Take this little QUIZ and reply and I will tell you who got the MOST right. Just number your answers TRUE or FALSE in numeric order.
**Myths of Grounding **
Electricity always takes the path of least resistance. True or False
In current flow, electrons leaving a grounded power supply are trying to get to back to earth ground. True or False
A 15-ampere breaker will ultimately trip if an energized 120-volt conductor is directly attached to an isolated ground rod that has a measured ground resistance of 25 ohms. True or False
A ground rod at a metal light pole with no equipment ground (green wire) serves to reduce the touch potential and makes the installation safer. True or False
The grounding electrode for a 480 x 120/208 volt transformer (separately derived service) is intended to help remove dangerous voltage (touch potential) that can be imposed on the metal parts of the electrical system from a line-to-ground fault. True or False
Ground rods are not required at a separate building supplied by a 60 ampere 4 wire feeder (120/240 V -, 2 hots, neutral and ground, with the neutral floated at the separate building disconnect). True or False
The connection to the earth (ground) is important for proper operation of Transient Voltage Surge Suppressors (TVSSs)? True or False
The connection to the earth (ground) creates a new zero reference. And the lower the ground, the lower the zero reference. True or False
Grounding (earth connection) metal parts to the earth helps reduce stray voltage. True or False
Grounding at service equipment helps in protecting interior wiring from transient voltage caused by lightning. True or False
I am not an electrical expert.
I’ll admit that I am not sure about my answers.
The questions are kind of like the ones on the drivers license test, kinda tricky.
The things you reference in the last 4 are all referring to common mode transients. They are all going to find another path to ground if you don’t give them one. #8 is poorly worded so it is hard to figure out what they are really asking but if you are saying the lower the “… impedance to ground…” I say it is true. The difference in potential between the EGC and the dirt will be lower. (zero?)
lol…these are not mine greg as I got them from Mike Holts site, he says they are all FALSE on his site and I wanted to get others opinions on them.
Electricity always takes the path of least resistance.
**False- A **[FONT=CMR10][size=2]**250 resistor will experience a current of 40 mA, while the 800 resistor will experience a current****of 12.5 mA. **
**[FONT=Palatino-Roman][size=2]False. In parallel paths, current divides and .ows through each individual parallel path in accordance with Kirchoff’s current law. So, when given multiple conductive paths on which to .ow, current will take all of the available paths. Yes, it’s true that more current will flow through the lower resistive path, as compared to a higher resistive path in a parallel circuit, but that’s not the question.
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In current flow, electrons leaving a grounded power supply are trying to get to back to earth ground. False- We know they are trying to get BACK to the source.
A 15-ampere breaker will ultimately trip if an energized 120-volt conductor is directly attached to an isolated ground rod that has a measured ground resistance of 25 ohms. False- We know that OHMS laws says 120 divided into 25 ohms is only 4.8 Amps, not enough to TRIP a 15 A Breaker.
A ground rod at a metal light pole with no equipment ground (green wire) serves to reduce the touch potential and makes the installation safer. False- because we know that the resistance of the earth is so great, very little current will return to the power source via earth if the earth is the only grounding path. The OCPD would not open much like in the example before.
The grounding electrode for a 480 x 120/208 volt transformer (separately derived service) is intended to help remove dangerous voltage (touch potential) that can be imposed on the metal parts of the electrical system from a line-to-ground fault.
[FONT=Palatino-Roman][size=2]False: Because all metal parts of electrical installations are required to be *bonded *to an *effective ground-fault current path *[250.4(A)(3)], there is no difference of potential [FONT=Palatino-Roman][size=2]between different separately derived systems.
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Ground rods are not required at a separate building supplied by a 60 ampere 4 wire feeder (120/240 V -, 2 hots, neutral and ground, with the neutral floated at the separate building disconnect). False- We know they are required at the separate building.
The connection to the earth (ground) is important for proper operation of Transient Voltage Surge Suppressors (TVSSs)?
[FONT=Palatino-Roman][size=2]False. The earth serves no purpose in the operation of a TVSS device. TVSS protection devices protect electrical equipment by shunting high-frequency impulse currents away from the load and back to the source via the circuit conductors, not via the earth.
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The connection to the earth (ground) creates a new zero reference. And the lower the ground, the lower the zero reference. False- Hmmm I find that zero reference is zero reference…not sure HOW low the ground is has anything to do with this reference point.
Grounding (earth connection) metal parts to the earth helps reduce stray voltage.
[FONT=Palatino-Roman][size=2]False: *Grounding *metal parts to the earth serves no purpose in reducing stray or NEV voltage.
However, *bonding *metal parts together reduces the difference of potential between the metal parts, but the stray or NEV voltage, as measured between the metal parts and the earth, will not be reduced.
Stray voltage or neutral-to-earth voltage can come from the electric utility’s distribution system, the building’s electric system, or both of these sources.
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Grounding at service equipment helps in protecting interior wiring from transient voltage caused by lightning.
**[FONT=Palatino-Roman][size=2]False. *Grounding *metal parts to the earth does not assist in protecting interior wiring or equipment from lightning.
However, *grounding *the building disconnecting means to the earth does reduce the likelihood of a fire caused by elevated lightning voltage seeking a path to the earth by arcing across combustible materials.
Interior wiring and equipment can be protected from lightning- induced voltage transients on the circuit conductors by the use of properly designed cascading TVSS protection devices: one at the service equipment, one at each downstream panelboard, and one at each point of use.
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ahh…you are good Wesley…it was just a fun thing and to be honest with you…this is why I idol Mike Holt…I did not get them all right myself when I did it…he explains things very well…
Perhaps Mike Holt has never really studied lightning.
IBM did a lot of studies on this in the lightning capital of the world and concluded it is very important to have a good grounding electrode and that it be used by all utilities. We went from blowing up 50 boxes a week to virtually zero. Our stuff works. The easy example is your modem. An ungrounded TVSS might do a great job of shunting the voltage on the supply side and preventing the circuitry from seeing a line/neutral surge but if the whole modem rises several hundred volts from ground that difference will be made up on the telephone side that has it’s own grounding coinnection and it goes boom.
As for #8, if your EGC is bonded to the neutral and not well grounded the difference in potential between the case of your stove and the terazzo floor in your kitchen (on a slab) can be shocking. (a defect in this house when I moved in) My “zero” standing on the floor was not the same as the utility “zero”. In the computer biz I have measured 35 volts between the ground of buildings 20 feet apart. “Local” ground is very important. It is why 250 says we drive a rod (or 2) at every building. In the computer biz we also bonded all of these together with a fat wire. If a data cable went somewhere we also ran a big bond.
All I can say about # 9 is to read Article 547 and the USDA articles about bonding grids and the ground rods you should have in a dairy barn. That is all to prevent stray current. The stray current is between “ground” where the power comes from and the dirt where you are using it.
They say, if you have a stock tank warmer, you should bury a ground ring around it and bond the tank. At the door of a barn you should be burying rods out in the dirt on the apron bonded to the steel in the floor.
Perhaps I misunderstood 10 since my thoughts were the same as the statement starting with “however…”
I think he interntionally made the wording vague on a few so he could make the case for “false”. It was my first impression that he was going for all false but about #7 I saw he may be going astray.
I think you are probably right as sometimes he does that on some questions, I just posted it because I found it interesting and to be honest with you when they start on the TVSS’s I don’t follow much since I don’t deal in them much.
in # 8 Greg I think mike was talking about TRUE ground…and the elevation of ground…not sure but I think he was speaking of that.
on # 9 I am not sure and BTW mike welcomes debate on these things as the one thing I like about Mike is he will change his view honestly if presented with facts to the otherwise.
I think in # 9 in your example, we are talking about fault voltage which the EQ. Plane should help give a stable potential and solve the problem which is the reason for the EQ.Plane in this environment.
Cattle are very " wizzied" by stray voltage usually coming from the issues with the “grounded” conductor but if the installation is followed in Art 547 should not be a problem…hopefully but I am not sure it reduces stray voltage versus giving a even potential to all metal parts and earth in itself…the stray voltage still might be present.
Hey…man I love this stuff…not your everyday electrical jargon mind you. Let me upload the articles for you.
The questions were written so badly that it is hard to decide exactly what he is going for but the reality is the purpose of the grounding electrode system is to provide a local reference and reduce the chance that the dirt, concrete floor etc is at a different potential than the EGC. The ground also provides a path to bleed off common mode transients. (things that affect all conductors)
Pierre,
Not a badly worded question…it is not a true statement if their are parr. paths…please look at the inpedance example I posted.
When doing questions you MUST read the question at face value, I am not defending Mr. Holt but if you read the question at face value it is truly a false statement making his question false.
Paul, Pierre is right. Go back and read the question at face value. “Electricity always takes the path of least resistance.”
In every case where current flows through multiple paths at different resistances, it is in fact always flowing through the path of least resistance. It flows through the other paths also, but that does not make the answer false.
