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sk3erkrou
01-22-2015, 05:08 PM
Hello everyone,
In my WIP, my MC has a metal prosthetic arm with some very advanced electronics in it. Is there any way to ground this prosthesis to minimize the chance of electrocution should he touch something with a current flowing through it?
My original idea was for a grounding wire to attach to it and be sewn into his clothing in a rubber casing, but I am not very familiar with how electricity works and could really use some help with this.
Thanks in advance!

Bolero
01-22-2015, 08:07 PM
So the prosthetic is metal on the outside? Not covered in flesh covered rubbery plastic? (which would insulate it)

Random thoughts arising

If it is all metal, and arm shaped, it would be quite heavy.
You need something rubbery for the grip bits.

Electricity thoughts
When you are talking about electronics and electrocution, are you concerned about damage to the advanced (delicate) electronics or damage to the person?

Grounding wire - that would need to be grounded to the ground at least, or better still an earth socket. When a person is handling electronic components - say computer chip being added to a mother board - it is good practice to wear a wrist band that grounds you - but the other end of the wire coming off the wrist band needs to be attached to earth - the computer casing if it is earthed for example.
You have grounding strips on some cars where people don't like getting a static electricity shock when touching the car - they hang down at the back of the car and drag along the ground (getting worn out). So if you wanted to ground by a wire down a person, it would need to be peeking out their shoe sole. Not sure that would work at all well on electrical current.

On the flip side, in general, if a person is standing on an insulated surface and not grounded, then the current doesn't flow to ground through them and electrocute them. So now not sure if the grounding wire is a good idea at all.

Anyway, I'm a bit out of practice on the details of these things. Someone more in practice will hopefully be along shortly.

If you can clarify what is your concern regarding "electrocute" that will probably help them.

Note that electronics of the sort you are talking about are generally fairly low on electricity conduction and usage, so if you are picturing the electronics as a route for electrocution of the person, I wouldn't be that convinced. I think they'd burn out (the soldered connections overheat and melt for example) more than the person being injured.

CrastersBabies
01-22-2015, 09:43 PM
This is one of those questions that you always wanted to know, but didn't KNOW that you wanted to know until someone asked it.

sk3erkrou
01-22-2015, 09:55 PM
If it is all metal, and arm shaped, it would be quite heavy.
You need something rubbery for the grip bits.

I know it would be heavy, but the arm being metal, particularly grounded, is an important plot point later on



When you are talking about electronics and electrocution, are you concerned about damage to the advanced (delicate) electronics or damage to the person?

I'm more concerned about damage to the person later on in the story, but the setup is that the grounding is for some advanced electronics in the prosthetic itself. This is technology more advanced than what we have currently, so I'm emphasizing the need for the ground a bit.


Grounding wire - that would need to be grounded to the ground at least, or better still an earth socket.

What if the wire eventually led to a small metal spike on the bottom of their shoe that would contact the ground?


On the flip side, in general, if a person is standing on an insulated surface and not grounded, then the current doesn't flow to ground through them and electrocute them. So now not sure if the grounding wire is a good idea at all.

I posed this question to r/AskPhysics as well, and one thing they mentioned was that if there was the metal on the bottom of the shoe to complete the ground, then they would not be grounded when that foot is raised, possibly solving this issue. Then it seems to become more a point of having the MC recognize what type of situation they are in if electricity is involved so they know whether or not to maintain being grounded.

King Neptune
01-22-2015, 11:01 PM
If you are picturing the prosthetic arm brushing against an uninsulated wire and that electrocuting the person, because there was not an adequate ground, then I think it could work. The ground plate on the sole of a shoe won't do much good when the person is standing on a thick rubber rug, as are sometimes used in the lobbies of buildings. The metal in the arm need not be complete; a steel (or other metal) frame with wiring, padding, etc. attached to that would make perfect sense. The frame could be modelled on the bones of the lower arm for esthetic reasons.

Bolero
01-22-2015, 11:09 PM
I'm more concerned about damage to the person later on in the story, but the setup is that the grounding is for some advanced electronics in the prosthetic itself. This is technology more advanced than what we have currently, so I'm emphasizing the need for the ground a bit.



I'm not really clear on why advanced electronics need grounding at all - as in why are you coupling "advanced" and "grounding"? If they are so advanced, why aren't they either built with protection or more robust than current electronics?

An alternative design with electronics at a contemporary level of fragile, would be that the electronics inside the arm are held in place on rods of plastic which are insulators and any current flowing would go up the metal of the arm.

Now returning to the metal of the arm - as a reader I picture things. So metal is rigid unless it is jointed like a suit of armour. If it is jointed, then you get moisture and dust coming in the joints and affecting the electronics.

Also the electronics - is that for the purpose of animating the arm, or is it hidden in the arm?
For a heavy metal arm, you will need quite meaty little electronic motors to animate it. Then you need meaty batteries.




What if the wire eventually led to a small metal spike on the bottom of their shoe that would contact the ground?



Broadly yes - but if the floor is say covered in rubber floor tiles then it will be insulated from any other ground so not any use.

Also, think about when you walk across a nylon carpet then get a shock when you touch a metal cabinet or door knob. Would it be possible to get electricity going up into the arm's electronics? I'm not sure - need to have someone else check that. But it is something I'd wonder about as a reader.

(Posted in parallel with Neptune)

CWatts
01-23-2015, 12:59 AM
What type of metal is the arm made of? Not all are conductive. Titanium isn't but I could definitely see it being used to make it lightweight - aluminum is another option, and it is conductive. Further confusing matters the two can be alloyed together. Plus hey if it's scifi you could always create your own materials with special properties.

PeteMC
01-23-2015, 02:46 AM
This is one of those questions that you always wanted to know, but didn't KNOW that you wanted to know until someone asked it.

Heh, that's exactly what I thought too!

But yeah, Titanium isn't conductive and is a lot lighter than say steel or something similar.

sk3erkrou
01-23-2015, 03:55 AM
Wow, everyone. This is all some really good information. I was going to remain ambiguous as to exactly what the metal was. However, there is a very important plot point that involves my MC being the only person who is able to touch something that has electrocuted everyone else due to the grounding of his prosthetic. The scene will be taking place outside. In a cave, to be exact, if that helps any.

King Neptune
01-23-2015, 04:55 AM
Wow, everyone. This is all some really good information. I was going to remain ambiguous as to exactly what the metal was. However, there is a very important plot point that involves my MC being the only person who is able to touch something that has electrocuted everyone else due to the grounding of his prosthetic. The scene will be taking place outside. In a cave, to be exact, if that helps any.

That should work just fine, as long as the prosthetic is partly of a conductive metal. It could aluminum with steel fingers and joints. The surfaces of the bearings can be bronze, but steel is good enough a conductor. If the prosthetic starts at the elbow, then the elbow could be a ball and socket joint with the bearings being grounded to each other and to a ground that runs outside the body to the sole of a shoe.

When that character touches something with an electrical charge, the charge will follow the connections between sections and drain off through the foot ground. The character might feel a twinge, but the bulk of the electricity would follow the conductor.

BDSEmpire
01-23-2015, 08:13 AM
What type of metal is the arm made of? Not all are conductive. Titanium isn't but I could definitely see it being used to make it lightweight - aluminum is another option, and it is conductive.

Maybe you're thinking of magnetism of metals but all metals can conduct electricity, that's one of the physical properties of a metal. Titanium is a very poor conductor, but it conducts nonetheless.

If you want your super dude to be able to touch a live wire you *don't* want him to have a clean path to ground. That would send the current right through him and wreck his day.

Instead, having the roboarm be covered in nonconductive rubberized UltraPlastic would be preferred. He could touch a live wire that would normally kill a person and move it safely.

Electricity is going to seek a path to ground so you want to eliminate that whenever you have to deal with live voltage. The big bucket trucks that linemen use to get up and work on high voltage lines have several tricks to make sure there is no path to ground. They also use big fiberglass rods to work with some of the gear so that they don't give the electricity any way to arc to ground.

sabindanjoup
01-23-2015, 09:41 AM
Okay, I just wanted to add that having a little spike in the shoe wouldn't probably be enough.
Real grounding would involve a steel rod buried a couple of feet beneath the ground. I have forgotten the exact figures, but I doubt whether a spike just touching the soil would do any good.

Mark HJ
01-26-2015, 12:42 AM
If you want your super dude to be able to touch a live wire you *don't* want him to have a clean path to ground. That would send the current right through him and wreck his day.

Instead, having the roboarm be covered in nonconductive rubberized UltraPlastic would be preferred. He could touch a live wire that would normally kill a person and move it safely.


And if you dump all that current through a grounding wire it is going to get hot. Very hot.
OK, that depends on the power source, but if you are talking the equivalent of mains power line, then we are talking enough current to melt the wire, unless a suitable circuit-breaker trips.

If the current is high enough, your grounding wire will not just get hot and melt, it will do it fast enough to literally explode.

Insulation is the way to go.

WeaselFire
01-26-2015, 10:52 PM
What do you need to happen for the story? A person could grab a high voltage line with one hand, never be grounded and have no ill effects, if that's what you need. Electricity needs to complete a circuit to flow. A wire in one side of an outlet is basically inert until you ground it or connect it to the other side of the outlet.

Jeff

Bolero
01-28-2015, 12:07 AM
One thought on your grounding wire (which you may well not be using now given what was said in the thread) - you would need a reason for the arm to have a grounding wire that was nothing to do with being able to grasp live wires. Just a thought - as in the gizmos in the story need to be solidly based for ordinary life in the story and have a lucky feature for the cool plot moment which is not just there for the cool plot moment.