Grounding, soil resistivity, ohm's law, safety, etc - Page 1
Forum section: electronics / beginners

author: eigenvictor
author_rank: Contributor
timestamp: February 24, 2015, 10:58:55 pm
content: Hi Guys, new here. I have a few related questions about the mains voltage, grounding, and voltage measurement. Have any of you ever tried measuring voltage indoors from the hot terminal to the floor, let say in a room or in a garage or in a basement?Since the floor is considered ground, the potential difference between the hot wire and the floor should theoretically be equal to the mains voltage, say, 120V AC. At the same time, soil has some resistance which depends on:1) the soil resistivity (which varies a lot)2) length, i.e. the distance from the measurement point to the "true" ground, i.e. the grounding bar from the center tap.3) cross sectional area of the "conductor"Though I am not sure what would be the "length" and the "area" in this case, since that would depend on the shortest path for the electron flow. But since the grounding bar is buried deep underground, would that mean that the "cross sectional area" in this case will be very large so the resistance between the grounding bar some 30 feet away and the measurement point indoors would be very low and if there is a current flow, the voltage drop will also be very low, so soil could be thought of as a conductor?Now back to the measurement. When I simply touch the probe to the floor, the voltmeter measures about 10V. However if I measure to a sheet of metal on the floor or just step on the grounded probe, it reads a much higher voltage, about 70 to 90V.At first I thought that may depend on how the voltmeter measures voltage. If it is an analog meter, I think it measures current and then calculates the voltage drop based on the internal resistance. But since I used a digital multimeter, I think it measures the time it takes to charge a capacitor to some reference voltage.So I am not sure, when I get a higher reading by measuring to a wide area with higher conductance than the floor material, what causes the higher reading and how is it related to the measurement method? Is it about the cross sectional area, or it is about capacitance?Could someone describe this from the physics point of view, what is going on? Surprisingly, I couldn't find any references or discussions of this topic anywhere.Thanks.

author: RJFreeman
author_rank: Regular Contributor
timestamp: February 25, 2015, 01:36:52 am
content: ok first thing to realise is that a 'Good Ground' stake can have up to 25 Ohms to ground.And then your first misunderstanding is presuming that floor is the same as the electrical ground.Resistance to ground depends on a number of factors, but of significance in explaining your observations (with the difference between the multimeter probe tip and the sheet of metal) is the surface area of your conductor in contact with the 'ground' (although as I said a floor is not usually considered an ideal ground either).your Probe has very little surface area in contact with the floor (electrically this may or may not be electrical 'ground', e.g if you have a wooden floor this would be less conductive to ground than say a metal floor), whereas a sheet of metal has a much greater surface area in contact with the floor.So you describe your measurement as measuring voltage between mains Active and 'floor' don't over think this - what is happening here is that current is flowing from Active, through your meter, through the floor, from floor floor to ground, and from there back to the Neutral-ground bond.both analogue and digital meters measure voltage drop across them, BUT THIS voltage can be calculated by ohms law (E=IR) where R is the resistance of the multimeter.The difference is that an analogue meter typically has a much lower resistance than a Digital meter - with an analogue meter typically being 20,000 Ohms /volt (so on the 100V range this would be 20,000 x 100 = 2,000,000 Ohms) whereas a Digital multimeter can have resistance of between 1,000,000 ohms and 10,000,000 ohms - so in fact a 1M Ohm input digital meter may actually load a test circuit more than a 20,000 ohm per volt analogue meter set to 100V.So lets just throw some numbers together to try an explain your observations:say your meter has an input resistance of 1M ohm.you connect it between Active (120V) and push the probe to the floor - measuring 10V10V across your meter means current flow is (10/1,000,000) roughly 10 micro amps.This also means the other 110V is now between the end of your probe on the floor and Mains 'Ground' (Actually the Neutral-ground bonding point) so we can calculate ground resistance as:(120-10)/0.00001 = 11,000,000 (11M) Ohms.Now you place a sheet of metal on the floor, this has a much greater surface area in contact with the floor (for the sake of this discussion the resistance of the metal is insignificant, so I am not even attempting to take it into account) - think of this as like going from one resistor to a bunch of resistors in parallel.now you are measuring 70v between active and the sheet of metal. using the same calculations we now have 70/1,000,000 or 70 micro amps flowing through your meter so the resistance between your sheet of metal and mains ground is now:(120-70)/0.00007 = 715,285 ohmsThis might suggest that the surface area (in contact with the ground) of your sheet of metal is 15.4 times the surface area of your probe tip, but the reality is more likely that the resistance between your 'floor' and 'mains ground' is somewhere around 700,000 ohms.

author: eigenvictor
author_rank: Contributor
timestamp: February 25, 2015, 04:04:35 am
content: Thanks a lot. Yes, I guess you are right I was overthinking it. I just need to think about measuring voltage between two points, active to "some other point", such as "floor", without assuming that it is the actual electrical ground. So the equivalent circuit would then contain some "unknown resistor" (or resistors in series and in parallel) between the voltmeter probe and the electrical ground. And the value of that resistor can vary depending on many different factors. It is just I got used to measuring voltage in a regular circuit with copper conductors, pretty straightforward. So it is kind of weird having to think of an equivalent circuit when measuring high resistivity materials and having to take the cross sectional area and other factors into account... (I always thought of the cross sectional area in the context of current or heat flow but never voltage!) And it is equally confusing to think of a voltmeter CLOSING the circuit resulting in a current flow and a voltage drop. I tend to think about the V meter as an infinite resistance... Somehow it didn't matter to me in regular low voltage circuits. And it also follows that there is no simple answer to the question what is the potential difference between the hot wire and the floor? This question came to my mind when I accidentally touched a hot wire with one hand and absolutely nothing happened. But I think it is safe to assume the worst case, i.e. you can get shocked if the "unknown resistance" is low (for example when standing on a wet floor).

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 25, 2015, 04:47:03 am
content: Reminds me of that old telephone-man's story...QuoteLong-since debunked by Snopes as fiction.

author: RJFreeman
author_rank: Regular Contributor
timestamp: February 25, 2015, 04:51:53 am
content: context of course is everything, and if you hit a higher voltage things could very well be completely different, which reminds me of a 'discussion' I had some years ago on a Newsgroup regarding Earthing electric toasters.....I made an innocent (I thought) remark, to the effect that of course, the metal case of an electric toaster was earthed (in the context of being connected to the earth pin on the mains lead) .Well, didn't I get called an idiot, and all sorts of other names for making such a such a stupid comment, since "of course toasters are not earthed"In fact the torrent of abuse confused me to the point where I tested my Electric toaster (case was connected to earth) and even took my multimeter to an appliance store and tested the toasters on display (which were also all earthed).By the time I returned to confirm all the toasters I had tested were earthed, some other folk had entered the discussion to agree that toasters should be earthed, and, well to cut a long story slightly less long it transpired that:In the USA (110V country) the body of a toaster is NOT earthed, and the reason given was so that when you hold a toaster in one hand and stick a fork in it with the otherand if the body of the electric toaster was earthed this would result in having 110V across your chest.....Well I live in Australia (240V country*) and other folk with earthed electric toasters lived in the UK (230V country) and we were always taught that sticking a fork or knife in an electric toaster that was plugged into the wall was a silly thing to do and our toasters have an earthed case so when/if the heating element breaks and comes in contact with the case this will ensure that the toaster remains safe.....and, well, I will let you draw your own conclusion regarding the two different approaches to Electrical safety.*At the time Australia WAS a 240V country, with mains variations between IRC +/-6% (225V to 254V), now Australia is a 230V country with allowed mains variations of +10/-5% or from 218.5V to 253V so further sag is allowed but about the same maximum voltage.

author: eigenvictor
author_rank: Contributor
timestamp: February 25, 2015, 05:03:10 am
content: Quote from: Richard Crowley on February 25, 2015, 04:47:03 amThat's an awesome story, fiction or not. It should be included as a problem in an EE textbook. And the question should be draw the equivalent circuit and determine if it is a true story

author: eigenvictor
author_rank: Contributor
timestamp: February 25, 2015, 05:59:14 am
content: Quote from: RJFreeman on February 25, 2015, 04:51:53 amYes, it always amazed me how people would start heated arguments about grounding even going as far as trying to prove themselves right and getting zapped as a result. Somehow the topics of high voltage, AC, grounding etc are the most misunderstood. (I am still working on that).And your story about the toaster just shook the very foundations of my fragmented knowledge of the grounding basics.You see, I am in the U.S. but I honestly never bothered to check if the toasters here are grounded but every article on grounding I read on the web mentions the ubiquitous toaster as an example of the proper grounding. So I took their word and thought that the toasters are ALWAYS grounded, period. Well, maybe those were the U.K. websites, haha. (Now I need to disassemble my toaster). I have to admit that I never tried sticking a fork into a toaster but I will observe my coworkers on Friday.I didn't know that Australia lost 10 volts. So at midnight did you guys have to add a couple turns to the secondaries of your transformers?

author: RJFreeman
author_rank: Regular Contributor
timestamp: February 25, 2015, 06:36:15 am
content: QuoteTo be fair, I have neverchecked if a US Toaster was grounded either, but rather accepted the word of ravening psychopaths on a News group, so that probably justifiably calls into question the accuracy of that claim.....QuoteWell there is evidence the power fairies (close relatives of the ARP fairies) snuck out one night and chained down all the surplus volts or something, as I later noticed this on an 11KV distribution line down the street from where I live:

author: RJFreeman
author_rank: Regular Contributor
timestamp: February 25, 2015, 06:40:33 am
content: Quoteyou couldn't get a Video for my entertainment and education???

author: eigenvictor
author_rank: Contributor
timestamp: March 02, 2015, 05:04:39 am
content: Quote from: RJFreeman on February 25, 2015, 06:40:33 amOK, I think I know what is going on. No need to use foreign objects to pull out a regular toast as it will pop out just fine unless the toaster is bad or dirty. That's why I never seen anyone do that. But my wife reminded me of another "use case" : toaster strudels! Those are heavy and won't pop so you need to fish them out with a fork! However, using both hands is problematic for a simple reason: the metal casing gets pretty hot! Perhaps that depends on a toaster model...But I got real curious about different grounding preferences for different appliances. There must be a special committee that decides on that. In our kitchen: a toaster (not grounded), a toaster oven is grounded, a waffle maker is not grounded, a (plastic) coffee machine is grounded, an electric kettle is not grounded. All of them (except for the coffee machine which is plastic but grounded) either have a metal housing or a metal "finish". Go figure.

author: RJFreeman
author_rank: Regular Contributor
timestamp: March 02, 2015, 10:29:18 am
content: Ok, the fact I had to look upwhat a "toaster strudel" was probably tells you how common that sort of conundrum is around here.....I am presuming that it is still considered better/safer practise or an act of sanity even in the US, to unplug a toaster from the mains before extracting such a comestible delicacy with the aid of an item of metallic cutlery?(for the record unplugging is always safer than simply turning it off at the wall, since while most power points would be correctly wired - with the switch in the active line, it is always possible (and safer to assume) a mistake has been made, with active and neutral transposed).Down here all the examples I can find of those appliances are Grounded. In fact there is little wriggle room for manufacturers in regards to these sorts of appliances as they are all 'prescribed' items, so there are strict rules and standards these appliances have to meet in order to be approved for sale in Australia.

author: amyk
author_rank: Super Contributor
timestamp: March 02, 2015, 02:33:57 pm
content: The toaster may be double-insulated, since some of the ones I've seen have only a 2-prong plug, and grounding would be impossible with that configuration.

author: ruffy91
author_rank: Regular Contributor
timestamp: March 02, 2015, 04:30:52 pm
content: I don't think you can double insulate a toaster. As the heating element would have to be completely insulated and there would have to be an isolation tranformer.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2015, 04:49:10 pm
content: Quote from: RJFreeman on February 25, 2015, 06:36:15 amCommon practice in the US is to fit domestic appliances with a single-insulated figure 8 power cord and a molded on two pin plug. This applies in my kitchen to the (metal) toaster, the (metal) tea kettle, the coffee maker, and around the house the vacuum cleaner and all electric table lamps and standard lamps, including those with metal housings.In fact the only item I routinely see fitted with a grounded three pin plug is a laptop power brick.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 02, 2015, 05:07:47 pm
content: The only unsafe thing is assumptions. I distinctly recall the dorm at university where all the plugs had ground nicely placed between the neutral and hot. About 55 volts each way. I have run into similar things in many places. Causes range from incompetent installation, failure over time, incompetent rework and some bizarre accidents.Grounded cases are good, double insulation is good, but neither are either necessary or sufficient.

author: SeanB
author_rank: Super Contributor
timestamp: March 02, 2015, 05:10:36 pm
content: Pretty much every toaster, other than those which were old when my father was born, I have ever taken apart have one very important safety feature. They all, even the $3 China special that might work a week or a loaf of bread ( whichever comes first), have a double pole isolator for the elements inside, so that as the lifter moves more than 3mm off the bottom both contacts disconnect the element from the mains.Though if you are stupid enough ( There must be enough to keep daytime soapies popular) to stick a fork into a glowing toaster to fish out the jammed slab of fat you have stuffed into it you probably do deserve to get a shock. A plus of having the earthed case is the fork will most likely also tough it as well so blow a fuse before the current goes through you.

author: electr_peter
author_rank: Unknown
timestamp: March 02, 2015, 06:10:07 pm
content: Existence of electricity in the kitchen creates a potentially dangerous situation no matter double insulation or grounding (precautions just limit danger to a minimal level, original danger is always there). If there was no electricity, it would be not dangerous for a user to poke around with a fork.On the other hand, I am safe to use forks in a kitchen because I do not have a toaster. I simply do not like toasted white bread. No toaster -> no danger from toasters no matter what

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2015, 06:25:14 pm
content: Any fool knows you don't use a fork to fish stuff out of a toaster. It will rip the bread apart and just make a bigger mess. What you need to use is a pair of knives slid down each side of the item to free it up and lift it out without breaking it...