Twenty Log
Well-known member
Sandy soil is interesting for the measurement...
Simple AC Mains switch (power switch) question.
- Thread starter jwhmca
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Twenty Log said:
That is exactly my criticism of the article I saw, and resistance between ground rods is not very well characterized, or very low.
The correct connection as per speedskater looks safe and reliable, while it may still hum for poorly designed gear (pin 1 problem).
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FWIW a few months ago while dealing with a broken water main in my yard and enough standing water that I needed to pump out, I got to test my ground rod resistance when a killer miswired extension cord, put hot onto my sump pump ground, and incidentally through me. Luckily I was not standing down in the water filled hole, so I was able to release, but it didn't take out the mains fuse like a low impedance path to neutral would have. Surely enough current to take out my internal fuse, if it reached my primary circuit.
JR
Speedskater
Well-known member
The best way is a 3 point fall of potential tester. They cost about $2500 and take a technician almost 1/2 hour. Howerver there may be a lower budget meter available.Kingston said:Speedskater said:
How exactly is this measured by the way? Stick an alligator clip to the nearest bedrock?
http://support.fluke.com/find-sales/Download/Asset/2633834_6115_ENG_A_W.PDF
Speedskater
Well-known member
This what Mike Sokol wrote on the subject, on another audio forum:
I agree that a single ground rod can have way more than the code required max of 25 ohms, but modern "Fall of Potential" ground resistance testers have come down in price a lot over the last few years. I have a B&K Model 309 which list for $195 and takes about 10 minutes to do an earth resistance test. Plus it uses a 400 Hz test tone and a high-pass filter to ignore any 60 Hz currents flowing in the ground rod already. So you don't have to disconnect the ground rod from the building to do a test like in the old days.
See Model 309, Digital Earth Resistance Meter - B&K Precision
Here's a pretty good tutorial from Fluke about the Fall-of-Potential test procedure. Fall-of-potential test method
Most electricians will still just drive a second ground rod, but I feel that ground rods should be tested for earth resistance after any lightning strike on the building. I saw one last year that melted the sandy soil around the ground rod into a glassy popsicle with very high resistance. I talked to my local inspector and power company engineer, who both confirmed there was no requirement for testing ground rod resistance after a lightning strike.
Mike Sokol
[email protected]
www.noshockzone.org
I agree that a single ground rod can have way more than the code required max of 25 ohms, but modern "Fall of Potential" ground resistance testers have come down in price a lot over the last few years. I have a B&K Model 309 which list for $195 and takes about 10 minutes to do an earth resistance test. Plus it uses a 400 Hz test tone and a high-pass filter to ignore any 60 Hz currents flowing in the ground rod already. So you don't have to disconnect the ground rod from the building to do a test like in the old days.
See Model 309, Digital Earth Resistance Meter - B&K Precision
Here's a pretty good tutorial from Fluke about the Fall-of-Potential test procedure. Fall-of-potential test method
Most electricians will still just drive a second ground rod, but I feel that ground rods should be tested for earth resistance after any lightning strike on the building. I saw one last year that melted the sandy soil around the ground rod into a glassy popsicle with very high resistance. I talked to my local inspector and power company engineer, who both confirmed there was no requirement for testing ground rod resistance after a lightning strike.
Mike Sokol
[email protected]
www.noshockzone.org
Twenty Log
Well-known member
I wonder if that is my issue; the glass Popsicle... We have near-by lightning strikes all the time here at 1,100 feet above mean sea level.... (yes, the sea is mean!)....
Have had garage door openers die, POTS to Ethernet VOIP boxes die... etc... I unplug my PC, Ethernet, and phone during storms despite the money guarantee on my 2kW UPS....
Some barns around here have massive copper braid going into the earth; designed from the 1800s... the lightning rods apparently "repel" lightning by voltage gradient on the sharp points... Have heard of copper grids near/underneath radio towers... AM is different than FM of course... AM is an isolated tower typically because the tower is the antenna... jus sayin....
Have had garage door openers die, POTS to Ethernet VOIP boxes die... etc... I unplug my PC, Ethernet, and phone during storms despite the money guarantee on my 2kW UPS....
Some barns around here have massive copper braid going into the earth; designed from the 1800s... the lightning rods apparently "repel" lightning by voltage gradient on the sharp points... Have heard of copper grids near/underneath radio towers... AM is different than FM of course... AM is an isolated tower typically because the tower is the antenna... jus sayin....
Speedskater
Well-known member
One half of the AM broadcast radio transmitting antenna is the ground. Good AM radio coverage requires a good connection to Mother Earth.
Cool Facts about Lightning
#662 The EMC Journal September 2011
Lightning is essentially a gigantic electrical spark that results from billions of volts of natural static electricity. Lightning is usually associated with thunderstorms and rain. ..............
Lightning Safety Facts from NOAA:
• Each second there are 50 to 100 Cloud-to-Ground Lightning Strikes to the Earth world-wide.
• Most lightning strikes average 2 to 3 miles long and carry a current of 10,000 Amps at 100 million Volts.
• A “Positive Giant” is a lightning strike that hits the ground up to 20 miles away from the storm. Because it seems to strike from a clear sky it is known as “A Bolt from the Blue”. These “Positive Giant” flashes strike between the storm’s top “anvil” and the Earth and carry several times the destructive energy of a “regular” lightning strike.
• Thunder can only be heard about 12 miles away under good quiet outdoor conditions.
• Daytime lightning is difficult or impossible to see under local sun and/ or hazy conditions. Night-time “heat lightning” can be seen up to 100 miles away (depending on “seeing” conditions).
• “Lightning Crawlers” or “Spider Lightning” can travel over 35 miles as it “crawls” across the bottoms or through squall line “frontal” clouds. This rare type of lightning is very beautiful as it zaps from “horizon-to horizon”. However it can turn deadly if it happens to strike the ground at the end of its super long path! {Lightning Crawlers from The Blue!}
• Radar has detected Lightning “Crawlers” traveling at high altitudes (15,000 ft to 20,000 ft) as they zap from cloud-to-cloud.
• Lightning “Crawlers” over seventy five (75) miles long have been observed by Radar!
• The temperature of a typical lightning bolt is 5x hotter than the surface of the Sun!
• How big around is a typical lightning bolt? Answer: About the size of a Quarter to Half-Dollar! Lightning looks so much wider than it really is just because its light is so bright!
• Lightning Strikes create powerful radio waves in the frequency range of 3 kHz (audio, VLF) through 10 MHz (shortwave radio). The VLF (3 kHz to 30 MHz) “lightning signatures” can travel around the world, allowing monitoring of world-wide lightning. The shortwave “lightning signatures can travel half-way around the Earth (the night-time side of the Earth). The best region to listen for distant shortwave lightning signatures is from 2 MHz through 7 MHz. After 3 AM local time you can listen to 3 MHz and hear the beautiful dispersion ringing of the static as it bounces back and- forth between the earth and ionosphere. It can at times sound like hundreds of tiny bells ringing at once!
.....................
(Taken from “Cool Facts” by Nextek, www.nexteklightning.com/enews/408/ coolfacts.htm 23 May 2008)
Cool Facts about Lightning
#662 The EMC Journal September 2011
Lightning is essentially a gigantic electrical spark that results from billions of volts of natural static electricity. Lightning is usually associated with thunderstorms and rain. ..............
Lightning Safety Facts from NOAA:
• Each second there are 50 to 100 Cloud-to-Ground Lightning Strikes to the Earth world-wide.
• Most lightning strikes average 2 to 3 miles long and carry a current of 10,000 Amps at 100 million Volts.
• A “Positive Giant” is a lightning strike that hits the ground up to 20 miles away from the storm. Because it seems to strike from a clear sky it is known as “A Bolt from the Blue”. These “Positive Giant” flashes strike between the storm’s top “anvil” and the Earth and carry several times the destructive energy of a “regular” lightning strike.
• Thunder can only be heard about 12 miles away under good quiet outdoor conditions.
• Daytime lightning is difficult or impossible to see under local sun and/ or hazy conditions. Night-time “heat lightning” can be seen up to 100 miles away (depending on “seeing” conditions).
• “Lightning Crawlers” or “Spider Lightning” can travel over 35 miles as it “crawls” across the bottoms or through squall line “frontal” clouds. This rare type of lightning is very beautiful as it zaps from “horizon-to horizon”. However it can turn deadly if it happens to strike the ground at the end of its super long path! {Lightning Crawlers from The Blue!}
• Radar has detected Lightning “Crawlers” traveling at high altitudes (15,000 ft to 20,000 ft) as they zap from cloud-to-cloud.
• Lightning “Crawlers” over seventy five (75) miles long have been observed by Radar!
• The temperature of a typical lightning bolt is 5x hotter than the surface of the Sun!
• How big around is a typical lightning bolt? Answer: About the size of a Quarter to Half-Dollar! Lightning looks so much wider than it really is just because its light is so bright!
• Lightning Strikes create powerful radio waves in the frequency range of 3 kHz (audio, VLF) through 10 MHz (shortwave radio). The VLF (3 kHz to 30 MHz) “lightning signatures” can travel around the world, allowing monitoring of world-wide lightning. The shortwave “lightning signatures can travel half-way around the Earth (the night-time side of the Earth). The best region to listen for distant shortwave lightning signatures is from 2 MHz through 7 MHz. After 3 AM local time you can listen to 3 MHz and hear the beautiful dispersion ringing of the static as it bounces back and- forth between the earth and ionosphere. It can at times sound like hundreds of tiny bells ringing at once!
.....................
(Taken from “Cool Facts” by Nextek, www.nexteklightning.com/enews/408/ coolfacts.htm 23 May 2008)
Like Twenty, I've measured 20V AC between neutral & earth in the UK. Just checked my house (in Oz) and its less than 0.2V
The MIX article is my understanding of UK/Oz practice and I'm glad it seems to be US too.
The utility companies don't like you to connect neutral to a local earth cos it upsets the balance of their 3 phase distribution. If you want to do that, the correct way is to have an isolating transformer as recommended by Kelm. This is what happens at power substations.
The main circuit breakers or any fuse in gear will never protect you from electrocution. At best, they prevent (??) the gear from bursting into flames. Even 50mA is enough to kill you.
To guard (?) against electrocution, you need an Earth Leakage Circuit Breaker. ELCB
In Oz, this is highly recommended for domestic wiring and is mandatory equipment for tradesmen working with power tools. WHS police this quite rigorously.
The correct way to do this is to sense the difference between Live & Neutral and trigger (both lines) if this is greater than some small figure (I think 50mA but don't quote me) If there is a difference, it must be going somewhere else .. maybe through you.
You can't connect Neutral to Earth after such a device as it will trigger immediately.
There used to be some cheap devices that sensed current in Earth but I think these have all disappeared in Oz. They only work if the leakage is going to the earthed chassis.
Some of the BS/EU standards for Earth Chassis were designed to blow the mains fuse if there was big short to chassis but this didn't protect from small currents that were enough to kill you.
The MIX article is my understanding of UK/Oz practice and I'm glad it seems to be US too.
The utility companies don't like you to connect neutral to a local earth cos it upsets the balance of their 3 phase distribution. If you want to do that, the correct way is to have an isolating transformer as recommended by Kelm. This is what happens at power substations.
The main circuit breakers or any fuse in gear will never protect you from electrocution. At best, they prevent (??) the gear from bursting into flames. Even 50mA is enough to kill you.
To guard (?) against electrocution, you need an Earth Leakage Circuit Breaker. ELCB
In Oz, this is highly recommended for domestic wiring and is mandatory equipment for tradesmen working with power tools. WHS police this quite rigorously.
The correct way to do this is to sense the difference between Live & Neutral and trigger (both lines) if this is greater than some small figure (I think 50mA but don't quote me) If there is a difference, it must be going somewhere else .. maybe through you.
You can't connect Neutral to Earth after such a device as it will trigger immediately.
There used to be some cheap devices that sensed current in Earth but I think these have all disappeared in Oz. They only work if the leakage is going to the earthed chassis.
Some of the BS/EU standards for Earth Chassis were designed to blow the mains fuse if there was big short to chassis but this didn't protect from small currents that were enough to kill you.
> I would suggest a third fuse for the round/grounding pin
There is never ANY excuse for fusing (even breaking) Ground.
There's always another route to ground, through the user to pipes or concrete. So we "ALWAYS" have some better path, never broken. (Pay no attention to my collection of 3/2-pin ground lifters-- I know when I'm cheating, I don't want a fuse to cheat me.)
> so-called "Edison socket", a two pronged device with two equal sized blades on the plug
Edison is responsible for the lamp-socket/base where one side of the line is easy to touch.
The US 2-prong plug is not due to him, and has a long complicated and startlingly unregulated early history. The forerunner of NEC started to standardize it in ~~1912, and decided to let the market decide. UL had some part in banning woefully under-insulated designs, but as we know the plug which became common is too easy to touch both prongs while partly inserted.
> for AC power distro within a building (ie, the wiring to/from the breaker box to wall outlets, etc) requires a "solid neutral" per the National Electric Code.
NEC *stops* at the plug.
FWIW: several places in NEC, white may be switched *IF* ganged with black.
What happens inside the appliance is between you and maybe UL.
NEC can mandate a black/white distinction because in-wall wiring is (nominally) done by trained electricians.
However this is a dream.
In my last kitchen, 60% of outlets had black/white reversed. (Hardly mattered since all the green wires were cut-off.)
Changing an appliance's fuse while plugged-in is like changing a car's fan-belt while the engine is running; except unlike the car, you can't tell when electricity is "running".
*UN-PLUG* before you put your fingers inside, or even near removable live parts (fuseholders). This is what NEC calls (in other context) "disconnect within sight". Even my plumber demanded a switch ON/NEAR the oil-burner for safe servicing. When gear has a plug/socket, this is acceptable as a service disconnect. You got it; USE it.
> inside a LOT of UL listed electronics gear that had only the hot lead switched, and the neutral "solid".
Hot-chassis "AC/DC" radios generally break the return path, the line which is more-groundy when hum is minimum.
> measure the voltage between earth and neutral at your own house.
2.75V from fusebox common to an isolated dirt-rod with a typical small complement of 120V loads on a 240-120 line. (This is an extreme though not abnormal case. The line is 400 feet from the street, the service ground is 50 feet from the house.)
> If you want some fun, put an ammeter
In my case, I would not expect a lot more than 1 Amp. 25m ground-rings are uncommon here; standard is two 8-foot rods. The resistance is rarely under 100 ohms. In addition, I can't get 8 feet down, not even the 5.6' needed for the 45 degree alternate, because it's all rock under the surface.
There is never ANY excuse for fusing (even breaking) Ground.
There's always another route to ground, through the user to pipes or concrete. So we "ALWAYS" have some better path, never broken. (Pay no attention to my collection of 3/2-pin ground lifters-- I know when I'm cheating, I don't want a fuse to cheat me.)
> so-called "Edison socket", a two pronged device with two equal sized blades on the plug
Edison is responsible for the lamp-socket/base where one side of the line is easy to touch.
The US 2-prong plug is not due to him, and has a long complicated and startlingly unregulated early history. The forerunner of NEC started to standardize it in ~~1912, and decided to let the market decide. UL had some part in banning woefully under-insulated designs, but as we know the plug which became common is too easy to touch both prongs while partly inserted.
> for AC power distro within a building (ie, the wiring to/from the breaker box to wall outlets, etc) requires a "solid neutral" per the National Electric Code.
NEC *stops* at the plug.
FWIW: several places in NEC, white may be switched *IF* ganged with black.
What happens inside the appliance is between you and maybe UL.
NEC can mandate a black/white distinction because in-wall wiring is (nominally) done by trained electricians.
However this is a dream.
In my last kitchen, 60% of outlets had black/white reversed. (Hardly mattered since all the green wires were cut-off.)
Changing an appliance's fuse while plugged-in is like changing a car's fan-belt while the engine is running; except unlike the car, you can't tell when electricity is "running".
*UN-PLUG* before you put your fingers inside, or even near removable live parts (fuseholders). This is what NEC calls (in other context) "disconnect within sight". Even my plumber demanded a switch ON/NEAR the oil-burner for safe servicing. When gear has a plug/socket, this is acceptable as a service disconnect. You got it; USE it.
> inside a LOT of UL listed electronics gear that had only the hot lead switched, and the neutral "solid".
Hot-chassis "AC/DC" radios generally break the return path, the line which is more-groundy when hum is minimum.
> measure the voltage between earth and neutral at your own house.
2.75V from fusebox common to an isolated dirt-rod with a typical small complement of 120V loads on a 240-120 line. (This is an extreme though not abnormal case. The line is 400 feet from the street, the service ground is 50 feet from the house.)
> If you want some fun, put an ammeter
In my case, I would not expect a lot more than 1 Amp. 25m ground-rings are uncommon here; standard is two 8-foot rods. The resistance is rarely under 100 ohms. In addition, I can't get 8 feet down, not even the 5.6' needed for the 45 degree alternate, because it's all rock under the surface.
> main circuit breakers or any fuse in gear will never protect you from electrocution
IF the chassis is properly tied to safety "ground" (and you don't open the chassis), if the hot-wire touches chassis the fuse will blow. The TOTAL system makes electrocution much less likely.
Yes, if you stick your finger inside, you will be a toasted cinder long before a 15A fuse cares. (You probably can't get 15A flow through typical meat with 120-240V supply.)
> utility companies don't like you to connect neutral to a local earth cos it upsets the balance of their 3 phase distribution
Generally, utility power MUST be grounded. Overhead service is exposed to induction or outright strikes from lightning, also crosses from distribution lines on the same pole. Even in underground service there are lightning hits from customer premises.
The best thing to do with most of this crap, especially lightning, is to dump it to dirt as soon and as redundantly as possible.
Overhead lines usually have a dirt-rod every other pole, and dirt-rod at or near every customer.
In grounding service lines, you normally pick one wire and ground it. It is not economic to, as can be done on microphone lines, have a transformer-tap used only for grounding.
For US-style 240/120V power, the CT is grounded.
3-phase is more complicated. If the service is Y (star) connected, you ground the center of the star. If each leg is 120V, you can pull 208V from two legs. I think much European service is 230V from center and 400V available for large loads. 3-Phase can also be Delta, which saves a wire, but there's no obvious place to ground. Leave that to specialists.
OTOH there are the problems of broken-neutral and "objectionable ground current". There's no easy answer for all cases. Different jurisdictions (and different eras) have different rules where utility Neutral and Local Dirt Rod may be connected. In the US, at the Main Service-- in simple cases you just land all the white and green/bare wires on the same bus. But that's once per service (generally per meter)-- my garage should have a ground-bus isolated from white and a green run back to the main service. However that's expensive and in the 1980s it was omitted. Later it was required, yet the 2006 NEC seems to allow it, and the 2012 NEC apparently bans it again.
So yes: White and Green connect, but "where" is a question for local jurisdiction. And when "where" is not "very close", then any load current will induce voltage-drop in white and 0.2V-3V difference from white to green.
The white-green drop should be limited by total voltage drop to load. NEC suggests (does not generally mandate) 2% max drop from service to load. Half that would be in White. So anything over 1.2V (on 120V circuit) suggests A Problem.
Early in my career I learned what happens when white gets loose. To simplify: 115V loads on both sides of 230V/115V service, the CT (white) burned at the fusebox. My powered-mixer and a coffee-pot split the 230V. The coffee-pot was not getting hot, then my mixer *blew up*. It was getting perhaps 160V (the pot got the other 70V which explains why it was lame).
======================================
Anyway, back to 2-pole versus 1-pole power switch:
If building for yourself, do whatever you like. It's an on/off switch, not some magic gizmo.
If building for production, hire an expert; switching is the least of your headaches.
IF the chassis is properly tied to safety "ground" (and you don't open the chassis), if the hot-wire touches chassis the fuse will blow. The TOTAL system makes electrocution much less likely.
Yes, if you stick your finger inside, you will be a toasted cinder long before a 15A fuse cares. (You probably can't get 15A flow through typical meat with 120-240V supply.)
> utility companies don't like you to connect neutral to a local earth cos it upsets the balance of their 3 phase distribution
Generally, utility power MUST be grounded. Overhead service is exposed to induction or outright strikes from lightning, also crosses from distribution lines on the same pole. Even in underground service there are lightning hits from customer premises.
The best thing to do with most of this crap, especially lightning, is to dump it to dirt as soon and as redundantly as possible.
Overhead lines usually have a dirt-rod every other pole, and dirt-rod at or near every customer.
In grounding service lines, you normally pick one wire and ground it. It is not economic to, as can be done on microphone lines, have a transformer-tap used only for grounding.
For US-style 240/120V power, the CT is grounded.
3-phase is more complicated. If the service is Y (star) connected, you ground the center of the star. If each leg is 120V, you can pull 208V from two legs. I think much European service is 230V from center and 400V available for large loads. 3-Phase can also be Delta, which saves a wire, but there's no obvious place to ground. Leave that to specialists.
OTOH there are the problems of broken-neutral and "objectionable ground current". There's no easy answer for all cases. Different jurisdictions (and different eras) have different rules where utility Neutral and Local Dirt Rod may be connected. In the US, at the Main Service-- in simple cases you just land all the white and green/bare wires on the same bus. But that's once per service (generally per meter)-- my garage should have a ground-bus isolated from white and a green run back to the main service. However that's expensive and in the 1980s it was omitted. Later it was required, yet the 2006 NEC seems to allow it, and the 2012 NEC apparently bans it again.
So yes: White and Green connect, but "where" is a question for local jurisdiction. And when "where" is not "very close", then any load current will induce voltage-drop in white and 0.2V-3V difference from white to green.
The white-green drop should be limited by total voltage drop to load. NEC suggests (does not generally mandate) 2% max drop from service to load. Half that would be in White. So anything over 1.2V (on 120V circuit) suggests A Problem.
Early in my career I learned what happens when white gets loose. To simplify: 115V loads on both sides of 230V/115V service, the CT (white) burned at the fusebox. My powered-mixer and a coffee-pot split the 230V. The coffee-pot was not getting hot, then my mixer *blew up*. It was getting perhaps 160V (the pot got the other 70V which explains why it was lame).
======================================
Anyway, back to 2-pole versus 1-pole power switch:
If building for yourself, do whatever you like. It's an on/off switch, not some magic gizmo.
If building for production, hire an expert; switching is the least of your headaches.
