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I have seen that power is transmitted from power stations to households at high voltage and low current to minimize the power loss.

That means the current in the transmission line is less than the current in the household wiring as there using a transformer we decrease the voltage and current is increased to keep the same power.

As it's the current that is dangerous, as it means how much charge flows per time unit: why are transmission lines more dangerous than household lines, even though voltage is high, but current is less?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat, twice. $\endgroup$
    – rob ♦
    Commented Jan 1, 2021 at 14:57

7 Answers 7

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Current flowing in the wire is irrelevant to the danger.

It's the current flowing through your body that will hurt you, and the amount of current that flows through your body will be proportional to the voltage between the wire and anything else that you happened to be touching (e.g., the ground upon which you are standing.)

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    $\begingroup$ @Solomon Slow,m afraid if the current thru the body increases with voltage then net current flowing thru the wire will increase due to parallel connections,thanks. $\endgroup$
    – sachin
    Commented Dec 30, 2020 at 22:10
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    $\begingroup$ @sachin, what do you mean, more current flows in households? A high-voltage power transmission line is obviously carrying a lot more current than a single household. $\endgroup$
    – TonyK
    Commented Dec 31, 2020 at 13:50
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    $\begingroup$ @sachin I don't think you understand how current works in parallel circuits. If you cause a short that is parallel to another circuit, the current is not the same for those two branches, the current rises infinitely on the short (or until the voltage drops due to lack of power) and the current goes to nearly 0 on the normal circuit. Your body would be similar to a short circuit on that scale. $\endgroup$
    – Greg Schmit
    Commented Dec 31, 2020 at 17:45
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    $\begingroup$ Lets also not forget that sufficiently high voltage allows arching through the air, so one can get zapped even if one is not quite touching. $\endgroup$
    – Baard Kopperud
    Commented Dec 31, 2020 at 18:54
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    $\begingroup$ @candied_orange You have set off on a tangent, but as we are there: 1) if the neutral wire also contacts the water, it will normally be the lowest-impedance return path, especially where the plumbing is plastic. 2) Live wires in flood water (for example) present a danger to people in that water, even if they are not touching the wire, as their wet, salty bodies may form part of a low-impedance leakage path to the earth, sufficient for them to take a dangerous fraction of the total leakage current. Ditto for lightning strikes near swimmers, including divers. $\endgroup$
    – sdenham
    Commented Jan 2, 2021 at 21:58
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The current that damages a body is far below the current capability of any typical power line (milliamperes vs amperes). Also, the body's resistance is quite high. So, any power line with sufficient voltage will be capable of giving a body a deadly amount of current. Therefore it is the voltage of the power line, alone, that dictates how much current will go through the body. Only when the source of power has an extremely low maximum current (like, say, an electric fencer or a small bug zapper) will a body drain the power and thus cause the voltage to drop.

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  • $\begingroup$ ,m afraid if the current thru the body increases with voltage then net current flowing thru the wire will increase due to parallel connections,thanks. $\endgroup$
    – sachin
    Commented Dec 30, 2020 at 22:18
  • $\begingroup$ I think you're understanding now, sachin. To paraphrase, if you touch the power lines, you provide a lower resistance path between the wires. Following Ohm's Law, more current will go through the wires, and that amount of current will hurt you. $\endgroup$
    – Jetpack
    Commented Dec 30, 2020 at 22:48
  • $\begingroup$ @Jetpack,how can more current pass thru the wire when its already fixed for a particular power obeying P=V I,yes when it gets a path thru the body,more current flows thru the body but that current is actually a part of the low current in the main wire,resulting in further low current than the original one flowing thru the main wire,thanks. $\endgroup$
    – sachin
    Commented Dec 31, 2020 at 0:27
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    $\begingroup$ @sachin If the power is held constant, then the voltage will drop as the current rises. If the voltage is held constant, the higher current simply draws more power from the station. Notice that the power demand on the grid is naturally variable. Grid infrastructure works to maintain stable voltages by continuously adjusting the power produced. Now, depending on how much power you draw, you might cause a noticeably unusual power spike, in which case something might explode or cut the power (though probably not fast enough to save you). $\endgroup$
    – HTNW
    Commented Dec 31, 2020 at 1:57
  • $\begingroup$ I believe the current used in American 2kV electric chairs is only 1 or 2A, and that is to ensure thorough cooking of the condemned individual. Much less will cause fibrillation (around 0.1A). $\endgroup$
    – Michael Harvey
    Commented Dec 31, 2020 at 11:56
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First, a word of clarification. "Less current than there otherwise would have been" doesn't mean that the current is actually "low". Remember that HV lines service multiple houses, so the current flow through them is still typically higher than what flows into a single average home.

However, the current that flows through the wire isn't what makes it dangerous to touch the wire or not*. What's important is the current that flows through you when you touch the wire. So, the question becomes, "how much current would flow through you if you touched the wire?"

All else being equal, the current that flows through you is directly proportional to the voltage across you. Double the voltage, double the current. And that's why it's dangerous to touch a high-voltage low-current wire: The high voltage will push current through you regardless of what the previous current flow was.

However, that's not the whole story. The fact is, there's no such thing as a perfectly constant voltage source. Your body would represent an additional load to whatever the source of electricity is, which would tend to pull the voltage down. High-voltage air ionizers and ozone generators are (usually) perfectly safe to touch, because their power supply circuitry has next to zero current delivering capability, so touching it cuts the voltage to a tiny fraction of what it was with no load. Electric fences (for cattle or pets) have a somewhat higher capacity, so the voltage drops enough that its not dangerous, but still enough to give you a little zing. For electrical power lines, though, the load presented by your body is incredibly tiny compared to what they normally supply, so the voltage drop is undetectably minuscule, and you get the full voltage across your body with all the current that that implies.

* Well, high current flow through a wire results in a lot of waste heat being generated, so I suppose, technically speaking, a high current could be dangerous in that you could be burned by it.

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  • $\begingroup$ Take a 11kV/415v, 500KVA distribution system. Then maximum current on HV side = 500/{sqrt(3) * 11} = 26.24 A. Maximum current on LV side = 500/{sqrt(3) * 0.415} = 695.6 A. $\endgroup$
    – Michael Harvey
    Commented Dec 31, 2020 at 11:53
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    $\begingroup$ @Michael Harvey the OP did not say “maximum current on LV side”. They said “current in the household wiring”. As HiddenWindshield says that current is indeed less than the HV line current. Your calculation differs from that by a factor of the number of households served by the HV line. $\endgroup$
    – Dale
    Commented Dec 31, 2020 at 14:47
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    $\begingroup$ @In the UK, with 240V household supply,we used to have a 60A fuse as standard protecting each house, nowadays it could be 80A or 100A. A 132 kV distribution circuit may carry 300 A in each of its three phases which is MORE than any household. A 240 or 400 kV HV transmission line might have a normal current over 1000A which is, again more (not less, as the OP says) than any household. $\endgroup$
    – Michael Harvey
    Commented Dec 31, 2020 at 15:44
  • $\begingroup$ @MichaelHarvey When you say "OP", are you talking about what I posted, or what sachin posted? Because you're replying to my post, but you're saying the same thing I said, the current in an HV line is typically greater than a single household. $\endgroup$
    – HiddenWindshield
    Commented Dec 31, 2020 at 23:15
  • $\begingroup$ @HiddenWindshield - the convention is that 'OP' means 'Original Poster', that is 'the person who asked the original question'. In this case that is 'sachin'. $\endgroup$
    – Michael Harvey
    Commented Dec 31, 2020 at 23:39
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The current in the transmission wires is lower for a given amount of power being delivered than current in lower-voltage wires transmitting the same amount of power. It can still supply many, many times more current than is needed to electrocute you.

You seem to have a misconception that the current and power are fixed. The wire supplies whatever current that the attached loads allow to pass. With no load, there's no current and no power delivered, with a heavy load, there's a lot of current and a lot of power...it's voltage that's (roughly) fixed. By becoming part of the circuit, you become another load, and a high-voltage wire will easily supply a little more current to pass through you.

Higher voltages are more hazardous in general because a higher voltage can drive a lethal current through a higher-resistance path that includes your body, and high enough voltages can cause insulators (including air) to break down and stop protecting you.

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Because the increased voltage is better at overcoming what little electrical resistance the surface of the body (and any attempt at insulation) provides, and will drive a higher current through the squishy stuff that you presumably value.

And there's a risk of a flashover arc, and if that happens your remains will be sufficiently charred that they won't be getting an open-casket funeral.

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    $\begingroup$ You can get a runaway situation where the initial current heats up your body causing cells to leak water which lowers the body's resistance, which causes more current to flow, which heats it up even more, in a cycle. The current rises very fast until your body actually explodes (is blown to pieces). The whole thing would take a tiny fraction of a second. This happened to a boy who climbed on the roof of a 25 kV railway feeder station near my home and grasped a live bus bar. $\endgroup$
    – Michael Harvey
    Commented Dec 31, 2020 at 20:29
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It's for multiple reasons.

First of, 30 mA through the chest is the danger territory. Things can happen before this, but that's generally where it's really dangerous. Dry skin can have 10 kohm resistance or more - so with 110 V or 220 V it's borderline if you get into the really dangerous territory.

If you get current through an arm, or other body parts, but not through the chest, you'll almost certainly walk away from 220 V.

If we jump to a 22 kV line, and assume a 10 kohm skin resistance, the current is no longer under 30 mA - it's a whopping 2 A - well above what's fatal. And it's no longer depositing tens of watts. It's depositing tens of kilowatts. Close to 50 kW if we assume a static 10 kohm resistance.

This leads to internal and external burns. Those burns may lead to kidney failure as well. Even if you only get current through a body part other than your chest, you'll have severe burns.

That means the current in the transmission line is less than the current in the household wiring as there using a transformer we decrease the voltage and current is increased to keep the same power.

This is mistaken. The voltage is high to reduce losses, but you don't have a 22 kV distribution line to your home. You have it to your neighbourhood. It's not uncommon to have hundreds of amperes available at a high voltage line. In addition, the short circuit performance is likely to provide a higher short circuit power than what you receive in your outlets at home, as the impedance in the system is likely to be low.

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  • $\begingroup$ Great answer which includes the relevant numbers. The numbers assume familiarity with Ohm's law, but the reader can alternatively skip all the numbers are still get a good idea about the typical outcomes. The numbers just explain the outcomes. $\endgroup$
    – Jirka Hanika
    Commented Jan 1, 2021 at 13:45
  • $\begingroup$ It's written from the perspective of one working with electricity, not physics. And I don't really believe this is a pure physics question either; biology blends into it as well. $\endgroup$
    – vidarlo
    Commented Jan 1, 2021 at 13:47
  • $\begingroup$ The tens of kW offloaded into your body have the effect of the tens of Watt offloaded into an egg in the microwave. I really recommend touching a household line if you have a choice. $\endgroup$
    – Peter - Reinstate Monica
    Commented Jan 2, 2021 at 12:57
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    $\begingroup$ @Peter-ReinstateMonica - It may be way worse than the microwave power consumption, because the microwave is 50% efficient and those 50% are purely thermal (wrt eggs and vertebrates), while high voltage eletrocution (kilovolt and above) also causes electroporation, protein denaturation, and a number of other effects - tissues can end up drastically damaged by effects of voltage or the proportional current even without biologically significant temperature changes. I'm just emphatically agreeing with your preference to stick to the lowest voltage available. $\endgroup$
    – Jirka Hanika
    Commented Jan 5, 2021 at 14:24
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Your language is imprecise. Voltage is not a property of a location, it's a property of two locations; it's the difference between the electrical potential between two points. It's not quite true that "it's the current that is dangerous", as the current is not the only factor, and to the extent that it is accurate, it needs the qualifier "the current through your body". The current flowing through a point that you are touching, by itself, is completely irrelevant to how much danger you're in (a wire with more current may be more dangerous, but the danger comes from the voltage causing the current, not the current itself). If you have less resistance than the power line (including the transformers), which you almost certainly do, then the current through you will be higher than the current through the wire, so the low level of current in the wire says little about how much danger you are in.

That means the current in the transmission line is less than the current in the household wiring as there using a transformer we decrease the voltage and current is increased to keep the same power.

That doesn't follow. If several households are connected in parallel to the power lines, then the current in the households could be less than the current through the power line.

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