normal wiring scheme these days.

2011

My understanding is that 50mA has the potential to cause death at main voltage (230VAC in Ireland). This is why RCDs installed on socket circuits are no greater than 30mA (I Δ n).

Equipotential bonding is only part of the story. For it to make a difference something else will have to have failed, if not there would be no potential difference between the two points being bonded together.

There are number of ways to reduce the risk and severity of electric shock in a domestic installation.

This includes, but is not limited to:

1) Supplementry protection from an RCD

2) Protective devices such as MCBs and fuses

3) Equipotential bonding

4) Double insulation

5) Installing CPCs and verfiying the earth fault loop impedance is low enough to ensure that the protective device operates within the required time.

6) Mechanical protection.

7) Devices that automaticaly disconnect from the mains when the cover is removed.

Bruthal

mikejjp wrote: »

M cebee wrote: »

http://www.voltimum.ie/news/8332/cm/september-s-question-of-the-month---bathroom-bonding.html
... an interesting quote.

How about ... http://en.wikipedia.org/wiki/Electric_shock ... Facts claimed:

> "The world population 95 percentile has 2125 Ohm body resistance at 220 V" - see Body Resistance Table

> "A domestic power supply voltage (110 or 230 V), 50 or 60 Hz alternating current (AC) through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60 mA."

Ohm's Law: I = V/R = 220/2125 = 103 mA

From this you'd think 103 mA is enough to kill most people - at least a very nasty experience. Looks like a vulnerable, old person or someone with a dicky ticker is going to be toast. Btw, how to get real-life experimental evidence unless you live under a Nazi regime! Maybe from experiments on apes?
Thanks for the mercy of earth bonding.

A person touching only a live point, and nothing else except feet on the ground in shoes etc, will present a far higher resistance than 2k ohms, and in most cases, would perceive no shock. You were looking for real life experimental evidence. I did that test a few times. Perhaps a video is needed:eek:

And the skin contact resistance itself varies very widely, so the severity of shock received from 230v contact from one hand to the other can vary to a very high degree, depending on a number of factors such as sweaty hands, contact area in mm square etc. But 100ma is achievable easy enough. I tripped RCDs myself on a couple of occasions through unintended contact.

If you do your ohms calculation for 110v, it would show 50ma, but direct contact with 110v would be quite an achievement to result in electrocution. It would want very good contact area in both places and a sustained contact.

2011

robbie7730 wrote: »

A person touching only a live point, and nothing else except feet on the ground in shoes etc, will present a far higher resistance than 2k ohms, and in most cases, would perceive no shock. You were looking for real life experimental evidence. I did that test a few times. Perhaps a video is needed:eek:

I believe you, please don't post a video of this. Others may copy and then lean against a metal pipe :eek:

And the skin contact resistance itself varies very widely, so the severity of shock received from 230v contact from one hand to the other can vary to a very high degree

Very true. This is why bathrooms present increased risk.

Bruthal

2011 wrote: »

My understanding is that 50mA has the potential to cause death at main voltage (230VAC in Ireland). This is why RCDs installed on socket circuits are no greater than 30mA (I Δ n).

Mine would be that the 50 ma is 50 ma, and the level of voltage does not really matter. Obviously the voltage level affects the current, and 50ma wont be flowing from 12v ac. But if one person contacts 170v and 50ma flows, and another contacts 250v, and 50ma flows, they both perceive the same shock.

The person is like 3 resistors in series, 2 skin contact resistances which can widely vary, and internal resistance which is generally similar. So the 50ma will feel the same, regardless of the voltage of the item contacted.

Bruthal

2011 wrote: »

I believe you, please don't post a video of this. Others may copy and then lean against a metal pipe :eek:

I was going to lean against a pipe in the video:D

Interesting that you say you believe it. Would you not say its a fact? I know a fair few electricians that dont believe it, and assume you will perceive a current flow in that scenario.

Bruthal

2011 wrote: »

Very true. This is why bathrooms present increased risk.

True. Water is actually a poor conductor. But what it does is form a large mm square contact area on the fingers, but extremely short in length, as in the distance from finger tip to the contact point is through water only microns thick, but with a huge surface area compared to this conductor length, the resistance through this water is low, so it drastically reduces the skin contact resistance.

2011

Interesting that you say you believe it. Would you not say its a fact? I know a fair few electricians that dont believe it, and assume you will perceive a current flow in that scenario.

Just because something is a fact does not mean that it is university agreed

Current will only flow between 2 points if there is a potential difference between them. There would be so little potential difference across you due to your shoes almost no current would flow. Therefore no shock would be experienced.

2011

robbie7730 wrote: »

True. Water is actually a poor conductor.

Funny you should say that. I was working with an engineer recently who was installing an automatic system that would use deionized water to quench electrical fires in switchrooms. Water only conducts due to impurities, making deinonized water a good insulator.

Bruthal

2011 wrote: »

Funny you should say that. I was working with an engineer recently who was installing an automatic system that would use deionized water to quench electrical fires in switchrooms. Water only conducts due to impurities, making deinonized water a good insulator.

Yea its another assumption that it is a good conductor. If you can imagine a 10 meter square area of water in contact with 10 meter square copper plates connected to 10 square cable, but the layer of water is just 1mm thick between the large plates, that would conduct a fair current to power a shower etc no problem. But get a 10 meter column of water but 1cm square, and it wouldnt power a cfl.

The large surface area but very thin, is the way is works with wet hands in contact with a live item and earth or neutral. Sweat adds in salt as well, which greatly increases the conduction.

Bruthal

2011 wrote: »

Just because something is a fact does not mean that it is university agreed

And vice versa.

mikejjp

https://sites.google.com/site/4occasionaluse/shock ... diagrams.

Assumptions:
Rfloors = 1000 Ohm
Rfault = 0 Ohm
Rkichencupds = 10000 Ohm
Rbody = 5000 Ohm ... wet hands on the sink and bare feet on a wet kitchen floor
Calculations:
Rcct = Rfault + (Rkitchencupds x Rbody) / (Rkitchencupds + Rbody)
Rcct = 0 + (10000 x 5000) / (10000 + 5000)
Rcct = 3333 Ohm
Icct = 240/3333 = 0.072 A = 72 mA
Ibody = Icct x Rkitchencupds / (Rkitchencupds + Rbody)
Ibody = 72 x 10000 / (10000 + 5000)
Ibody = 48 mA
Conclusion:
In the appropriate place, enough to kill a fit person.
Bond their sinks, or rely on the RCB to save their life. :eek:

Mod Edit, comment removed, unrelated to the OP.

2011

mikejjp wrote: »

Mod Edit, comment removed, unrelated to the OP..

I don't think that these type of demonstrations are a good idea. A safer way to illustrate this is by disconnecting the neutral from a light bulb. It does not set a good example. Doing this where I work would guarantee permanent removal from site.

Apprentices often copy what they are shown. Before you know it one may attempt to touch 2 live wires while standing on an insulator, the only problem is they may be 2 different phases!

Bruthal

2011 wrote: »

A safer way to illustrate this is by disconnecting the neutral from a light bulb. Doing this where I work would guarantee permanent removal from site.

Well its fairly serious leaving everyone in the dark alright:pac: