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Q&A

Why is the resistance of water so high?

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Image alt textMeasuring the resistance of water from my tap I got 500kOhm. Adding considerable amount of salt brought it down to 20kOhm:

The question I have is why water is such a danger for short circuits if the resistance is so high?

Is there an issue with the measurement technique, or are there other reasons?

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why water is such a danger for short circuits if the resistance is so high?

Actually, the life safety danger for water is NOT a short circuit. In fact, a short circuit in a typical (all values here are US-centric, other areas of the world have somewhat different ways of handling these issues, though the electricity is the same everywhere) 120V or 240V circuit with a modern circuit breaker will trip in a small fraction of a second. For example, if an electrical device was off, got soaked in water to cause a short circuit, and was then turned on, the likelihood is that the breaker will trip and protect the people, devices and wiring involved quite well.

The problem is when you have a ground fault. The normal path for power is either between hot and neutral (120V) or between two hot wires (240V). If all the power goes from hot to ground instead, the breaker trips (this is a short circuit). The problem is if some of the power goes from hot to ground. This can happen in a number of different scenarios, including:

  • Insulation breakdown
  • Water providing a moderately high-resistance (because if it was low resistance it would be a short circuit) path to ground
  • People, pets or other critters (e.g., mice, rats) touching parts of the device that they shouldn't be touching
  • Component failure
  • Improper installation etc.

This combines especially with the metal frame/case of major appliances being connected to ground (though the problems can also happen with small appliances with plastic cases), because anyone touching that metal would be connected to the current running to ground.

People are not, generally speaking, good conductors. Skin has high resistance, plus if you are not standing on a conductive surface then current won't go into you because it has no way to get out of you. However, skin resistance goes way down when skin gets wet. Combine that with bare, wet feet and a person could easily become part of a circuit. As a result, the biggest concerns with ground fault situations are water areas - kitchens, bathrooms, pools, etc. - and have been gradually extended (as the protective technology has improved and become less expensive) to include garages, unfinished basements, all outside receptacles, etc.

In addition, the amount of current needed to injure or kill someone is very low, if that current happens to go through your heart. How can it go through the heart? In one hand and out the other (one hand on a device with a ground fault, the other hand on a metal pipe or other grounded appliance) or in one hand and out through the feet (classic pool and bathroom situation).

Also note that in addition to heart attacks or other direct injuries, electric shock, even at relatively small levels, can cause secondary problems. Two are particularly worth noting for their danger:

  • Pools

This includes swimming pools, hot tubs, large public fountains - any place where a person can have most of their body in water. A small amount of current may not cause direct damage, but it can cause temporary paralysis, so that the affected people (everyone in the pool at the time) are unable to get out. In addition, rescuers are affected the same way as soon as they walk into the water. This can quickly lead to drowning or actual electrocution (if the ground fault gets worse).

  • Ladders

If you are standing on the floor and get a shock, despite some paralysis you may be able to pull yourself back before you have a serious injury. However, if you are on a ladder when this happens, moving backwards means falling down - trading one type of injury for another!

The end result is, without going through all the math here, is that the combination of:

  • Water, provided it is not totally (distilled) pure, conducts some electricity
  • Water drastically increases the chance of any electricity getting into/out of a person
  • Ground faults should never happen, but when they do and they are combined with wet people, can be fatal

The solution is a Ground Fault Circuit Interrupter, or GFCI. This can be part of a circuit breaker or (familiar to most in the US) part of a receptacle with TEST/RESET buttons. A GFCI checks to see if the current going out on hot and neutral (or hot & hot for 240V) matches. If it doesn't, the current is going somewhere else (ground fault) and since that might be into a person, it trips fast enough to prevent serious injury or death.

Some parts of the world use a Residual Current Device, or RCD. This works much the same as a GFCI, but when it is installed for an entire house (or a large part of a house), it is set to trip at a relatively high level, which helps protect from a lot of problems but does not provide the same level of life safety protection that a GFCI normally provides. GFCI is also often provided on a plug-in appliance (e.g., portable hair dryer) to help protect people using the appliance in places that do not have GFCI protection installed.

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General comments (2 comments)
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Pure water does indeed have very high resistivity (different from "resistance"). Reasons water is dangerous around electricity include:

  1. Even small amounts of impurities greatly increase the conductivity of water.
  2. The water that you worry about that might contribute to a shock hazard is especially likely to contain impurities. A flask of highly pure water in a laboratory isn't the danger, its more like a puddle on the floor. Most of the dangerous water (in an electrocution context) is stuff you wouldn't want to drink.
  3. Even small amounts of moisture can make your skin much more conductive.
  4. A little liquid water greatly increases the effective contact area of a conductor touching your skin.
  5. At typical house power voltages, it doesn't take much resistance to cause enough current to hurt, or worse. At 110 V, 110 kΩ allows 1 mA to flow. You'd definitely feel that, making you jerk back involuntarily, possibly hurting yourself in the process. It can also cause muscles to contract uncontrollably. If that happened to your heart muscle for long enough, you're dead. It can also seriously mess up brain signals if running thru the head. And that was just 110 kΩ with 110 V.
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I think your measurement instrument is fine. What is not is your understanding of resistance: there is nothing like this "resistance of water". In your case, the measured resistance depends on the distance between the immersed probes. The relevant concept is "electrical resistivity" or "volume resistivity" of materials (see Wikipedia). For example, if a 1m x 1m x 1m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then a right 1m x 1m x 2m parallelotop will have a resistance of 2 Ω.

Now, usually, the resistivity of water is high, and depends, as you know, upon the concentration of minerals it contains (or more precisely and generally, upon the concentration of ions). Pure water is a relatively good dielectric, but still far from being as good as plastics because there is always a small percentage of ions inside water (google "ph of water").

So, often, water will not cause short circuit in A4 battery powered instruments or so. Still, as electronic components are often very sensitive, even very small amount of currents may cause important failures (think about op-amps that have usually input current in the nA or pA ranges). Furthermore, since we usually ignore the concentration of minerals, it is evident that these instruments shouldn't be immersed inside water. Furthermore, water causes other problems like oxidation etc.

Now, for high voltage powered circuit, working, e.g., on the 220V main, even 20 kΩ gives about 10 mA of current, at which the heart begins to fibrillate.

In conclusion, water is really undesirable in electrical circuits.

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