How I almost killed myself

[rrinker]

The Mythbusters "test" was flawed in so many ways. It also assumed you have a bath waste that is conductive, however in the majority of the world we have plastic wastes and plastic waste pipes.

 Maybe on all new construction. But my house was built in 1972, and the main waste pipes from the bathrooms are cast iron pipe, same as was in the house I grew up in, which was built in the 50's. Though it's not the electrical properties that really worry me, it's the way it weakens as it ages and then leaks, so at some point I may have them replaced as a preventative measure. Most of my under sink traps are that crappy metal, too. I much prefer plastic there as well, as it tends to not have major failures, just leak around the connections.

 

[Sal Ammoniac]

How about this?

[helius]

Again, what's the point you're trying to make here? If there was no heating element present or the load was different do you think the result would be different?

It's not warranted to assume that water acts as a purely ohmic load in these cases. In order for current to flow at all, there is some polarization (capacitance) in the ion gradient, which isn't easily separated from the heating element since it has area which alters that capacitance. You can't really model it as two parallel loads.

[zl2wrw]

[SNIP]
We had a beast of a transformer that we used for load testing; 575V in, something like 9V out with enormous lugs on the output due to the increase in current.
[SNIP]
He then tells me to stick my hand in the barrel. I declined, but he convinced me to do it. With only my hand in the barrel of (fairly warm) water, nothing much. But when I got to elbow-depth, I had the oddest feeling in my forearm, like someone was grabbing and releasing my arm 60 times a second.

My only explanation is that I was getting some kind of capacitive coupling through my shoes to the concrete.
[SNIP]

Was the 9V secondary of that transformer floating? (or was it earth referenced?)
Did that transformer have an earthed electrostatic screen between the 575V primary and 9V secondary windings?

I suspect that what was happening was that high frequency common mode was capacitively coupling through the insulation between the transformer primary and secondary windings and your arm was a path to earth!


I have had a plumbing related "shock". House was built in the 1940's, plumbed with Cu pipe and a galvanised iron connection to the old cast iron city water mains, and in NZ, conductive water pipes are bonded to protective earth, which is bonded to neutral (MEN variant of TN system). It was a hot summers day, so I was barefoot, washing the car and playing with the dog with the hose - when I was done with this, my hands and feet were thoroughly wet (pruned skin etc). When I grasped the handle of the brass outside tap to turn off the hose, I felt a rather noticeable 50 Hz tingle - thought "that shouldn't do that!".

After I got dried myself off and got changed, I got my trusty Fluke DMM and found that there was about 15 Vac RMS touch potential between the outside tap and local ground. A clip on amp meter indicated that there was a couple of amps flowing in the PE bond to the galv. iron water pipe coming into the front of the house. The power company came and had a look and basically said that there was nothing wrong with the wiring in the house (ie neutral hasn't gone open) and that the fault was somewhere else and that this house was just in the return path for the fault wherever it is (they basically gave up as no one had any idea where the fault was). A couple of years after that, water started leaking out of the old galv iron water pipe under the front lawn. The plumber replaced it with plastic pipe, and as a result, the touch potential on the outside tap at the back of the house went to zero

[IanB]

A couple of years after that, water started leaking out of the old galv iron water pipe under the front lawn.

Galvanic corrosion due to the fault current, perhaps?

[zl2wrw]

A couple of years after that, water started leaking out of the old galv iron water pipe under the front lawn.

Galvanic corrosion due to the fault current, perhaps?

Yes, I assume that is what happened to the pipe.

Somewhere in my neighbourhood, there probably was (and maybe still is?) a house with an open circuit neutral conductor, resulting in most of their return current flowing through the neutral-earth link in their switchboard, to get to the metallic city water mains, to get to the neutral-earth links in all the other houses, and thus back to the neutral conductor and the distribution transformer.

[HackedFridgeMagnet]

I have had a plumbing related "shock". House was built in the 1940's, plumbed with Cu pipe and a galvanised iron connection to the old cast iron city water mains, and in NZ, conductive water pipes are bonded to protective earth, which is bonded to neutral (MEN variant of TN system). It was a hot summers day, so I was barefoot, washing the car and playing with the dog with the hose - when I was done with this, my hands and feet were thoroughly wet (pruned skin etc). When I grasped the handle of the brass outside tap to turn off the hose, I felt a rather noticeable 50 Hz tingle - thought "that shouldn't do that!".

After I got dried myself off and got changed, I got my trusty Fluke DMM and found that there was about 15 Vac RMS touch potential between the outside tap and local ground. A clip on amp meter indicated that there was a couple of amps flowing in the PE bond to the galv. iron water pipe coming into the front of the house. The power company came and had a look and basically said that there was nothing wrong with the wiring in the house (ie neutral hasn't gone open) and that the fault was somewhere else and that this house was just in the return path for the fault wherever it is (they basically gave up as no one had any idea where the fault was). A couple of years after that, water started leaking out of the old galv iron water pipe under the front lawn. The plumber replaced it with plastic pipe, and as a result, the touch potential on the outside tap at the back of the house went to zero

Do you have RCDs on your house?
How good is your Earth?
Do you have a shed or outbuilding with a separate Earth stake?
If it was a neighbours house, I am guessing it would have to be pretty close. Or else something of yours, pool or a sump pump?
Where did the galv pipe go? Was it only attached to your house or was it incoming?

[zl2wrw]

Do you have RCDs on your house?
How good is your Earth?
Do you have a shed or outbuilding with a separate Earth stake?
If it was a neighbours house, I am guessing it would have to be pretty close. Or else something of yours, pool or a sump pump?
Where did the galv pipe go? Was it only attached to your house or was it incoming?

Yes, the house has 30 mA RCCBs for sub-circuits as per ASNZS 3000:2007 (hot water cylinder, range and heat pumps excluded).
Turning off the main switch made no difference to the current flowing in the bonding wire connected to the water pipe.
The people that the power company sent to investigate said that there was nothing wrong with the wiring of this house (yes, they did look at/test the main earth rod, and inside the switchboard too).

In any case, replacement of the metal water main to this house with plastic pipe eliminated the touch potential and stray current (if we had a bad earth rod and a bad neutral, I'd expect that replacing the metal pipe with plastic would make the touch potential much higher, not lower).

[HackedFridgeMagnet]

 

Yes, the house has 30 mA RCCBs for sub-circuits as per ASNZS 3000:2007 (hot water cylinder, range and heat pumps excluded).
Turning off the main switch made no difference to the current flowing in the bonding wire connected to the water pipe.
The people that the power company sent to investigate said that there was nothing wrong with the wiring of this house (yes, they did look at/test the main earth rod, and inside the switchboard too).

In any case, replacement of the metal water main to this house with plastic pipe eliminated the touch potential and stray current (if we had a bad earth rod and a bad neutral, I'd expect that replacing the metal pipe with plastic would make the touch potential much higher, not lower).

Just trying to think through how you got that tingle.
The only thing I can think of is, as you said before you have some non zero remnant voltage on the incoming Gal pipe but that this pipe has was not electrically well bonded to your water pipe and therefore your main earth.
Maybe your soil type is not very conductive, and it only became conductive with all the water.
Also bear in mind it is easy to have a good earth stake but not a good earth, all the power guys would have done is test the resistance to the earth stake, not to the ground as you need a special meter for this.
But I guess in those situations the earth itself becomes less important.
Ps. If you could be bothered, you can get some 2 part compound to improve an earth cant think of the name. But unfortunately I think plants don't like it.

[SteveyG]

Yes, the house has 30 mA RCCBs for sub-circuits as per ASNZS 3000:2007 (hot water cylinder, range and heat pumps excluded).
Turning off the main switch made no difference to the current flowing in the bonding wire connected to the water pipe.
The people that the power company sent to investigate said that there was nothing wrong with the wiring of this house (yes, they did look at/test the main earth rod, and inside the switchboard too).

In any case, replacement of the metal water main to this house with plastic pipe eliminated the touch potential and stray current (if we had a bad earth rod and a bad neutral, I'd expect that replacing the metal pipe with plastic would make the touch potential much higher, not lower).

Just trying to think through how you got that tingle.
The only thing I can think of is, as you said before you have some non zero remnant voltage on the incoming Gal pipe but that this pipe has was not electrically well bonded to your water pipe and therefore your main earth.
Maybe your soil type is not very conductive, and it only became conductive with all the water.
Also bear in mind it is easy to have a good earth stake but not a good earth, all the power guys would have done is test the resistance to the earth stake, not to the ground as you need a special meter for this.
But I guess in those situations the earth itself becomes less important.
Ps. If you could be bothered, you can get some 2 part compound to improve an earth cant think of the name. But unfortunately I think plants don't like it.

It's the typical PEN failure on a TN-C-S system. The mass of earth and the earth derived from the PEN conductor end up at dissimilar potentials. This is the reason TN-C-S supplies should not be exported to outbuildings any significant distance from the property.

[HackedFridgeMagnet]

It's the typical PEN failure on a TN-C-S system. The mass of earth and the earth derived from the PEN conductor end up at dissimilar potentials. This is the reason TN-C-S supplies should not be exported to outbuildings any significant distance from the property.

Well it probably isn't a typical PEN failure because AFAIK they found no broken PEN when they checked the premises.