ESD mat to mains??

[aqstephenson]

Hey all - setting up my first bench. at home. complete beginner. bought expensive bertech ESD mat (high temp/ conductive on bottom) and a wrist strap. it all made sense. the mat would discharge your static via the wrist strap. and now, low and behold: NOW CONNECT YOUR MAT TO MAINS... FOR SAFTEY!   

i guess i didn't do my research, and now i see that grounding the mat will protect components and possibly yourself. but here's the thing. i have no interest in grounding my mat to mains. i don't feel qualified to test mains, feel very uneasy about connecting myself to it (even if its just the ground component of it). so, two questions:

what can i expect if i simply connect my wrist strap to my mat, touch the mat before i touch components, and go from there? is stuff going to eventually blow up, me included? it's a lot of mat - 18 square feet.

assuming grounding to mains is mandatory, one of the plugs near my workbench is a GFCI, the other standard. however, neither have a center screw. both have screws on top and bottom. can i expect those screws to be connected to ground like a center screw? if so, then grounding to either top or bottom is ok? (see, i'm uncomfortable at this point even unscrewing the mains outlet cover!)    i think people teaching other people about setting up an e-bench need to be very explicit on this issue.

[Richard Crowley]

Remember that you are NOT actually connecting anything to "THE MAINS".  You are connecting to the green-wire safety ground.

If your location is new enough to have a GFCI, then it is probably safe to assume that the mains receptacles are likely wired properly with green-wire ground.

Go out and get an ordinary (3-prong) NEMA 5-15 plug and use a green-color single wire to connect the round GROUND PIN to your anti-static mat through a 1M resistor.  Leave the flat HOT and NEUTRAL pins floating (connected to nothing).  Then you don't have to fool around with the screws on the decorative plate which very well may NOT be grounded, especially if the receptacle is in a plastic box.



If your location is older construction and the GFCI was an "after-market" addition, then it may NOT be safe to assume that the power recptacles are properly grounded.  More research and details are necessary to determine whether you have a real ground connection available.

The problem with leaving the "anti-static" mat "floating" is that it will charge up to whatever highest voltage it last came into contact with, and it may take minutes, hours, or days to drain down to zero.  An "anti-static" mat that has a residual charge on it is WORSE than no mat at all.  It has the capacity of delivering a static discharge all by itself, and it gives a false sense of security.

You did not reveal any details of the location, surroundings, etc.  If you are nearby METAL water pipes, then you could connect your mat to those as a surrogate "ground". Of course, plastic pipes are useless for grounding.  And gas pluming should NEVER be used for grounding (for what is hopefully obvious reasons!)  Or, if you are near an outside wall, etc. you could possibly sink your own ground rod, etc.

[mtdoc]

Strictly speaking, connecting you ESD mat to the ground pin (the bottom semi-round hole in the US) is really "grounding to ground" not "grounding to mains" - but I get what you mean.

If that is inconvenient (takes up one outlet) or makes you uncomfortable, you could attach the ESD mat ground point to a different grounding point - for example the grounded metal frame of a piece of nearby equipment (I use the water heater which is next to my test bench).  You could probably even use grounding rod - pounded into the earth - dedicated to that purpose since it is likely to maintain the same potential as your home wiring's grounding point.

[wraper]

[Zero999]

Strictly speaking, connecting you ESD mat to the ground pin (the bottom semi-round hole in the US) is really "grounding to ground" not "grounding to mains" - but I get what you mean.

If that is inconvenient (takes up one outlet) or makes you uncomfortable, you could attach the ESD mat ground point to a different grounding point - for example the grounded metal frame of a piece of nearby equipment (I use the water heater which is next to my test bench).  You could probably even use grounding rod - pounded into the earth - dedicated to that purpose since it is likely to maintain the same potential as your home wiring's grounding point.

And if that's inconvenient and you live in a humid climate and have plaster walls, then take a strip of copper foil, <5mm wide <1m long and stick it it to the wall with a water based adhesive. It will have a resistance of between 1M to 100M with respect to earth which is good enough for ESD protection. I saw this trick where I used to work but I didn't believe it was a good ESD earth, until I tested using a multimeter and it was a few M Ohm, relative to the earth pin on a mains socket. I'm not sure if this would work as well if you had wooden walls and lived if a climate with very low humidity.

[aqstephenson]

thanks - were I to use this in one socket, and use the other socket for the power strip hosting my tools (power supply, scope etc) then everything would be grounded together which is the best practice, correct?

[aqstephenson]

I'm in USA, and my earth holes are on the bottom. were I too unscrew the case to take a look to verify the bottom hole is running to ground, what would I look for in terms of the wire? will it just be copper, with the black and white (hot and neutral) above it? or can you even see behind the box? also, I know you can buy plug testers, can these testers test exclusively for ground connection? thanks.

[Richard Crowley]

If you don't have experience with mains wiring, It is not really recommended that you poke around inside a power junction box.  And you can't really tell by looking, anyway.  But, as you say, having the anti-static mat connected to the same ground as your instruments is a very good start.

[tooki]

I have my mat connected to a chassis screw on my bench power supply, which is grounded.

[james_s]

Ground wire in the US is a bare copper wire. I agree with others here though, just use a 3 prong plug, connect your ESD mat to the green screw that goes to the round pin and leave the others unconnected. Plug it in and call it good.

[leham]

1meg ohm resistor in series is also a good idea

[james_s]

For a mat? The mat should already have a fairly high impedance, and wrist straps have it built in too. I connect mine directly, the metal cabinets of my gear are directly earthed too.

[Creative]

Hi my ESD mat is made by rosewell and it has two snaps that are grounded through a resistor.  One snap connects to a plug that is plugged in the wall; the other snap is used to ground my hand when handing parts

[Richard Crowley]

Connecting the mat directly to ground is OK. Because the mat is not highly conductive, so it will drain a static charge, but it will not provide a low-impedance path to ground in case of a fault.

The flip side of that is: connecting your wrist strap directly to ground is DANGEROUS. Because it provides a dead short from your wrist to ground. So, if you touch something the fault current will be much higher because of a short path to ground.  ALWAYS use a high resistance between ground and your wrist. Something between 1M and 10M will be safe while still providing a path to drain any static charges.

[GreyWoolfe]

Ground wire in the US is a bare copper wire. I agree with others here though, just use a 3 prong plug, connect your ESD mat to the green screw that goes to the round pin and leave the others unconnected. Plug it in and call it good.

This is what I did but I did pull out the other 2 pins.  It stays in the socket just fine.  I may go with the idea of adding a 1M  resistor at the plug.

[madires]

Connecting the mat directly to ground is OK. Because the mat is not highly conductive, so it will drain a static charge, but it will not provide a low-impedance path to ground in case of a fault.

Please measure the resistance of the back side of your mat, preferable between two snap connectors. My small mat is around 15kOhms side to side and a large mat is around 30k. The mat's top side is the dissipative layer. So I'd strongly recommend to connect the mat via a 1MOhm resistor to PE. Bonus points if you use an 1W resistor rated for 500V or more.

[aqstephenson]

very appreciated all. my wrist strap and my mat connector both have 1M resistors, and will be connected in series. i like the ground plug option, and until i can get one of those I will connect to power supply chassis screw, making sure my power supply stays is plugged in at all times. once its set up (in either of the above setups) what's a measurable way to test that the mat is grounded ie- discharge some static? I saw a youtube vid where the guy moonwalks in his socks then shocks himself on his power supply connector, then does it again but touches his grounded mat first then does not shock himself. that was an easy and effective demonstration, but I can't even get enough static from my carpet to recreate it!  that said, I still want to play it safe, set this up, and verify the mat is grounded. balloons? 

[james_s]

What's wrong with using a multimeter?

[ollihd]

Not sure if anyone covered this already, but it is good practice to actually make sure that the ground is actually present in the socket. This is not always the case (old houses for example).

[aqstephenson]

yes, and thanks. I'm not up to testing the socket, even though I have two brand new multimeters good up to 600V. I'm going to bring over an electrician.

[ollihd]

There are also these testers you just plug in to the socket and they will indicate how the wiring has been done, if ground is present, etc. Google "socket tester" and go to images. If you want to use a multimeter a really easy thing to check is: 1. Ground+common should read 0VAC, 2. Ground+live should read 110/220VAC (depending where you live).

[karoru]

i guess i didn't do my research, and now i see that grounding the mat will protect components and possibly yourself. but here's the thing. i have no interest in grounding my mat to mains. i don't feel qualified to test mains, feel very uneasy about connecting myself to it (even if its just the ground component of it). so, two questions:

In that case, are you scared to use your stove, kitchen toaster, fridge, washing machine or any other appliance in your house with metal case that you plug into the mains socket?

[rdl]

You can get outlet testers at any big box store. They're much cheaper than a visit from an electrician. It's also nice to have a non-contact voltage tester.


http://www.homedepot.com/p/Shock-Stop-Tester-CE-HGT6120/204501748


http://www.homedepot.com/p/Klein-Tools-Non-Contact-Voltage-Tester-NCVT-1SEN/100661787

[karoru]

There are also these testers you just plug in to the socket and they will indicate how the wiring has been done, if ground is present, etc. Google "socket tester" and go to images. If you want to use a multimeter a really easy thing to check is: 1. Ground+common should read 0VAC, 2. Ground+live should read 110/220VAC (depending where you live).

No it won't read 0VAC, unless you have very old 2-wire installation with neutral connected to ground in socket. There will be often few volts present , depending on wire gauge, distance from point where N and PE connects and loads that are currently applied on wiring that's being shared by that particular socket. More than half of sparkie science is about wires having non-zero resistance.

Another tip though: If ground-live and ground-neutral shows around half of your mains voltage it means someone made faux-grounded socket leaving ground connector unattached;) Unless you're in Norway for example which has both prongs "hot".

[aqstephenson]

these are the two outlets i have to work with. all outlets in the house are screwed like this, top and bottom, no center screw. when unscrewed, there is a kind of white mesh/cloth preventing visibility if the screws are connected to ground. anybody worked with these types of outlets before, or know if these screws would be connected to the ground wire? i'm planning on buying/using a 3prong plug that was mentioned before to ground my mat, but still wanted to post these pics in case someone recognizes this build.

[PointyOintment]

Maybe. If the boxes are metal, they should be. If the boxes are plastic, probably not. Check continuity/resistance between the screw and the ground contact that a plug would connect to.

[Richard Crowley]

It would be easy enough to measure the resistance between the round ground pin and the screw head(s).  Some receptacles are made so that the mounting strap is internally connected to the round-pin safety ground.  DO NOT under any conditions poke a meter probe into either of the two slots.  From your own accounts, it does not appear that you are experienced enough to go poking around like that.

BUT:
1) When you can just use a mains plug with only the ground pin connected, what is the point of fooling around with the screws?
2) Even if the screws are connected to the round pin, that doesn't ensure that it is actually grounded.

You seem to be concentrating on secondary issues, and ignoring the primary issue: Are your outlets REALLY grounded?

[aqstephenson]

tested both outlets with outlet specific testers (one gfci and one standard) and both read normal. neither registered an open ground, so they appear properly constructed. of note, however, is that when the gfci outlet tripped, it took out the normal one as well. it would be easier for me to ground to the non gfci, but with this 'series' setup is this an issue?

my mat-to-ground wire ends with that circular attachment shown in 1st pic, and it doesn't appear that there is a way to connect that to the green screw of the 5-15. is the little hole in 2nd photo showing where you were saying to connect a wire? use a screwdriver to bring the plates (gold and silver colored) together to capture the end of the wire, and then connect the other end of the wire to my mat wire's circular end? if so, whats the best way to connect a wire end to that circular mat-to-ground end? or would you recommend stripping the mat-to-ground wire and connecting directly to the 5-15? might be easiest, i'd prefer not to do that unless ultimately it was safest. very much appreciate your patience and input!

[Richard Crowley]

Safest and most straightforward is to cut off the ring-lug crimped terminal, strip the wire and terminate it to the 5-15 plug as it was designed.  The plug is constructed like that to greatly reduce the possibility of any stray strands of wire shorting from one pin to another.  If at some future point, you need the ring terminal back, it is trivial to crimp another one on the end of the wire.

[karoru]

tested both outlets with outlet specific testers (one gfci and one standard) and both read normal. neither registered an open ground, so they appear properly constructed. of note, however, is that when the gfci outlet tripped, it took out the normal one as well. it would be easier for me to ground to the non gfci, but with this 'series' setup is this an issue?

Why did the GFCI outlet tripped? If it was because you or your tester shorted live and earth then it's absolutely okay - modern circuit breakers are not selective (it's very hard to do equivalent of old "slow-blow" fuse with predictable outcome).

Go to your nearest hardware store and buy a mains plug. Connect only earth terminal with your ESD mat grounding wire, plug it into whatever socket you got at your house and you're okay. It's safe unless you can't discriminate between a connector that goes to the "mouth" of "sad face" outlet and others, in which case you got bigger problem than ESD safety:)