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smckean (Tosca)
Joined: 18 Jan 2014
Posts: 975
City/Region: Guemes Island (Anacortes)
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
Vessel Name: Tosca
Photos: Tosca
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 Posted: Tue Jun 13, 2017 10:20 pm Post subject: |
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thataway,
Turns out I have been in and out of town like a yo-yo, so I am just now getting to this...........
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I said: "Only from shorts located upstream from the fuse, not from overloads (as I have often heard said)."
You said: "Sorry but overloads will cause fuses to blow. The fuse protects wires downstream, not upstream." |
I believe this is a misunderstanding. Of course overloads cause fuses to blow, but my point was that in a purely overload situation [that is, not a full short, but an in-between situation where too much current (amps) is flowing for the wire size, so the wire begins to heat up, perhaps eventually to a level where the wire insulation will melt, or perhaps a fire can start], it does not matter where in the circuit the fuse is located. Once the current is above the rating of the fuse, the fuse will blow, regardless of its location, thereby protecting the circuit. In this situation there is no additional protection by placing the fuse near the source. Note that physics, being what it is, requires that the current is the same every where in the circuit.
OTOH, in a short situation, the current only flows in the wire up to the short, then it flows back via the other conductor involved in the short. Shorts are particularly dangerous since there is no load so resistance is near zero and large currents will flow until something gives (wire melts, batteries explode, or whatever), and may well start a fire. Note that the wire in the circuit downstream of the short carrys no current in a short situation. So here is where it is important that the fuse be located near the source since only the wire from the source to the short is involved. By placing the fuse as close as possible to the source (battery) one minimizes the length of wire where a short can be dangerous since only wire upstream of the fuse is not protected by the fuse.
| Code: | I said: "My point was that shocks, especially dangerous shocks, are all about voltage. 120v is dangerous and 12v is not.....AC or DC."
You said: "The skin's high resistance protects us from serious damage from 12 volt, for the most part. However, I have seen some very serious direct injuries from 12 volts. (As a Diener for Los Angeles Co; Coroner during college, working in Emergency rooms for over 35 years, and also during my academic and private practice of medicine.)" |
I don't have medical training or experience, so your knowledge must prevail here. However, I would assume that whatever injury you have seen from 12v applications does not come from electrical current flowing in the human body alone (as it might in a high voltage application). Correct me if I'm wrong, but I would expect 12v injuries must result from secondary effects of high amperage flows. That is, high currents in 12v systems can cause equipment to explode, or high heat, or fire to occur. I would think that it is from heat, fire, explosions, acid spills, and other secondary effects that harm. I can't imagine that a human could be injured from electrical flow resulting from applying a 12v source to the skin alone (e.g., an electrocution)
| Quote: | | Under the correct circumstances even the power from a D battery can kill. |
Can you give me an example of such a set of correct circumstances?
BTW, my entire point of discussing all this, now and before, is to dissuade folks from what I expect is a common belief (a belief I once had) that somehow there is an iron-clad rule that one does not have to worry about electrical damage downstream of a fuse. Also, my purpose is to dissuade the belief that fuses near the load instead of the source never protect wire upstream of the fuse. Fuse location is not magic. It is not so that only wire upstream of a fuse need be worried about. |
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