A frayed typical USB cable: Dangerous or not?

Factual Questions
1

So, I only have a vauge understanding of volts vs. amps.

From what I understand, voltage is the “pressure” of the electron flow, and amps is actual “volume” of electrons flowing.

I get that amps are the aspect we measure as to which we consider the lethal potential in humans.

All that said, I have a peripheral device hooked up to my computer to its USB port (a Wacom tablet), and the wire’s plastic insulation has split, exposing the wire it connects to the tablet. It still works, because the wire isn’t severed, but how dangerous might this be, if left unexposed (I taped it up)?

I looked up that a typical USB cable carries 5 volts, and 500mA.

2

you generally can’t even feel 5 volts on unbroken skin. The only risk is possible damage to the device or port if the 5 VDC wire shorts to one of the data or ground pins.

3

Okay, I sort of figured. So, is it the voltage relative to the amps that determine the resistance in a material (like a human hand), to be dangerous?

4

it’s a matter of the voltage present, the resistance through the particular region of the body, and the current available. from there it’s generally just appying Ohm’s law. From what I remember from classes, there’s a particularly critical range of current (100 to 200 mA) that if passed through the heart can disrupt the heartbeat and cause fibrillation which won’t correct itself; you need immediate medical intervention. below and above that may temporarily disrupt it, but it should resume beating if the shock is removed. further above that is where you get unpleasantness such as burning of internal organs.

5

It isn’t dangerous to you, but you damage something on your computer if the wires short out.

6

Pretty much exactly what I was going to say.

7

Thanks jz, a simply put explanation. And thanks everyone else… time to get a new tablet.

8

why? If you “see” split insulation, it’s only the cable’s jacket. Which is the outside cover over the individual wires inside the USB cable (+5V, D+, D-, GND) along with either a foil or braided shield. those wires are still insulated from each other, so really, I would just tape it enough to provide some additional strain relief and keep using it.

9

Well, it was an old one I handed down to my 12 year old daughter. She’s taken to sketching on her laptop with it. I was only half-joking, but there were some wires fraying at that spot, and it seemes very unstable, as it’s a flexing point. My daughter has a bad habit of being rough with cords. Only a matter of time before… zzzzt!

I have a friend with a heat gun and some shrink-wrap sleeves… hmmmm…

10

The human body has a very non-linear voltage/current response. Ohm’s law only applies to devices with a (mostly) linear response.

A very simple model of the human body is a resistor in series with a resistor and a capacitor. The resistor kinda simulates the skin and the rest simulates your internals (it’s very much an over-simplification). However, the value of these resistors varies depending on the applied voltage.

At a low voltage like USB voltages (5 volts) the human body has a very high impedance. It varies a bit from person to person, but generally we’re talking several hundred thousand ohms to several million ohms. If you take your handy dandy multimeter (which also uses a very low voltage) and measure the resistance from one hand to another, you’ll get a very high value. If you use this value to calculate how much current you’ll get from a 120 volt line though, you’ll be way off. At 120 volts, the human body’s impedance drops down to about 1,000 ohms, which again varies a bit from person to person and the conditions.

Generally speaking, voltages start getting dangerous to touch above about 50 volts. You can grab both terminals of a 12 volt car battery and not get shocked. 5 volts from a USB cable is pretty darn harmless, and (if the USB port complies to the spec) the current should be limited to 500 mA or so anyway, which further prevents possible damage. Even so, there is a small risk of fire. 500 mA through a small enough piece of metal in a bad connection can get quite hot.

As others have said, there’s also a risk of damage to the equipment or the port.

By the way, the official “safe” current as defined by most safety standards is 5 mA. A 10 mA current isn’t likely to cause your heart to go into fibrillation, but it’s not guaranteed not to either.

If the insulation is damaged but the wire underneath is fine, simply covering the damaged insulation with electrical tape is sufficient to repair it. If the wire underneath has been damaged, it needs to be repaired or replaced.

ETA: A heat gun and some heat shrink tubing, as cmyk said, works great to repair insulation damage as well.

11

Tape it up and don’t worry about it.
On the other hand, the local Salvation Army here sells them for $2, so what’s the point of not replacing it now.

12

If the current on a USB port is seriously over-drawn, the OS can detect this, and will (or is supposed to) shut that port down.

In my limited anecdotal experience, Linux will toggle the port off and on again, while Windows (XP?) will shut the port down until rebooted.

13

The “volume” of electricity (or the number of electrons) is measured in Coulombs. Amps are the rate of flow: one Amp is a rate of one Coulomb per second.

14

Nevertheless, my girlfriend is convinced that some of her data might leak out if the insulation is broken. “No…”…“Ni una palabra?”

15

I’ve just recently gone through some USB compliance testing for the 500mA limit…be aware that many PCs actually tie USB V+ to the power supply, and are capable of sourcing several amps.

It’s actually the USB client’s responsibility not to draw more current than allowed.
-D/a

16

Too bad you didn’t say it was pulled into a tight mess then we know what the answer would be.