3
\$\begingroup\$

In general, when should free-floating electrical contacts be considered potentially dangerous? Danger may be direct (e.g., due to electrical shock) or indirect (e.g., due to fire risk).

In this question on SE.SuperUser, a poster asks if it's safe to have two wires for a power switch exposed in their household. Their concern seemed to focus on electrical shock to a person or pet.

The dangers of electrical shock to humans is discussed in these questions:

This YouTube video claims that 12V can start a fire:

Can 12 volts start a fire? Yes! Wake up people. Every day I hear people say "oh it's only 12v it can't do much". Shock wise, you're safe. Fire wise, treat it like line voltage, do it right, and safely. Because this can happen in your car, or even a charger. Electricity is electricity. It causes heat, and under the right condition, a fire...
Keep this in mind with your car audio installation too! Do it right, and make sure it's fuse protected!!

-"Can 12 volts start a fire?", YouTube

Presumably, we can construct hypothetical scenarios in which extremely tiny voltages or currents can trigger an unstable system. This question intends to focus on realistically possible scenarios that could accidentally happen in everyday environments, e.g. in households.

Question: In general, when should free-floating electrical contacts be considered potentially dangerous? Danger may be direct (e.g., due to electrical shock) or indirect (e.g., due to fire risk).

Alternative statement: Bob has an open circuit with two exposed contacts on his bedroom floor, built into into clothing (as part of some prototype wearable device he's working on), or in closet, pantry, desk drawer, or some other messy location where he/someone-else/misc.-items may come into contact with the electrodes. The power source behind the electrodes maintains a potential difference of \$v\$ between them, up to a maximum current of \$i_{\text{max}}\$. As a reasonably educated electrical engineer, Bob understands the potential danger of coming into contact with these electrodes. Over the domain of voltages/current-limits \$\left(v,~i_{\text{max}}\right)\$, in what regions would Bob be:

  • unconcerned with any potential danger;

  • mildly concerned with potential danger;

  • worried;

  • reasonably certain something bad'll come of it?


Clarifications

Something is "dangerous" if any of the following apply:

  1. Any person or pet coming into contact with the free-floating electrical contacts might suffer physical pain or/and any type of observable physical harm.

  2. It's potentially possible for the electrical contacts to cause any sort of common household item to catch fire when exposed to the electrical contacts at length.

  3. The electrical contacts could foreseeably cause observable damage to common household items, the house itself, etc..

As for risks:

  1. I'm primarily looking for heuristics. The goal is to separate "reasonable safety concerns" from "absurdly paranoid concerns".

  2. Contrived risks may be omitted. This is, I'm sure that we can come up with hypothetical scenarios, e.g. where the electrical contacts trigger a measuring device that intentionally starts a fire; however, I'm concerned with risks that might be accidentally encountered.

  3. Risk-qualification's going to be fuzzy, and that's okay. Perfectly valid answers may declare there to be a grey zones in which the presence-or-absence of danger is unclear/fuzzy.

The electrical contacts may be qualified by:

  1. Being either AC or DC current.

  2. Having any constant voltage between them.

  3. Having any current-limit.

An ideal answer might provide a heuristic that a normal person could use to assess how they should feel about the potential danger from two exposed electrical contacts randomly scattered in their home of a known voltage-difference with a known current-limit for both AC and DC.

\$\endgroup\$
  • 1
    \$\begingroup\$ Is there a question here? \$\endgroup\$ – Trevor_G Mar 7 '18 at 15:20
  • 1
    \$\begingroup\$ @Trevor_G The first sentence. I'll copy/paste it at the bottom for clarity. \$\endgroup\$ – Nat Mar 7 '18 at 15:21
  • 1
    \$\begingroup\$ In can start a fire with a 1.5V AA battery, that doesn't make it an unsafe device to handle though \$\endgroup\$ – PlasmaHH Mar 7 '18 at 21:02
  • 1
    \$\begingroup\$ Bit of googling suggests that some people have had battery fires in pockets, although usually the 3.7V e-cig ones which really do need to be treated with respect. \$\endgroup\$ – pjc50 Mar 7 '18 at 21:26
  • 1
    \$\begingroup\$ Vaguely related amusing anecdote: I once somehow inherited a box full of screws and spare CMOS batteries (more accurately - coin cells of the type used as a backup power source on PC motherboards). Whoever gave me that clearly did not think it through. I put all of them in series and the stack measured precisely zero volts. \$\endgroup\$ – user253751 Mar 8 '18 at 2:22
8
\$\begingroup\$

The answer is going to depend a lot on what you mean by dangerous. You have to pick a pretty clear definition to measure against before you can answer this question. So let's use EN61010 Safety requirements for electrical equipment for measurement, control, and laboratory use. General requirements. Because I have that to hand. Safety standards for other electronic equipment have similar requirements.

Shock risk

EN61010 defines something as hazardous live if the voltage is over 33Vrms (or 70Vdc) and either the current is over 500uArms (2mA DC) or there is more than 45uC stored in a capacitor.

Fire risk

The standard considers circuits of less than 30Vrms (or 60Vdc) and limited to low currents such that power is less than 150W, and well separated from other higher energy circuits to be a low fire risk. Circuits which don't meet this level must use extra insulation against shorts, or a fireproof container etc. This level is based on there not being anything particularly flammable inside the device where the potential source of ignition is, the levels would be lower around a pool of petrol or a load of dry paper.

Single Fault

This is a more complicated concept. No one failure in the device should allow something dangerous to happen. For example, if there is some mains wiring, and that is separated from the case only by an air gap, that is not OK. The wire could break loose, touch the case, and give the user a shock. You would have to add some extra insulation, or attach the case to a low-impedance safety ground. The point is that a 5V/1A logic supply is not automatically considered safe if it is derived from mains with a transformer and rectifier. To be considered safe, the transformer must meet certain standards so a failure could not result in the low-voltage side getting mains voltage.

\$\endgroup\$
  • \$\begingroup\$ Would you happen to know why the \$V_\text{DC}\$ figures tend to be so much higher than the \$V_\text{RMS}\$ figures? I mean I get that \$V_\text{peak}{\approx}{\sqrt{2}}V_\text{RMS}\$ for sinusoidal current, but that'd still seem to put the peak AC voltages below the sustained DC voltages. \$\endgroup\$ – Nat Mar 7 '18 at 18:03
  • 1
    \$\begingroup\$ I believe this is a biology thing. DC tends to stop the heart completely, but in a way in which it will often restart. AC sends the heart into rapid uncontrolled contractions (called fibrillation) which it does not naturally recover from. A defibrillator can help though. \$\endgroup\$ – Jack B Mar 7 '18 at 18:09
  • \$\begingroup\$ Only electrical study (i know of) which studied electrical effects on human subjects. See p24. \$\endgroup\$ – StainlessSteelRat Mar 7 '18 at 18:20
  • 1
    \$\begingroup\$ Note that "low fire risk" is not the same thing as "no fire risk". I started a fire using 1.2 volts once, by shorting a Ni-Cd battery through a strand of steel wool sitting on a clump of dryer lint. \$\endgroup\$ – Mark Mar 8 '18 at 2:25
5
\$\begingroup\$

The key question is, "how much power could this source deliver into a short circuit, or a human?"

The is a function of both source voltage and available current. You can do a lot of damage with high-current low-voltage supplies: an arc welder or a car battery.

Even extremely low voltages are capable of putting out a lot of heat, e.g. the 3.7V lithium batteries used in e-cigarettes. The batteries themselves are also flammable.

In the specific case mentioned on superuser, that is a PC ATX power supply "on" button with exposed wires, the voltage present is logic-level and current-limited to a very low value, so there's no danger.

In general you should always try to avoid exposed wires unless it's necessary to work with something that's switched on. All sorts of absurd accidents are possible which may result in damage to the equipment, but dropped tools are a particular risk. Once I dropped a screwdriver into a floppy drive that was running and it neatly blew a whole track off the PCB with a puff of smoke.

\$\endgroup\$
2
\$\begingroup\$

This is basically impossible to answer since dangerous is a term with no real meaning.

Any terminal that exposes a voltage to the environment can potentially present harm either to the external world or to the device itself.

How much danger is present depends on how much power that terminal can deliver if shorted to a return path.

There are measures you can take to limit the exposure but it's really all a matter of degrees.

The other issue you need to be concerned about is whether a single fault can potentially expose a much more dangerous voltage through the exposed terminals.

\$\endgroup\$
  • \$\begingroup\$ If the question's unclear, I'd prefer to clarify it to get meaningful answers. To that end, I can appreciate that "dangerous" is pretty open-ended; would it help if I asked about specific risks, e.g. potential to cause a fire when such electrical contacts touch common household items? \$\endgroup\$ – Nat Mar 7 '18 at 15:33
  • \$\begingroup\$ @Nat unfortunately you would have to list every possible cause. It really is never that simple. Electricity is dangerous period, acceptable risk is the governing factor. \$\endgroup\$ – Trevor_G Mar 7 '18 at 15:35
  • 1
    \$\begingroup\$ I tend to agree with Trevor on this. Under the right conditions, any source of electricity can be dangerous. Free electrons do not care, they want to fill an atom, on the positive terminal or ground. Can a charger create a fire? Yes. Am I going to race home and disconnect all my chargers? No. But none are in high use areas where the cord can be damaged and I replace cords when wire is frayed/damaged. \$\endgroup\$ – StainlessSteelRat Mar 7 '18 at 18:15
2
\$\begingroup\$

Speaking as a person who has worked with industrial controls and residential power for ~20 years, IN GENERAL all free floating electrical contacts should be considered dangerous. For instance, if there were 2 wires that I KNEW only carried 24VDC, I wouldn't worry about being shocked from touching them, but if they touched each other they could cause any number of possible failures on the systems they are connected to. Those failures could then create significant other hazards to people, equipment, or buildings.

\$\endgroup\$
  • 1
    \$\begingroup\$ Very good point; there might also be mishaps e.g. a short to case inside a non-earthed unit putting 240V onto a supposedly low voltage line. \$\endgroup\$ – pjc50 Mar 7 '18 at 15:46
2
\$\begingroup\$

Dangers:

  • someone can get electric shock; generally 100V or more is needed for dangerously high current through the body. The effect can escalate to multiperson accident when someone comes to try to help the electrocuted person
  • someone can feel something not actually dangerous, but gets frightened and does something too fast and out of control, for ex. hits himself with a knife
  • there's a high current source, say 12V car battery, a metallic tool, your ring or any well placed metallic thing gan get red hot in a second.
  • someone sees your "free electrified wire ends", thinks that this is your normal behaviour everywhere and some doors in front of you get closed permanently
  • another people see that you are still alive and do well. They also start to have free electrified wire ends.
  • you accidentally start a strong machine which starts to rotate, sucks your hair or clothes and soon the rest of you
\$\endgroup\$
  • 2
    \$\begingroup\$ I like the out-of-the-box risks. While clearly the raw "can this electrocute/catch fire" risks are important to the OP, it's good to think of the other things that happen, especially when the human element is brought into the mix. \$\endgroup\$ – Cort Ammon - Reinstate Monica Mar 7 '18 at 17:06
2
\$\begingroup\$

The YouTube video explanation was technically incorrect and was finessed carefully with a precision gap to strike an arc, avoid shutdown, sustain an arc and adjust the gap to sustain a bigger arc to raise the temperature to red hot levels of burning air.

It was not simply voltage that caused the fire. If it were simply voltage, we would never be driving in 12V cars.

Rather it was a (carefully) shorted (point contact) circuit current and arc of sustained high energy density below an example of a primitive converter protection threshold.

The same might be possible with an inductor from a 1.5V D cell battery or a single LiPo cell with needle electrodes. The battery may also overheat and explode. Products shipped with batteries are required to have plastic sleeves to insulate the battery terminal from exposure of the product wires being crushed during shipping.

So it was not the voltage that posed the risk, rather the resonant arc of voltage & reduced current with a plasma arc load near the source impedance of the battery for maximal power transfer, sustained for a time duration to raise the plasma temperature to 3000'K red hot not 10000K .

Similar to the confusion created in the video (Voltage is unsafe )the Power Industry has educated workers in the last 2 decades about Arc Flash Protection. Even at low grid voltage like 600V. Workers must use flame retardant clothing and face guards in case of such a V*I power arc with power flash risks as bad as a direct hit by lightning current.

Similar to AC line voltage, yes it unsafe to ground yourself and touch line voltage, but it the high resistance of conductor contacts and the low (leakage) resistance of insulation that is unsafe , not (just) the voltage.

Relay contact reliability

This is also related to the de-rating of relay contacts from similar questions in this forum. The more energy storage in the load (R stores none, L stores some, moving solenoids store more and moving DC motors store the most) and the frequency of switching these loads the dramatically reduces the life span of relays.

Arc Flash

The safety issue here is the amount of energy stored in the source available to be delivered upon an unintended short caused by a breakdown of insulation. residential circuit breakers are rated for 10kA short circuit but then trip to protect the circuit. But a corroded contact in an old house drawing 15A from a 15A breaker can cause a fire at the corroded wiring contact. Fire Marshalls educate the public about these safety issues.

\$\endgroup\$
  • 1
    \$\begingroup\$ Not sure how "we would never be driving in 12V cars" makes sense, considering that lead-acid batteries are probably one of the most dangerous 12V sources you could find. Short-circuit that and you'll know you did. \$\endgroup\$ – jpa Mar 7 '18 at 18:35
  • \$\begingroup\$ Because this is the safest voltage for fuel cars where e-cars must use extra protection to run at 300V. Raising exposed battery terminal voltages is possible with 24V vehicles but surge dump voltages rise (ACU turn off) and lowering voltage only increases the risk of higher current issues from high resistance. Risk can never be zero, but with proper design can be made safe as possible. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 7 '18 at 19:12
1
\$\begingroup\$

I like the answer of @Jack B, as it serves us some literature numbers. But you need to keep in mind, that these numbers are only valid in the environment they are specified for.

I do not know, for what environment the EN61010 is intended, but OP also asked about closet/drawer/messy location.

If your drawer is in your workshop, it's not entirely impossible to contain steel wool. And it's not impossible for your circuit to contain a 9V block battery. You can already see where I'm going.

We know the idle voltage, and we know that a maximum current of 5-6A can be expected. This leads to an output power of <60W. Well within the limits of what should not be considered a fire hazard, regarding to EN61010.

Aside from these real dangers, a non-short connection may just empty your battery. But that's also annoying.

For these reasons I always provide proper insulation for any contacts, when storing them away. A battery may be kept in its package, or in my battery box. When preparing a hike, my GPS's spare batteries are insulated with paper/tape before going into the backpack.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.