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In the United States in many state jurisdictions, wiring that operates either exactly at or below 100 volts is not required to be installed by a licensed electrician, and no site plan, building permit, or followup inspection is required for electrical work involving these voltages.

I am trying to determine where this concept first originated, because I've tried looking through various editions of the National Fire Protection Association (NFPA) National Electric Code (NEC) going back about 1980, and I don't see a specific designation of "circuits above 100 volts" and a separate one for below.

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There are many electrical standards that seem to be based on this..

  • about 90 volt AC ringing voltage of landline telephones
  • 25 volt / 70 volt constant voltage speaker wiring systems (70.7v RMS * 1.41 = 99.6v .. so it's just barely "low voltage", no license needed to install in some jurisdictions)
  • 48 volts DC for phone battery backup in central offices and remote terminals
  • up to 48 volts DC for phantom power in balanced audio microphone systems
  • up to 48 volts DC for Power Over Ethernet (though there has been a recent change to the NEC regarding network POE in excess of 60 watts per device)
  • up to 48 volts for signalling on computer communications cables.

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It is also directly written into the statutory law of some states. For example, Wisconsin statutes 101.80 to 101.862:

https://docs.legis.wisconsin.gov/statutes/statutes/101/IV/86/1

  • 101.80 Electrical Wiring and Electricians
  • (paraphrasing the weirdly written language) 101.862(4)(d) for equipment or systems that operate at 100 volts or less:
    • Anyone may engage in the business of installing, repairing, or maintaining this electrical wiring. They are not required to be licensed as an electrical contractor with the WI DSPS (Dept of Safety and Professional Services).
    • Anyone may engage in installing, repairing, or maintaining these systems. No licensing as an electrician or enrollment as a registered electrician with the WI DSPS is required.
    • A master electrician is not required to be at all times responsible for the person working on these systems.

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In a search for historical references to 100 volts, I found this:

https://americanradiohistory.com/Archive-Electrical-Experimenter/EE-1915-08.pdf

The Electrical Experimenter, August 1915, Page 134: How Electricity Kills - [...] The average man can take a direct current of 100 volts and scarcely notice it. 200 to 400 volts will give rise to muscular cramp, while respiration is suddenly stopped at 550 volts. [...]

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Is there a specific definitive point in history where this division of "0 - 100 volts is safe, but 101+ volts is unsafe" started?

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  • \$\begingroup\$ I recall Telco could make its own rules as it's own employees were trained for installations with 105 Vac ring voltage which were often higher in some areas \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 27 '19 at 0:45
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    \$\begingroup\$ I'm voting to close this question as off-topic because it is a question about the reasons behind laws and administrative rules not about electrical engineering design and theory. \$\endgroup\$ – Charles Cowie Apr 27 '19 at 1:05
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    \$\begingroup\$ The reason why you don't see any reference to 100V or less in the NEC is because the threshold is 50V or less, Article 720. "Low Voltage" system installers do not need to be licenses electricians in most jurisdictions if the voltages are 50V or less, which allows for Telephone technicians that work with 48V to not need to be licensed electricians. That also covers alarm, HVAC and fire protection technicians. \$\endgroup\$ – JRaef Apr 27 '19 at 1:23
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    \$\begingroup\$ The NFPA is a very old organization.. it may be possible that 100 volts was a recognized threshold far back in history. But I have not been able to determine how to access the oldest NEC editions to research this. \$\endgroup\$ – Dale Mahalko Apr 27 '19 at 2:44
  • \$\begingroup\$ Just guessing, but the rule may have been intended to require licensing for working on the ubiquitous standard power cabling in buildings, at 110V, while not being too restrictive for other, less common, (thus) less dangerous and possibly then-still-to-be-invented systems and machines. \$\endgroup\$ – JimmyB Apr 27 '19 at 17:46
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There are undoubtedly various points in the past when different thresholds were set for classifying voltages in terms of safety, but I don't think this is what you are asking.

Different standards employ different thresholds. For example, so-called "extra low voltage" is defined as under 50VRMS AC or 120VDC (see https://en.wikipedia.org/wiki/Extra-low_voltage). We can say that in general, the voltage range between 50 and 100V is where it becomes more than infinitesimally likely that the voltage could induce enough current through a human body to injure it.

It would indeed be absurd to claim that any voltage at or above a certain threshold will injure or kill, while anything one volt below is unconditionally safe. Individual people and the circumstances under which they encounter the voltages in question introduce plenty of variability into real-world situations.

The point of setting a limit at 100V, at 50VRMS, or at 120VDC is not to claim that there is a sharp line of demarcation in terms of the actual effects. It is simply a matter that whenever you make a standard, you have to draw a somewhat arbitrary line somewhere.

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That's odd, since shock-hazard typically starts at 60-70VDC and above, and is lower for AC. Are those 100V ratings involving fire-hazard, rather than for electric shock?

Perhaps this is about commonly non-lethal shocks, rather than about actual "safe" circuits in the usual sense. Is the word "safe" taken to mean "no electrocution hazard?" (The English definition of "safe" isn't quite the same as "extremely painful but only kills you when it knocks you off the ladder.")

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Looking briefly, I see the Euro IEC ratings are for detectable-not-painful when below 60VDC or 30Vrms, and painful-noninjury below 120VDC and 50Vrms. (From articles about IEC 62368-1:2018, I don't have access to the actual doc.) Anyone selling low-voltage equipment has to obey IEC, otherwise their products will be banned from Europe.

Perhaps you'll get more document hits when searching for "safety" and "shock hazard" with "60V" or "70V" rather than 100V.

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