General > General Technical Chat

Grounding boxes in home wiring are totally safe for you?

<< < (5/6) > >>

Shock:
The video was mainly to illustrate that most of the current flow is through the path with the least resistance, but current still flows in parallel paths. The 0.001 path is basically a dead short so when the guy grabs onto the grounded chassis and is grounded through his feet or whatever, just like if measuring down the live wire in a functioning circuit he would see very little voltage drop.

I think he made the 1ohm wire to match his 1k resistance and make the math look good. But in reality he won't be standing on a perfect ground either so his resistance could be even higher. His appliance will likely have a fuse and the mains or rcd will trip, but it doesn't mean he gets away without feeling it.

Edited for clarity.

Shock:
I re-watched RSDs first video and it reminded me why I never watched them fully, he flipped to electron flow mid video and started increasing negative voltage, it got a little squirrely afterwards with terms like "backed up voltage". At that point I had an aneurysm and was forced to close the video. :D

I'll admit though tackling it unscripted is hard work.

mag_therm:
Jonpaul said: "There is NO such thing as a ground"

I agree,also suggest careful calculations before using "Earth Ground Rods " for safety grounds.
( I saw a lot of discussion about "good" ground rods on internet forums, in my opinion, usually doubtful)

Refer to Dwight "Electrical Coils and Conductors" , 1945,  Chapter 10, where he did a method for ground earth rod resistance based on capacitor analogy,  which is still used these days, re-published with ISO units.
 Similar calculations can be done in FEM. (I did some)
Chapter 10 Eqn 15 for the resistance between two vertical ground rods is given in the old CGS (physics) units:
In ISO units following the conversion is by  this reference:
https://blog.nvent.com/calculating-on-ground-electrode-resistance-of-a-single-rod-ground-electrode-design-principles-and-testing/

R = ( rho / 2*pi* L) * [ln (4*L/ radius) -1 ]
where L is driven rod length [metre]
 L is also the distance apart of two rods , between which the resistance is calculated
radius is of the rod [metre]
rho is soil resistivity  [ Ohm/metre], reciprocal of the more common conductivity values

Example for Michigan USA where the soil rarely dries , using a map of national measurements done in 1975:
 https://www.fcc.gov/media/radio/m3-ground-conductivity-map
conductivity : 8 milli Siemen/metre   : rho = 125 Ohm.metre

L is the driven rod length, and there are 2 of them at distance L apart, L = 2 metre

substituting:
R = 38 Ohm That is the DC resistance between the two rods. 50/60 Hz resistance is similar.

In residential and industrial locations , the distance between the utility PCC will be greater than mentioned above.
But optimistically assuming 2 metre, the short circuit currents through the protection device via "Earth Ground Rods"
for a " 1 phase Bolted Fault" can be calculated:

120 Volt : Isc = 3.2 Amp
240 Volt Isc = 6.4 Amp
4160 Volt single phase ( common in older USA cities) : Isc = 109 Amp

13000 Volt : Isc = 342 Amp

The above is why, in my opinion, there must always be a properly sized metallic earth/ground conductor between PCC source and point of ground fault.

Jester:
Around 1980 I worked for Manitoba Hydro, they generate most of the power for the province about 500 miles north of the border (Nelson river) where they convert it to DC and then send it South along two conductors instead of three. Conversion station near Winnipeg than converts it back to conventional three phase ac.  I think they generate about 6000MW now. Normally it operates bipole, however they can take one side down and use a single conductor plus ground. Apparently the ground path has much lower resistance than the actual conductor, if memory serves me ~0.1 Ohm vs. 5 Ohms? They have an array of ground rods at either end. Now that’s a serious ground!

Berni:
Dang that must be some serious grounding there to use as a conductor.

I am guessing the one line operation is for backup or maintenance. Since id imagine sending DC trough the ground rods would not make them happy in the long term. The lucky side would have some nice corrosion protection from the DC current while the other side gets eaten away by corrosion at an accelerated rate.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod