Having read the cat pee thread I was wondering as I have no knowledge of Ohms/Watts/resistance etc. At what point does a damp/wet socket become dangerous in terms of distance eg Lets say an external floodlamp has a loose live lead and it touches the metal casing on a wet day when the walls of the building are wet if I touch the wall at ground level (lets say 10 feet below) would I get a shock? assuming it wasnt earthed.
Well, a wet building is earthed. So, most of the current would go through you rather than the 15/20 feet of foundations *only if* you are a lower resistance. You won't be. At least, not unless you lie full length on the ground and douse the whole area in salt water. You might feel a tingle.
If it has an RCD it would trip and you wouldn't get a shock.
If its earthed and no RCD the fuse would blow.
If it has no RCD then its time you fitted one as you would probably get a shock depending on what else you were in contact with and a lot of other factors.
OK thanks guys I just didnt kno, question answered.
What about the damp proof course?
If the lamp was unearthed, and there was no RCD protecting the circuit, then the "conductive" wall will behave a bit like a large potentiometer wire - the voltage present on the wall will be roughly proportional to the distance from the point where the current is being injected.
Hence you would have close to 240V next to the lamp, and the voltage falling away the further you get from the lamp.
So you could expose yourself to a dangerous potential difference by simply having your feet on the ground, and then touching high enough up the wall, (or even touching the wall in two places). Its a situation where you will have potential "shells" or a gradient travelling through the wall. You can get a similar situation when you have a fault in a buried cable causing an area of soil to become "live" - there are plenty of cases where livstock have been killed simply due to the distance between their legs - they can straddle a wide enough voltage drop to pass a fatal current through their bodies.
How much current will flow will depend on the conductivity of the wall (wet alone would be slightly conductive, saturated and full of salts, could be quite a low resistance). The more conductive it is, the lower the source impedance - the more current could be available to flow through you. However since you are making a parallel conductive path, the less preference there would be for electrical the route through you. So there is a trade off. However you may well be able to find locations where you could be exposed to dangerous shock currents.
Only if the damp course isn't insulating the wall from ground.
If it is, its quite different with the resistance of the wall determining the current that flows through his finger and him. And that also depends on what shoes he is wearing if any.
A voltage gradient appears on the ground around a fault to earth. You can imagine it as a series of rings around the fault point. For you to get an electric shock, you have to be in contact with two points of different voltage. (eg the "live " thing you'retouching and your feet. The exact voltage appearing across you depends how far apart these points are, the resistance of the ground (and your footwear) and the voltage appearing at the fault point.
This is why cows are more likely than a human to die in a lightening strike, their feet are further apart. And in better contact with the ground. So, you don't even have to be touching the casing of your floodlight to get a shock. Just stand nearby with bare, wet feet as far apartas possible.
What about the earthing rod strapped to mains earth and connected to all the copper pipes buried in the structure above it?
Your house is earthed. Well earthed.
The wall likely has dozens of earthed metal back boxes buried in it. Even if it's a cavity wall, the ties are metal.
Its likely to be far more complicated as walls tend not to be uniform in construction. A modern damp proof membrane screws it up as they are high resistance plastic. Bricks are less absorbent than mortar so the current would flow differently depending on the bond. Etc.
Not necessarily with the outside leaf of a cavity wall.
But the internal wall won't be soaking wet so that wont necessarily provide a relatively low resistance path to ground.
Probably not, but that would very much depend on the material the wall was made of and if it absorbed or repelled water. I have felt the buzzing as one might detect on a metal case of a double isolated appliance very often on things like pipes when it was just a cable that ran along with it. That is electrostatic coupling, but likewise, I could feel it on a wooden mount on an outdoor mains lamp when the housing leaked and filled partly up in water!
Surely, on a modern system if this happened now some circuit breaker would trip ahead of any personal injury.
Brian
The difficulty is that you have what amounts to a 3D matrix of resistors
- since its not a good conductor, you can tie it to ground potential in multiple places, and still have significantly higher touch voltages present on other parts of it. Much the same as it can be simultaneously
0 deg C on its outside surface and 20 deg C on its inside.
Indeed - even humidity on the day would affect it. You see similar voltage gradients round earth spikes etc.
Also why the wiring regs recommend 10mA trip RCDs for circuits in use near livestock.
You'd be surprised - even a "dry" wall has moisture content and there's a LOT of it to compensate for the very high resistivity.
Not surprised...
Fuck all would happen. No shocks nothing at all.
You could piss on the wall if you wanted to (or get the cat to do it for you)
How about capacitative coupling through the dpc?
C = keA/d
permittivity k = about 2 for polythene e (really epsilon 0) = 8.8e-12 F/m A = area = say 0.1m wide times 40m perimeter = 4 d = thickness = (??) about 0.001m (1mm)
So capacitance between ground and house outer leaf = about 70nF
Reactance R at 50Hz is 1/(2.PI.f.C)
R = about 45k Ohms
Current passed at an RMS of 240V is V/R = 5mA
OK - lots and lots of assumptions there. But I do believe a typical person would feel that, though it would not be dangerous.
However, if we couple in all the other parallel earth paths (resistive and capacitative) it's not totally beyond the bounds of a reasonable guess that you'd get to the point of tripping a 30mA RCD.
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