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Old 12-14-2007, 01:58 PM
steveparrott steveparrott is offline
Join Date: Jun 2005
Location: Tampa, FL
Posts: 1,111
Originally Posted by johnh View Post
Hi Joey, I didn't just make this up. This is an excerpt from the IAEI magazine, Nov/Dec 2004 issue.
"A non-sinusoidal peak voltage of 42.4 volts for an ac circuit is considered a safe voltage under dry conditions and would not normally constitute a shock or fire hazard. In wet conditions, this voltage must be limited to 15 volts for a sinusoidal ac and 21.2 volts peak for a non-sinusoidal ac. Since the bodyís resistance is decreased by water, the permissible voltage of the output of the low-voltage system is decreased accordingly to ensure safe handling and operation of these circuits where water is a concern. Using low-voltage systems in close proximity to or in fountains and swimming pools should only be considered when all the NEC requirements are applied"
Link to full document here
The outdoor landscape is generally considered to be wet conditions, although this may not be the case in some desert climes.
This is substantiated by the IEC Technical Specification 60479-1 on touch voltages.
30V is the maximum safe voltage for ventricular fibrulation. 15V is the maximum safe voltage for painful "Let-Go" current. By limiting to 15V, UL1838 has eliminated 15V of pain!

Your assessment of the IEC TS 60479-1 is correct on the assertion that 30V is the maximum safe voltage for ventricular fibrulation (assuming a bodly resistance of 1000 ohms). Note that this corresponds to large surface area contact between two sweaty hands. In practice, the scenario would need to be something like grasping an energized wrench firmly in each hand. How that would ever happen during an installation is a mystery to me.

Much more likely (as a worst case scenario) would be holding (with sweaty hands) onto an energized screwdriver shank and a live bare wire. The bodily resistance of such a contact would be about 9 times the above - resulting in a current exposure of about 2 mA (at 22 volts) that would deliver a shock but nowhere near the let-go threshold.

Let's not forget that there has never been a serious injury reported from contact with any circuit under 30 volts. In fact, in a 12-year study from OSHA, there were zero deaths reported from contact with any volltage under 110 volts.

The 1838 voltage limitation of 15 volts has no authentic scientific basis.
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