Grounding - How does it work? How not to die. - Page 1
Forum section: electronics / general

author: PrairieChicken
author_rank: Newbie
timestamp: November 03, 2014, 07:15:03 pm
content: Grounding is mysterious to me. I see the anti-static mat is grounded but to where is the ground lead going? How does the 1K resistor help one not to die. If the ground lead is going to the ground connection in the wall supply (mains connection) and some other person in the building has electricity dumped to ground why doesn't the killer electricity now in the ground wire go up my wrist strap and fry my wings? Why does a star configuration for ground prevent hot wings? Should I be using the ground connections on the back of my equipment to connect to ground?

author: tszaboo
author_rank: Super Contributor
timestamp: November 03, 2014, 07:35:21 pm
content: It is 1 Megaohm. If someone dumps current into the protective earth, at a few mA, the "Residual-current device" will trip (which is highly recommended, maybe even an extra just for the workbench.). So on one direction, you have 1Mohm+ you (100Koh)+the resistance of floor the other direction you have some 10 ohms for the current.

author: tggzzz
author_rank: Super Contributor
timestamp: November 03, 2014, 07:40:06 pm
content: Quote from: PrairieChicken on November 03, 2014, 07:15:03 pmWhat have you done to find out the answers to your questions? What is it about those answers you don't understand?Sure, one answer leads to another question - such is the way of life of an engineer.

author: timb
author_rank: Super Contributor
timestamp: November 03, 2014, 07:43:45 pm
content: The ground connections on an outlet are (in the US at least) tied together in the breaker box and then run outdoors to a stake driven far into the ground.If another piece of equipment in the building is shorted to the ground connection, you don't get fried because electricity always takes the least resistant path to ground. In a properly wired building, that usually means the outdoor ground and nothing else.It's also normally a 1M resistor and it's there to slow the static discharge a bit.Try this: Rub your bare feet on the carpet and touch a doorknob or other source of ground, feel that jolt and see the spark? Now do the same, except hold one end of a resistor and touch the other end to the ground point. The result? A controlled discharge.The resistor works the other direction as well, if you happen to touch a hot wire, it limits the amount of voltage/current that can go through the strap to ground, helping you not to die. (The same applies to your question about faulty equipment elsewhere in the building if your main ground point is also faulty.)Keep in mind that not-dying is *not* the primary reason for the resistor, so don't rely on it to save your bacon.Sent from my Smartphone

author: nanofrog
author_rank: Super Contributor
timestamp: November 04, 2014, 05:18:42 am
content: TheFUNDAMENTALS OF ESD(.pdf) might be of interest.

author: LabSpokane
author_rank: Super Contributor
timestamp: November 04, 2014, 06:24:18 am
content: Quote from: PrairieChicken on November 03, 2014, 07:15:03 pmA star config is to ensure proper conductivity into the earth with several connections. That is but one of several possible arrangements. For example, your local substation likely has a grid of grounding conductors beneath it to ensure an equipotential ground across the facility. One still connects to a common point for ground to your facility or equipment. The idea is for the ground to have a very low resistance so that if a fault occurs, that current does not feed back into the network and use you as the low impedance path to earth. There are special meters that are designed for testing earth grounding as well as all sorts of techniques to ensure good conductivity to earth regardless of soil conditions.If your equipment has a three prong plug and is in good, working order, and your electrical system is properly grounded, the ground lug in back is probably already grounded. But do check to be sure. I always ohm out new equipment to make sure how it is grounded. If the equipment properly uses the earth ground provided by your mains there is no need to double up. In fact, if you double ground to a different ground point, you will likely create a ground loop and create more problems for yourself.Grounding is its own science and well worth reading up on. Good question!

author: PrairieChicken
author_rank: Newbie
timestamp: November 05, 2014, 11:16:48 pm
content: Thank you for all the replies. At timb's suggestion I've been having fun rubbing my shoes on my vinyl chair mat to build up a static charge and then discharging in various ways.NANDBlog's summary of resistance in each direction was initially confusing but after reading timb and LabSpokane's response I understood. The static discharge follows the path of least resistance, namely to the earth ground.Thanks to nanofrog, the Fundamentals of ESD pdf was a good short read. I liked the table about the small voltages which cause damage to the different device types and also the table of static generation voltages for different activities.

author: tszaboo
author_rank: Super Contributor
timestamp: November 06, 2014, 07:42:47 am
content: I was actually answer the question: "some other person in the building has electricity dumped to ground why doesn't the killer electricity now in the ground wire go up my wrist strap and fry my wings?"

author: DisasterGuy
author_rank: Newbie
timestamp: November 11, 2014, 08:25:13 pm
content: Let me ask this question a slightly different way. I am pretty familiar with grounding as it applies to the RF and lightning protection arena. I am however working on setting up a new bench and scratching my head a bit as it relates to incorporating ESD grounding with common point grounding for the purposes of equalizing potential. I currently have the ESD mat on the bench, ESD mat on the floor, bench frame and wrist strap bonded together and then earthed through the building (electrical) safety ground via a 1 MOhm resistor. I want to also bond the earth connection on my service and test equipment (DMM, Scope, Frequency Counter, Power Supply, RF Service Monitor, etc) to the ESD system to provide for common potential but doing so would then negate the resistor since each piece of service equipment is also safety grounded through the mains. I am tempted to place the test equipment on an isolation transformer and allow it and the ESD system to be on a floating ground connected only through the single resistor.Thoughts?

author: LabSpokane
author_rank: Super Contributor
timestamp: November 11, 2014, 09:20:10 pm
content: When in doubt, ohm it out.If your equipment has three prong plugs, it likely is already earthed to the ground rod at your mains panel. If this is the same ground that you have used for your bench and mats, then you are doing fine ... except ....the 1 Meg Ohm....are you talking about the wrist strap only, or the entire grounding system? The 1 Meg resistor is only for your wrist strap. Most have the resistor built in.I want the metal portion of my bench grounded to earth directly - i.e. without a resistor in the path. That way, the bench can't accidentally become "hot" - the breaker goes first. The spec to trip a GFCI is 6mA, which won't even be close with 120/240 through a 1 MegOhm resistor. So, putting that resistor in your ground circuit completely defeats all circuit protection devices at 0.1-0.2 mA of fault current.I do "belt and suspenders" and have a local GFCI and all of my bench metal and panels solidly earthed back to the ground rod via a dedicated 10AWG conductor.As for the isolation transformer, that is for your device under test (DUT), *NOT* for your test equipment. If your test equipment is designed to float, it will have an isolation transformer already. Things such as oscilloscopes are designed to operate with a common earth ground. (It is done for safety, so that for example, the outer ring of a BNC connector won't be live when you plug in a probe.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 12, 2014, 12:03:04 am
content: Quote from: DisasterGuy on November 11, 2014, 08:25:13 pmYou want the earth ground of your equipment to be connected directly to earth ground, without the 1 MOhm resistor. If a wiring fault develops in your equipment, and the chassis gets connected to the live line of the mains, you don't want the chassis to become hot. If the chassis is only grounded through a 1MOhm resistor, the chassis will be connected to the live line via a very low impedance connection, while being grounded through a very high impedance connection. In this scenario, it's effectively not grounded at all, but only connected to the mains live line. If someone touches the hot chassis while touching a good solid ground, they'll be shocked.True, with the chassis of your equipment grounded through a low impedance connection, if you took a chip that had a high static charge on it, and then directly touched it to the equipment chassis, you might damage the chip. But if you've got an ESD mat, and an ESD wrist strap, and these are grounded through 1MOhm, then your body and your chips will all be at ground potential anyway, so you'll never face the scenario of having a highly charged chip touch a well-grounded chassis.A 1 MOhm resistor is useful between an anti-static mat and ground, and it's a good thing to have between your wrist strap and whatever you've connected the strap to. The resistance makes a "weak ground", strong enough to safely dissipate damaging high voltage static, but weak enough to act as a high impedance connection as far as mains are concerned.

author: Tandy
author_rank: Frequent Contributor
timestamp: November 17, 2014, 01:22:23 pm
content: To answer your question about ground electrocution.Imagine for one moment that you had no fuses or RCD and you were holding on to the end of a wire from the Earth/Ground connection and standing in a pool of water. Someone on the circuit touches the live line to the earth connection and without the circuit protection devices hundreds of amps could flow from live to Earth. You would not get electrocuted for two reasons..1. The lowest resistance path to earth is through the nice copper cable that has a connection directly into the earth. Where as your body has a relatively high resistance that is a less preferable path for the current to travel through.2. The copper earth cable is creating a link where the ground you are standing on is now at almost the same potential as the supply voltage. So for example +110V is going into the ground so both the ground you are standing on and the supply voltage are at +110V. For electricity to flow there needs to be a difference in potential.In reality you would have circuit protection that would shut off the source in anyway.The reason you need a 1Meg resistor in your ESD wrist strap is because if you were to have an Earth wire in one wrist and touched the mains live wire in the other hand there is a big potential difference between the two and your body is a much lower resistance path for the current to take than the air. So your 110V supply will go in one hand and out the other causing all sorts of damage on the way (passing through your heart). So by putting a 1Meg resistor in series with your wrist band and you then grab a live wire the amount of current that can travel through the wrist band is severely limited reducing the potential for harm to you. NOTE that this only protects you from this path to earth, if your body is in contact with the ground for example holding the grounded chassis then the wrist strap won't save you.

author: Rigby
author_rank: Super Contributor
timestamp: November 17, 2014, 01:41:57 pm
content: Quote from: mswhin63 on November 11, 2014, 11:37:04 pmThat's one thing I've never gotten my head around. Why is isolation required on CRTs?

author: Tandy
author_rank: Frequent Contributor
timestamp: November 17, 2014, 01:57:06 pm
content: Quote from: Rigby on November 17, 2014, 01:41:57 pmIt isn't required but is a prudent safety procedure when working at such high voltages.Imagine this scenario you have 100V battery and grab the +ve terminal nothing happens, no current flows even if you are standing in a pool of water in bare feet on the ground. The reason being that the - terminal is not connected anywhere. The air resistance is to high to create a circuit with the ground.Now connect the negative side of the battery to the ground using an earthing stake. If you now grab the + terminal you will create a circuit as your feet will be at the ground potential of 0V in this case and your hand at +100V. You have created a circuit.The same is true with mains electricity although we are dealing with AC. If you had an AC generator; generating 100V and held the live wire there would be no circuit. Connect the neutral line to earth and again you would create the circuit.So if you have a high voltage on a CRT TV with a common ground you could have 1000s of volts going through you if you touch the wrong thing. If however they are isolated you will not create a path for the current to travel, the same reason isolation transformers are used in bathrooms and on building sites.

author: Rigby
author_rank: Super Contributor
timestamp: November 17, 2014, 02:12:18 pm
content: I get that, but when I speak in the arcade community, who prefer CRT displays, they state something along the lines of "no isolation=death" even when you aren't working on the equipment. I've never seen a good explanation as to why.

author: Tandy
author_rank: Frequent Contributor
timestamp: November 17, 2014, 02:24:04 pm
content: Sounds like exaggeration of good practice really.Of course it is good practice to isolate high voltages as they carry higher risks than lower voltages. I expect this is just that the safety benefits of isolation have been pushed so hard in this particular case that it has become considered essential.

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 17, 2014, 02:33:20 pm
content: Quote from: Rigby on November 17, 2014, 02:12:18 pmMost CRT displays and TV sets utilized a low-cost power supply system which effectively tied one side of the incoming AC line to circuit common (chassis ground). Without an isolation transformer, connecting an earth grounded oscilloscope or other piece of gear to the chassis can result in a big bang, flash and letting the magic smoke out of things.This isn't inherent to CRT displays themselves, but the majority of them use such a non-isolated power scheme. That is why they need to be plugged into an isolation transformer for servicing.

author: Rigby
author_rank: Super Contributor
timestamp: November 17, 2014, 02:36:32 pm
content: For servicing, I get it.For regular use, I've heard that, basically, you'll cause an apocalypse if you don't use isolation transformers EVERYWHERE.Being humble, I've attributed my lack of understanding here to my own ignorance. My experience has been screaming at me that it is THEIR inexperience that places such high importance on full-time isolation.I should listen to my experience more.

author: janengelbrecht
author_rank: Regular Contributor
timestamp: November 17, 2014, 02:58:54 pm
content: when anything has live on it i use rubber glovesI once was electricuted by an old CRT television....so i learned my lesson ....rubber gloves and one hand in your pocket

author: i4004
author_rank: Regular Contributor
timestamp: November 17, 2014, 03:24:09 pm
content: Quoteyeah. the paradox of grounding is as follows:you can protect the humansOR(electricity generating) machines.not both.grounding was used when they figured out too much damage is wreaked upon machines that didn't have it. so you just lead the electricity surplus to earth, earth won't mind.but it also means most of the humans dying form electricty today die because that neutral wire is grounded at the power company's side.so this leads to conclusion that most of the deaths are caused because you need to protect the gear....heh...and also counter-intuitive to thread subject line: you die because of grounding.(although some other equipment is used to protect you on your side of the electricity delivery chain)Quotetrue, you won't. if you don't touch one point with one hand, and another with other hand, and sufficient voltage and current exists between the two.to that extent, iso. trafo doesn't protect from such cases anyway.it (paradoxically again) protects from...heh...grounding!a simple suggestion would be totreat metal that you suspect is carrying electrictiy with respect and don't touch it.if you're not sure about it, measure with dmm. measure between points that you need to touch (if you need to touch it) and ground ie earth potential.because it really takes a small amount of current to kill you if it passes thru your heart....n2ixkQuotefirst parahpraph i like, because it's past tense, 'utilised'.2nd paragraph not so much because you make it seem the live chassis ground is used these days too.it's not, although SOME parts are "hot", the're usually visibly marked as such on the pcb.that means you're working on mains volatges and should be more carefull.offcourse that is not to say isolation trafo is not a good practice...it is.but those that "respect the metal" and check prior to touching (again, IF THEY NEED TO TOUCH IT AT ALL! most of the time you don't.) probably won't die doing it.

author: SteveyG
author_rank: Unknown
timestamp: November 21, 2014, 08:03:27 am
content: Quote from: i4004 on November 17, 2014, 03:24:09 pmDon't measure with a DMM unless it has a low impedance mode. A test lamp or dedicated two probe voltage tester should always be used if you're testing for dead. DMMs have multiple mechanisms which could fool a user into thinking something was safe to touch.