

I am currently looking into the issue of what is a "safe to touch" voltage. Can someone enlighten me on:
1. What is the maximum AC voltage deemed "safe to touch"?
2. When this subject was investigated how was the "safe to touch voltage" arrived at?

 

[felixc]

This is a tough question, as what's dangerous isn't the voltage but the current that passes through a body, and where it passes through.
Just try placing a fresh 9V battery on your tongue will show that the voltage isn't the issue.

 

[BrianG]

This is a rather variable question since it depends very much on the application. For example in "wet" areas, such as bathrooms, swimming pools, etc., the "safe" voltage will be very low. If the person likely to touch the voltage source is assumed to be clothed and wearing normal shoes, so that the current path is via the shoes to ground, then the voltage can be very much higher.

A starting point might be the IEC 60905 standard which defines any voltage above 30V r.m.s, or 60V d.c as being "hazardous" and therefore not safe to touch.

Hope that helps!

 

[ICman]

I agree with felixc - it's the current. There are probably some standards for medical equipment you'll want to check out, or maybe contact a manufacturer.

I remember something like 0.100 - 0.200 Amps will cause fibrillation of the heart and will likely kill if it is not stopped quickly. Current above that tends to just stop the heart, but it can be revived easier. Otherwise, I guess you need a de-fibrillator?

Anyway, the NEC places a limit of 10 Volts on anything contacting a body.

 

[resqcapt19]

ICman,
You said: "Anyway, the NEC places a limit of 10 Volts on anything contacting a body."
Can you cite the code section that says that? Thanks.
Don

 

[mc5w]

I discovered when I was a kid that if I did not do hourly hand washing then enough perspiration salts would build up that picking up a 1.5 volt D cell by the ends would get my attention.

Any voltage that is enough to light a light bulb is unsafe under the right circumstance.

Article 110 of the National Electrical Code considers anything over 50 volts to be a hazardous voltage. That is correct if you keep your hands clean and dry. If you are wet enough all bets are off.

Mike Cole, mc5w@earthlink.net

 

[dspDad]

Safety is defined by the regulating authority for commerical applications, and is different for Europe, US, consumer grade, commercial grade and medical grade devices.

Most of the agencies specify in detail how measurements are to be made. This includes leakage or voltage versus frequency, and various failure modes within the device and in the power and ground connections.

Medical grade devices also have different requirements for patient contact vs non-patient contact, and have requirements to prevent patient contact by choice of connectors.

DspDad

 

[jon1]

An old textbook (which I am currently looking for) claimed that 8 milliamps DC can stop the human heart. It also claimed that a pulse of 100 millijoules could also do the job. When a current is injected through the epidermis, no matter how close to the heart, a small fraction of the total current passes through the heart. The reason 40 to 60 VDC is called out in certain standards is because of the typical insulation and dispersion characteristics of the skin. As it so happens, 60 cycles per second tuned to stopping the human heart, so I would lean towards the low end of 'safe' voltages for 60Hz.

 

[biff44]

I know, thru experience, that 250 volts DC is not safe to touch.

 

[rbalex]

Question 1.

The primary US safety consensus document for worker safety, NFPA 70E, does not consider any voltage "safe to touch;" however it does not specify a “limited approach boundary” for voltages below 50V.

Section 725.2 of the National Electrical Code states that Class 2 circuits “…provides acceptable protection from electric shock.” A Class 2 circuit is primarily defined in terms of the power supply, rather than a specific voltage.

Question 2.

Basically the 50V is roughly 80% of 60V, which is the lowest documented “lethal” voltage. There have been fatalities involved with voltages below 60V; however they have not been proven to be the actual cause of death. One specific “Class 2” voltage is 30V DC or less for a dry cell battery with a capacity equal to or less than a No. 6 carbon zinc cell.

 

[jon1]

A No.6 carbon zinc cell has enough milliamp-hour capacity to stop hundrends of human hearts, so what is most important is the actual proximity to the heart and the spacing and voltage on any exposed terminals. This is why implantable electronics are so stringently controlled. Most safety regulations just throw up thier hands and tell you to label anything that needs servicing to be powered down and fully discharged first (uaually within 2 seconds of power off).

 

[RalphChristie]

It depend on a few factors like regulating authority, codes/standards in use and place where you want to determine the safety limit.

First, it is important to remember that it is current that kills, not voltage.


From IEC report 479-2 1987, Effect of current passing through the human body:
There are four major factors which determine the seriousness of electric shock:

Path taken through body.
Most dangerous and most common path is through the hart, arm to arm, or arm to leg.
Persons are not normally accidentally electrocuted between phases or phase to neutral, almost all accidents are phase to earth.
Amount of current.
Perception - tingling - about 1mA
Let-go currents - about 1...6mA
Painful, difficult to release energised objects - 9...25mA
Muscular contractions, breathing difficult - 25...60mA
Ventricular fibrillation - 60...100mA
Time the current is flowing
Normal electrocardiogram - one pulse beat - at 80 beats per minute = 750msecs
Consists of the normally pumping action and the refractory or rest phase (150msecs)
Death could occur if within this very short period of 150msecs a current flow was at the fibrillation level.
The body electrical resistance
Moist skin reduces to the flow of electric charge about a factor of 500 times compared to dry skin.

Earth-leakages are therefore expressed in mA.




For higher voltages, step and touch potential limits are normally determined to ensure personal safety under abnormal conditions.

Body limit
From experimental data the upper limit for max. body current was derived as a function of current duration (t in sec) given by:

Ik = 0.116/?t for 0.03sec < t < 3sec

Touch potential limit is the potential difference between the grid potential rise (GPR) and the surface potential at the point where a person is standing, while at the same time having his hands in contact with an earthed structure.

The safe touch potential is a function of the safe current duration and the resistance of the body, the resistance under the feet and the surface layer resistivity in ohm-metre.


Safe touch Potential = Ik (Rk + 0.5Rf)

where Ik = 0.116/?t for 0.03sec < t < 3sec

thus Safe touch Potential = [116 + (0.7 x ?s)] / ?t


Rk = body resistance = 1000ohms
Rf = resistance under feet = 3 x ?s
?s = Surface layer resistivity in ohm-metre

This is from the Eskom standard (local South African electricity suplier) Other countries will use similar approaches.

Revelant documents:
IEEE Std 80/1986 IEEE guide for safety in AC substation grounding.
IEC report 479-2 1987, Effect of current passing through the human body.



Regards
Ralph

 

[RalphChristie]

Aaaaaaaarg!!!!

Let me try again:

For higher voltages, step and touch potential limits are normally determined to ensure personal safety under abnormal conditions.

Body limit
From experimental data the upper limit for max. body current was derived as a function of current duration (t in sec) given by:

Ik = 0.116/(t^0.5) for 0.03sec < t < 3sec

Touch potential limit is the potential difference between the grid potential rise (GPR) and the surface potential at the point where a person is standing, while at the same time having his hands in contact with an earthed structure.

The safe touch potential is a function of the safe current duration and the resistance of the body, the resistance under the feet and the surface layer resistivity in ohm-metre.


Safe touch Potential = Ik (Rk + 0.5Rf)

where Ik = 0.116/(t^0.5) for 0.03sec < t < 3sec

thus Safe touch Potential = [116 + (0.7 x Ps)] / (t^0.5)


Rk = body resistance = 1000ohms
Rf = resistance under feet = 3 x Ps
Ps = Surface layer resistivity in ohm-metre