Why is a high resistance of Megohms suitable for grounding static electricity?

Why is a high resistance of Megohms suitable for grounding static electricity?

Engineers are often surprised by the high levels of resistance that give an adequate ground in static electricity work. The reason is simple - static electricity charge generation is effectively a small current generator in the microamp or nanoamp range. We are usually happy to achieve limitation of voltages to a few volts. Simple consideration of Ohms law shows that for, say, 1 microA current generated (which is average level) a 1 M ohm resistance will only show 1V buildup. Increase that to 10⁹ ohms and it might start to get more problematic, showing 1kV!

You can now start to easily see why with modern insulating materials static charge build-up is common - with a 10¹²ohm material (which are quite common) only 10nA charge generation rate would give 10kV. Modern polymers can be well over 10¹³ ohm.

Of course life is not so simple in reality but it gives a good first approximation. The second important parameter is charge storage (capacitance) which with resistance forms a characteristic RC charge decay time. If this gets above about a second or so, static voltages stay around long enough for use to notice them.

So for a static dissipative floor in an ESD Protected area, 10⁹ ohms resistance or below is all that is needed to keep static voltages on chairs, trolleys and other items to a low level. Humans are more problematic as they move around and generate a higher current - someone found that 35 Mohms resistance from body to ground would keep body voltage below 100V in most cases with some margin of safety.

People start feeling shocks if their body voltage goes above about 3-4 kV. This can start to happen if the floor resistance goes much above about 10¹⁰ ohms. An average body capacitance might be of the order 100pF, so the time constant is around 1 sec. (People like me who have big feet have higher capacitance). Many modern laminates, glass, plastics, synthetic stone etc have resistance well over 10¹² ohms. So the decay time can be hundreds of seconds, and voltage reach tens of kilovolts. Any high voltages generated sty on the body for several minutes under these conditions.

There's some more information on this in my on-line article on Why static charge builds up on people

Is cardboard a problem in my EPA?

We have various types of cardboards (dissipative & insulating types) in our ESD protected manufacturing area. Is the cardboard static-generating material? Is it ok to have them in the EPA?

Any insulating material is likely to be static-generating. One problem with cardboard and paper is that they are very variable materials and their electrical properties vary with air humidity by several orders of magnitude. Under humid air conditions they could be dissipative but under dry air conditions they could be insulating and cause a problem. So, unless you have specific ESD grades, it is better to keep them out of the ESD Protected Area.

While vaccuuming sand with a 6m long and 5cm dia. pvc pipe connected to a flexible pvc hose attached to the vaccuum truck, a considerable amount of static is produced, shocking the worker holding the apparatus. How can I reduce the shocks?

It's difficult to say for sure without examining the situation, But there are two main possibilities. The main one is that the operator is getting charged up and eventually discharges to some nearby object, feeling a shock. If the operator is standing on a concrete floor, or other conductive material, and they wear "antistatic", static dissipative or conductive shoes, then their body should not charge up and the shocks can often be avoided.

However the root cause of the charging is the dust in the pipe. Dust particles impact the pipe walls and create static electricity on the pipe walls. If there are any isolated (not earthed) metal parts these will charge up and can be the source of shocks. The best way to get rid of these risks may be to replace the pipe with conductive or static dissipative pipe, and earth it. Any metal parts in the pipe system should also be earthed (grounded).

Can we use ESD chairs instead of wrist straps?

In one area in our factory they want to buy ESD chairs so that they can eliminate their wrist straps.

Using ESD chairs is not a substitute for personal grounding through wrist straps. Personal grounding requires a reliable electrical connection between the body and ground, achieving a resistance less than 35 M ohm. Generally this cannot be achieved by a person sitting on a chair for two reasons. Firstly the resistance of the chair is usually too high, and secondly the contact between the body and chair cannot be guaranteed due to the clothing worn.

In my view the use of an ESD chair is to prevent the chair itself becoming a source of electrostatic fields which could cause ESD risks, and could increase the charging on personnel sitting on the chair. If you have chairs in your EPA they should be ESD chairs in any case.

Using ESD gloves

Why should I have to use ESD gloves if we are using wrist bands? How can the ESD gloves help us? How can I explain to the people the importance of using esd gloves?

There may be various reasons for wearing ESD gloves. The wrist band grounds your body and prevents it acting as an ESD source. If you are not wearing gloves, anything you hold (e.g.tools or boards) are grounded through your body. If you wear non-ESD gloves this grounding is prevented - the tool or other item held in the hand could become charged and be an ESD source. So if you need to wear gloves when handling ESD sensitive components in an EPA, the gloves should be ESD gloves.

Why don´t I get static shock when I touch somethings like a wall or a tree or door?

Shocks are only felt if your body is charged to over about 4000V, and you touch something conductive.

If the wall or door is made of wood, concrete or some other material that has low or intermediate conductivity, any static charge on your body escapes slowly and does not cause a shock. In contrast if you touch metal, water, or another person when your body is highly charged, the charge is discharged quickly as the material is highly conductive. In this case you may feel a shock.

How does a vacuum cleaner cause static electricity?

How does using a vacuum cleaner cause static electricity?

When dust travels in the air sucked through a vacuum cleaner it impacts on the pipe walls and other internal parts. These impacts generate static charges on the particles and on the pipe walls. If these parts are made from plastics or other insulating materials they can charge up and give static shocks. Rotating parts such as carpet beaters can also charge up through rubbing action. If the suction pipe has a metal coil and is not earthed, this can charge up and give quite an energetic spark.

If there are flammable vapours (for example solvent fumes) present, these sparks could cause a fire or explosion risk. In larger vacuum cleaners (above about 1 m3) if the dust can give a flammable atmosphere, there may be a risk of fire or explosion in the dust collector.

What is high voltage for the purposes of ESD?

I have read that personnel should not be grounded when working on or around high voltages. Can you tell me what is considered high voltages for the purposes of ESD?

That may depend on your local Health and Safety regulations. However the IEC 61340-5-1 ESD prevention standard regard over 250 V.a.c. and 500 V.d.c. as high voltage. Conventional wrist straps and footwear usually have some level of protection up to those voltages afforded by the resistance built into the wrist band cord or footwear. 61340-5-1 recommends that above these voltages the minimum resistance-to-ground from the person's body should be increased, with a minimum of 750 kohm per 250 V.a.c. (500 V.d.c.). Whether you are happy to do that may depend on your safety analysis and regulations. The manufacturers of your ESD equipment may give some further information on their partcular products.

Can a supermarket shopping trolley really build up a static charge?

Can a supermarket shopping trolley really build up a static charge? I can see the insulated wheels having chance, but would have thought the very large surface area of metal would discharge this to the atmophere quickly enough so that a shock from the trolley would be very unlikely. I would have thought any shock from a trolley would have been a discharge of static built up on the person.

Yes, a trolley can charge up. Air is a very good insulator and does not allow charge on the trolley to escape easily unless quite high voltages (thousands of volts) are reached. The main paths that charge can leak away are through the tyres and floor, both of which can often be highly insulating, or through the person touching the trolley, through their shoes and the floor.

There are at least 3 ways in which a person could get a shock when they touch a trolley - either the person is charged, or the trolley is charged, or both are charged.

You may be interested to read my on-line articles

Static shocks and how to avoid them
Why static builds up on people

How to prevent ESD damage

When you work with solid-state devices,what may help prevent ESD damage?

A device outside and ESD Protected Area (EPA) must be protected by ESD shielding packaging. Devices should only be taken out of their ESD protective packaging when inside an ESD Protected Area in which electrostatic risks are controlled to an insignificant level. In manual handling and assembly, it is most important to ground the body of the person who is working with the ESD susceptible devices. There are many other precautions that might be needed. Please look at our ESD guide

Can electrostatic discharges harm people?

Can static electricity cause any physical damage to people?

Static electricity discharges do have significant current flow, which can be several amps or tens of amps for a few hundred nanoseconds. The stored energy which is released in a discharge is also important.

Small static discharges do not do damage to a person and may not even be felt. At the other extreme lightning is a static electricity discharge and can certainly kill. So between the two extremes we can expect there to be a range over which a person could be injured in some ways. Where that range lies, and what the effects are, is not well documented as far as I know.

There are standards which may be helpful, PD 6519-2:1988 probably being the most relevant in this case:

PD 6519-3:1999 (IEC 60479-3:1998).Guide to effects of current on human beings and livestock. Effects of currents passing through the body of livestock.

PD IEC/TR 60479-4:2004. Effects of current on human beings and livestock. Effects of lightning strokes on human beings and livestock.

DD IEC/TS 60479-1:2005. Effects of current on human beings and livestock. General aspects.

PD 6519-2:1988, (IEC 60479-2:1987). Guide to effects of current on human beings and livestock. Special aspects relating to human beings. (Under review)

Wanted - antistatic or conductive road surface material or coating

I have a number of client who's customers suffer shocks when they touch a car park ticket machine. In most cases this is due to the electrostatic characteristics of the road surface materials. The solution is far from easy.

I am looking for a conductive coating material that can be applied to, or instead of it, asphalt or epoxy coatings on car park approach road surfaces.

Another possibility might be to fit conductive speed bumps before the barrier - I am also looking for any manufacturer who may be interested in supplying such things.

If you can help, please get in touch via my web site.

Kelvin electrostatic generator generates static from water

I have been told of a simple apparatus that can generate an electrical potential out of drops of water. Can you give me any guidance as to where I can find details of the construction and the underlying science. I am told that it is called a storm in a teacup.

You may be thinking of the Kelvin electrostatic generator. This takes water from one container and drops it inoto two others. There is a system of loops and wires connected to the "output" cans which feeds back voltages to the input, resulting in the output cans charging to a high positive and negative voltages respectively

http://physicsnt.clemson.edu/physdemo/cat/elecstat/kelvinel.htm

http://www.physics.umd.edu/lecdem/services/demos/demosj2/j2-02.htm

If you search on google you will find other references.

Can rubbing the contacts damage a DIMM module?

A friend told me to clean the finger contacs of a DIMM memory module with an eraser :))).. I told him don't do it that way because you will get esd damage.... am I wrong????

To be honest, I don't know the answer to this. I think the main thing is that you should wear a wrist strap to ground your body while handling the module. If you are not doing this the risk is far greater that ESD from your body will damage the module.

If you are wearing a grounded wrist strap while handling the board, and working on a grounded work surface (if you are using a work surface), then I think the ESD risk from static generation as you rub the contacts is probably small.

There must be a better way to clean the contacts, but I don't know it.

Did I damage my RAM stick?

I'm recently going to upgrade my computer with a 200GB HD and 1GB RAM. I've been "messing" for months with the inside of my computer, but something happened yesterday. I opened it and took out both RAM sticks (256 + 128). Did some other stuff also, like cleaning and moving some cables, in order to make room for the HDD. When I closed my computer, opened it, it went directly from the POST screen to a black one. Then windows couldn\'t start because of a file missing. Then I couldn't start the Repair console or the Linux Live CD. It turned out to be the 128MB RAM stick. I took it out and it worked. I have some questions though.
1. Certainly it was damaged by static right?
2. Is there a chance that I ruined something else, including the RAM slot? I ran a 2h stress test and no errors came by.

We can rarely say for definite that ESD is the cause of damage unless expensive failure analysis confirms it to be so. However it certainly sounds as if it is possible, even likely. ESD can weaken semiconductor components and give later failures - so it's possible that another component could be damaged. You'll only know if it fails.


3. I\'m now buying a wrist strap from amazon now... to avoid more problems. Will this prevent any damage from ESD? I have to clip the wire to the metal chasis of the pc, unplugged?

Connect the wrist strap to the PC chassis and wear it in good contact with the skin of your wrist. For safety, the pc should not be plugged in. Don't touch and electronic parts until you are connected in this way.

I may have to work on a carpeted room... but if I absolutely have to, I'll work in some other room.

If you use the wrist strap it will remove most of the risk of damage, carpets or not. The biggest cause of ESD damage is from a charged person to a component at a different voltage. By connecting to the pc via the wrist strap you are "equipotential bonding" yourself to the pc and there will be no voltage difference between you and it - no voltage difference means no ESD.

Try to keep other sources of static such as polythene or foam packaging well away from the pc while the covers are off. Once the covers are back on, it is well protected.

Shocks in winter

As a baby i would give my mother shocks when she picked me up, and if i touched a car and someone was leaning against it they would receive a small shock too. I am now 40yrs old and have had many many static shock free years but at xmas it returned with a vengeance. The shocks i receive closing my car door cause a heavy sensation in the muscle of my arm, sometimes elbow down, sometimes into the upper arm and the pain can last for up to 8hrs... this now extends to receiving shocks from payment machines in car parks, from my allow wheels when checking tyre pressure, from light switches in my cottage, from my lamp. It sounds over dramatic and daft, i know, but it is really getting me down now and i have a certain amount of trepidation about touching some items that cannot be avoided - let alone the shocks i give from time to time to fellow colleagues. Brushing my hair is a theatrical act in itself.

Have you read my on-line article ? Most shocks of this type are due to the person becoming charged up with static electricity because of the materials of their shoes, floor material and furnishing materials. This often becomes worse in winter due to dry air conditions.

It's hard to say what can help in any particular case without investigation of the particular case. Sometimes choosing different types of shoes can help. Unfortunately the real cure may be in changing floor or furniture materials which is often not practical or expensive.

These problems often become less or disappear in spring when the weather becomes warmer and damper.

Why does static electricity increase in cold weather?

I'm hoping you could give me some information about why static electricity increases in the cold weather?

The reason static is more problematic in cold weather is because of low humidity of the air. "Relative humidity" is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. So 50% rh means the air has only half the amount of moisture it could hold. It turns out that static is promoted if rh drops below about 30%.

Cold air can hold less moisture than warm air. So cold outside air at say 0oC and 100% rh is taken into a building and heated up to make it comfortable. The relative humidity drops by one half for every 10oC rise in temperature - so if no moisture is added, the air will be 50%rh at 10oC and 25%rh if heated to 20oC. 25% is certainly dry enough to promote static electricity!.

The main factors conributing to static electricity indoors are floor covering and shoe sole materials, and furniture covers materials, and dry air conditions.

Grounding through ESD footwear and flooring

We have a very peculiar problem while using the ESD ( Static Dissipative ) Shoes. The the pass band - lower limit on the tester is 0.75M Ohm and the upper limit is 35M Ohm) some shoes pass and some fail. Are the limits set on the footwear tester ok or should we increase the upper limit?

The limits on your tester are correct for measuring resistance from the person's body to ground via the footwear and flooring, for a person who is handling ESD susceptible parts. This is recommended by IEC 61340-5-1 to be between 750 k ohm and 35 M ohm.

However the limits might not be correct for testing footwear as worn, to a metal plate electrode. Under the IEC 61340-5-1 standard the range limit for this is between 100k ohm per shoe, or 50 k ohm testing both shoes together, and 10^8 ohms. Footwear can be within this range (and so compliant with the standard) and yet fail your test.

For primary grounding of personel (in other words, the main means of grounding people who are handling ESD susceptible parts) the resistance for shoes needs to be lower than the maximum required resistance for grounding through footwear and flooring. There must be some margin for the resistance of the floor. Also, the floor resistance needs to be low enough to achieve a resistance to ground less than 35 M ohm - this will require floor resistance considerably less than 10^9 ohm allowed by the standard. In this case the best procedure in my view is to measure the resistance from person's body to ground while they stand on the floor - this is recommended to be between 35 M ohm and 750 k ohm. This measurement, and compliance with this resistance range, is not specifically required by the standard.

In summary:
1) 61340-5-1 allows footwear and floors to have higher resistance than would be needed to achieve 35 M ohm resistance from a person's body to ground
2) In order to achieve 35 M ohm resistance from body to ground you would have to choose footwear and flooring types that would reliably give low enough resistance
3) It is recommended, but not required, by 61340-5-1 that the resistance from a persons body to ground should be between 750 k ohm and 35 M ohm when grounded by footwear and flooring - if they are relying on this as the primary means of grounding while they are handling ESD susceptible components.

What is the average static shock (in volts) that you get from touching a doorknob?

I cannot answer this question directly but I can say that body voltages can vary from zero up to 35000 V and above. In general you don't feel shocks unless your body voltage is above about 3000-4000 V. Getting out of a car, about 8000-10000 V is not uncommon. In general the likelihood of achieving a certain body voltage decreases as the voltage increases, so voltages of a few hundred volts are "normal", and a few thousand volts not uncommon, but tens of thousands of volts are more unusual.

How much of a charge can a human body build up from static electricity?

This question is a bit like "How long is a piece of string?" The answer depends on various factors and circumstances. We tend to think in terms of voltage rather than charge, because as the saying goes, "It's volts that jolts" - in other words voltage, rather than charge, causes us to feel a shock. About 3000-4000 Volts on the body will cause us to feel a shock when we touch some object. A shock felt when getting out of the car may be caused by a body voltage of about 10000 V. Body voltages up to 35000 V have been reported, but I see no reason that higher voltages might not be possible in exceptional circumstances. Body voltages up to 10 kV (10000 V) are commonly responsible for the shocks we feel in daily life, and are largely caused by insulating flooring, footwear and furnishings. For more information on this see my article on Static shocks and how to avoid them.