31 Aug

Common Sources of ESD Damage You May Have Overlooked

Common Sources of ESD Damage

As we’ve talked about previously, often the sources of static electricity go completely unnoticed. This is because we humans can’t even detect a static charge until it gets up to about 3,000 volts. The problem is that sensitive electronics can be damaged by a charge down in the 100-volt range.

If you’re in the business of manufacturing printed circuit boards or other electronics, you already know how serious a problem electrostatic discharge in the work environment can be. But for the rest of us, we may not recognize just how big of a problem ESD can be, and how much it is affecting our critical devices.

Once you become aware, then you’ll want to start addressing the problem by setting up a static-free zone, or an ESD protected area (EPA).

One of the first and biggest areas that you’ll probably begin to address in more critical environments is the flooring itself. Putting in one of the many types of ESD floors will help carry static charges away to ground right through the floor, rather than allowing it to end up damaging your sensitive equipment.

Another method of controlling ESD involves using specially designed ESD shoes that will allow static charges to dissipate.

Using ESD flooring and ESD shoes addresses a major source of static discharge buildup.

But where else do static charges come from?

Believe it or not, the typical work environment is loaded with sources of static electricity. Here are some examples you may not have thought about:

  • a typical “scotch tape” dispenser:  the tape itself builds up a static charge coming off the roll (which can be up to a few thousand volts!)and the dispenser is usually made up of insulating materials that can build up a charge
  • plastic baggies (for components) can have a few hundred to over 1,000 volts of static charge built up on them with simple handling.
  • plastic water bottles (or any plastic bottle containing liquid) can build up thousands of static volts of electricity
  • any other products made out of insulating, rather than conductive materials (plastic cups, bins, organizers, etc.)

As you can see, often the sources of static electricity are so close at hand, and so seemingly benign, that they’re very easy to miss.

What Can You Do About It?

Bringing static electricity under control at a given workstation can be accomplished using a few simple tools:

  • ESD work mats will carry the charge away from any conductive materials you set on them
  • ESD grounding straps or wrist straps allow the wearer to stay grounded at all times, preventing the buildup of a static charge on the human body
  • The use of monitors and meters will allow for quick and easy measurement of static buildup, which will help identify and eliminate sources of ESD before they become a problem

Need help setting up your ESD protected areas? Give us a call today, and one of our ESD control experts will be happy to provide all the help you may need!

24 Aug

What Causes Static Electricity?

What Causes Static Electricity?

Static electricity, the phenomenon responsible for electrostatic discharge, is responsible for everything from simple, often harmless effects like a child’s hair standing up on a playground slide to much more dangerous incidents like fires at the gas pump and even the Hindenburg disaster.

We’ve all experienced the shock of touching a doorknob—or perhaps even another person—after walking across carpeted floors, but perhaps few of us really understand what “static electricity” really is… and fewer understand what causes it.

What is Static Electricity?

“Static” electricity is latent electrification of an object. Unlike “current” electricity, which flows through substances, a “static” buildup involves acquiring an electrical charge which remains until it is discharged.

In the simplest of terms, “static electricity” refers to a positive or negative electrical charge. An object, or indeed a person, can build up excess electrons (a “negative” charge) or can lose electrons (a “positive” charge).

More specifically, the surface of an object is where this electrical charge resides.

How Does Static Electricity Build Up?

The most common means of a person or an object acquiring an electrical charge is a process known as “triboelectric” charging. Triboelectric is derived from the Greek, tribo- (‘rub’) and ēlektron (‘amber’), which is one of the earlier substances known to have been responsible for the “triboelectric” effect, and is thus the origin of the terms, “electron” and “electricity.”

As the Greek origins of the word imply, triboelectricity is often the result of rubbing, or friction. However, friction is not required to produce the charging effect.

In reality, simple contact followed by separation can produce an electrical charge in many substances. Amber and wool, for example, will produce triboelectricity when brought into contact with one another and then separated.

Other examples of substances known to produce the triboelectric effect:

  • glass and silk
  • rubber and fur
  • hair/skin and certain plastics & vinyls

All that is needed for the build-up of an electrical charge is contact, followed by separation. In this simple example, a woman starts fueling her vehicle, then gets inside. Presumably, all it took was contact between her shoes and the vehicle’s carpet, or perhaps her skin and the fabric on the seat, to produce a buildup of static electricity:

What Causes Electrostatic Discharge?

As you can see in the video above, the woman has picked up a static charge by contact with the interior of her vehicle.

A “discharge” occurs when the object (or person, in this case) which has built up a negative or positive surface charge comes into close proximity or contact with another object that has a different charge. In the video above, the woman comes into contact with metal (either in the pump or the vehicle itself) and a small spark (or electrical arc) is produced.

Given her close proximity to the fumes of the gasoline, the results are predictable.

A similar discharge occurs when you walk across a carpeted floor in shoes in rubber-soled shoes and then touch a doorknob or other metal.

The real problem for businesses comes in when that electrostatic discharge occurs near sensitive equipment. This type of discharge can produce immediate, catastrophic failure, or even minor damage that doesn’t produce failure for a long time to come.

In future blog posts, we’ll talk about various aspects of electrostatic discharge, and why prevention is so important.

In the mean time, remember that electrostatic discharge is the reason that you always want to place your gas can on the ground before filling it up. Your truck bed or carpeted trunk is a recipe for disaster!

 

15 Aug

Taming the Electrostatic Discharge Beast

Taming the Electrostatic Discharge Beast

Electrostatic discharge, the rapid (and often unforeseen) flow of electricity that results when two electrically charged objects come into contact with one another, is an incredibly common phenomenon that wreaks havoc to the tune of billions of dollars each year in costs.

Whether your business produces ESD-sensitive equipment, or uses ESD-sensitive equipment in your operation, chances are you are dealing with significant costs associated with ESD—whether you’re aware of it or not.

Most electronic equipment has some degree or another of sensitivity to electrostatic discharge, and of course it’s hard to imagine many situations in which we do not come into contact with electronics on a daily basis. In fact, we’re often not aware of the most basic scenarios in which we generate an electrostatic discharge in everyday life, including plugging a charged USB cable into a laptop or even touching a cellphone when we carry a charge.

The manufacturers of the most ubiquitous devices have been working for decades to reduce vulnerability to ESD, and yet the costs associated with it continue to skyrocket.

The Costs of ESD

Sometimes the costs associated with Electrostatic Discharge (ESD) are obvious. For manufacturers who produce integrated circuits (ICs) or any products that contain them, product failures are perhaps the most significant source of ESD costs.

Cisco estimates that the costs of ESD damage are:

  • 1x the cost of assembly and labor if found during assembly,
  • 10x the cost of assembly and labor if found during testing, and
  • 100x the cost of assembly and labor if found at the customer site.

The problem is that ESD damage can be hard to detect. Even with more than $5 billion in costs associated with ESD each year, it is likely that the majority of true costs are not properly attributed to ESD.

In the event of catastrophic or complete failure of a given device subsequent to a known ESD event, responsibility for the failure is easy to discover. The problem is that human beings cannot detect the familiar ESD “arc” until approximately 3000 volts. The threshold for damage to sensitive circuits can be as low as 100 volts, meaning that the vast majority of electrostatic discharge events may be going unnoticed.

Even more insidious, however, is the latent damage that can be caused by electrostatic discharge. Often the damage produced by ESD isn’t discoverable until much later, meaning that the true cause of a product or system failure may not be known, and therefore cannot be accounted for in any meaningful way.

Killing ESD at the Source

Often, companies invest in reducing the effects of ESD rather than trying to stop the causes of it. Even so, return on investment can be measurably very high.

To truly gain a significant ROI, however, the investment must be made in preventing ESD by eliminating the causes of it, not just mitigating its effects.

In manufacturing or handling sensitive electronic equipment, creating an Electrostatic Discharge Protected Area is a great starting point, with all manner of ESD protection, including:

  • ESD flooring
  • Grounding devices like wrist straps
  • Grounded static dissipative work surfaces
  • ESD clothing and shoes
  • ESD packaging
  • Signage

Whether you’re designing manufacturing facilities, operating a production line, or interacting with electronic circuits at any level, we’re here to assist you with any and all of the questions you deal with. As experts in the prevention and elimination of ESD for more than 15 years, we have the expertise, know-how, and depth of products to help you solve any of your ESD challenges.

In the coming weeks and months, we’re excited to be able to begin to share our expertise with you via this blog, and we look forward to interacting with you here!

07 Jul

Can we make our own ESD tools?

Q: Can we make our own ESD Tools?

A: Perhaps.  Since common hand tools can be at risk of creating an ESD event via CDM, what do we do about it?  I was able to treat the screw driver in my last installation with our ESD anti-stat chemical Shock Stop.  We also have Ultra Spray which lasts for a good deal longer than gimmicky products on the market, which are probably based on a fabric softener.

Our industrial products mentioned above will last longer, but need to be re-applied depending on usage.

I was brainstorming and came up with a more permanent solution to the ESD Screw Driver.  Pretty effective, huh?

1-measuring-resistance-of-non-esd-screw-driver3

1-measuring-resistance-of-non-esd-screw-driver3

2-esd-chemical-shock-stop

2-esd-chemical-shock-stop

3-treating-non-esd-screw-driver-with-anti-stat-shock-stop

3-treating-non-esd-screw-driver-with-anti-stat-shock-stop

4-remeasuring-screw-driver-resistance

4-remeasuring-screw-driver-resistance

5-looking-for-a-more-permanent-esd-screw-driver

5-looking-for-a-more-permanent-esd-screw-driver

6-ground-zero-esd-modified-screw-driver

6-ground-zero-esd-modified-screw-driver

6.5 esd-modified

6.5 esd-modified

7-measuring-resistance-of-gz-esd-modified-sd

7-measuring-resistance-of-gz-esd-modified-sd

Until I can work out a deal with Western Forge in Colorado Springs (dreaming a bit here), ESD Finger Cots!  Ground Zero ElectroStatics, Inc. has got them.  We’ve also got GZ-Shock Stop and GZ Ultra Spray for industrial uses in the EPA.

www.gndzero.com

1-719-676-2548

Order a sample today.

07 Jul

Demonstrating CDM Discharge using Common Hand Tools

Q:

What is the significance of the time to the charge generation in tribocharging?

Why is it that in tribocharging, there is a big charge produce in short period of time while small charge will be generated at long time? ( at the same force)

(I took the liberty here to respond to the question and go a bit further and look at CDM Testing as described in a recent issue of Conformity.)

A: First a little background about charge as it relates to ESD (ElectroStatic Discharge).

Triboelectric charge is merely the contact and separation of materials.  “It involves the transfer of electrons between materials.”  Which materials lose electrons and which gain them depends on the materials.

Static electricity can be measured in coulombs, and related to voltage potential via the equation: q=CV.  q = charge in coulombs, C = Capacitance, V = Voltage

The industry typically uses electrostatic potential and thus uses voltage to look at this energy form.  Voltage is merely charge potential with respect to a ground point or reference and measured in volts (v).

Insulators or materials with high resistance restricts or prevents flow of electrons across (surface) or through (volume) it’s material.

Conductors or materials with low resistance easily allows the flow of electrons across it (surface) or through (volume)  it’s material.

Insulators and isolated conductors can tribocharge to high voltages and will remain for a long time… so long as energy is not transferred via induction (isolated conductors) by bringing other objects into it’s vicinity and grounding the other object, by grounding the isolated conductor, or by balanced ionization (isolated conductors or insulators).

When isolated conductors are grounded, they (becoming grounded conductors) will enable electrons to flow easily to ground and the charge upon it will become neutralized and reduced to near zero.

Insulators cannot be grounded.  They can induce charge to isolated conductors and can cause electrical overstress/ESD events to isolated conductors at the time they are grounded via the charge field and do not need to contact the isolated conductors in order to do so.

Here’s another way to say that; “CDM (Charged Device Model) charging can produce two separate discharge events.  Here’s how it works.  If you ground a conductor (the conductive blade of a screwdriver for example) while it is in the presence of any item carrying an electrostatic field ( a charged piece of plastic or clothing), the conductor will acquire an electrostatic charge that may be sufficient to cause damage when discharged.”

Human Body Model, as is described in ANSI/ESD S20.20-2007… and the ESD control thereof, is concerned with limiting the voltage in the EPA for the protection of ESDS devices (ESD sensitive devices) to 100 volts and a discharge to within that level in less than 0.3 seconds for ESD Technical Elements (some quicker) at minimum.

I need to know what specifically are you interested in; the HBM, MM (Machine Model), or CDM (Charged Device Model)?  Keep in mind, that “volt per volt, MM discharge is an order magnitude more powerful than HBM discharge because the resistance of human body has been removed from the equation.”

In the article in Conformity, “Demonstrating CDM Discharge Using Common Hand Tools” provided by the ESDA, they state; “The damage threat from hand tools is CDM charging of the hand tool, accompanied by MM discharge to the component or device.”

Source: Conformity : ESD Open Forum April 2009 pg 20.

The following pics depict the testing I did in my lab in accordance with what I’d learned from a recent Conformity article from the ESD Open Forum entitled Demonstrating CDM Discharge using Common Hand Tools.  It involves charge, not by contact, but by induction;

1-non-esd-screwdriver2

1-non-esd-screwdriver2

2-shockstop-treated-screw-driver2

2-shockstop-treated-screw-driver2

3-gz-shock-stop-sample1

3-gz-shock-stop-sample1

4-charging-dp-with-silk1

4-charging-dp-with-silk1

5-zeroing-field-meter1

5-zeroing-field-meter1

6-donning-wrist-strap1

6-donning-wrist-strap1

7-confirm-blade-is-at-zero-on-non-esd-sd4

7-confirm-blade-is-at-zero-on-non-esd-sd4

8-touching-blade-for-cdm-charge2

8-touching-blade-for-cdm-charge2

9-cdm-charge-potential-measurement1

9-cdm-charge-potential-measurement1

10-zeroing-shock-stop-treated-sd-blade4

10-zeroing-shock-stop-treated-sd-blade4

11-confirm-zero-volts-on-shock-stop-treated-sd1

11-confirm-zero-volts-on-shock-stop-treated-sd1

12-recharging-dp-with-silk1

12-recharging-dp-with-silk1

13-touching-blade-on-esd-treated-sd-for-cdm-charge1

13-touching-blade-on-esd-treated-sd-for-cdm-charge1

14-cdm-measurement-for-esd-treated-sd1

14-cdm-measurement-for-esd-treated-sd1

15-rechecking-cdm-on-non-esd-sd1

15-rechecking-cdm-on-non-esd-sd1

16-remeasuring-cdm-on-non-esd-sd1

16-remeasuring-cdm-on-non-esd-sd1

07 Jul

Are ESD shoes and Conductive shoes the same thing?

 

Q:

Are ESD shoes and Conductive shoes the same thing?

A: There are two types of ESD shoes, Static Dissipative and Static Conductive.

The Static Conductive shoe will guarantee a combined resistance of personnel and footwear of less than 1.0E6 Ohms.  I have a pair of Static Conductive shoes that when I’m standing on a static conductive flooring system (2.5E4 Ω to 1.0E6 Ω), my combined resistance from my body through the ESD footwear and through the ESD conductive flooring system to electrical ground or earth is less than 1.0E6 ohms per DoD 4145.26-M, C6.4.7.5.1: “The maximum resistance of a body, plus the resistance of conductive shoes, plus the resistance of the floor to the ground system shall not exceed 1,000,000 ohms total”… “The contractor can set the maximum resistance limits for the floor to the ground system and for the combined resistance of a person’s body plus the shoes, as long as the total resistance does not exceed 1,000,000 ohms.”

This Static Conductive shoe is typically used for electrical safety requirements for facilities that deal with explosive environments such as ordinance, munitions, explosive powders, flammable liquids, etc.  This is outside of the realm of ANSI/ESD S20.20-2007 and MIL-HDBK-263B.

If you’re goal is the protection of static sensitive devices, then Static dissipative shoes on a static conductive flooring system or a static dissipative flooring system will suffice so long as the combined resistance of personnel, footwear, and flooring to electrical or earth ground is less than 3.5E7 Ω as per ANSI/ESD STM97.1-2006.  In that case, a good static dissipative shoe will be more than 1.0E6 or a meg ohm, but the resistance will probably be less than 35 Meg ohms.  The best way to measure the footwear is to have personnel wear them for at least 10 minutes prior to going to the tester and checking for pass/fail low/fail high, as that’s the most practical way to test them.  You can measure the resistance of the shoe from insole to outsole, but they aren’t used that way on the ESD flooring system.  The ESD shoe relies on sweat from the personnel that wears them.

My combined resistance from my body, through my Static Conductive C4327 (men’s) or C437 (woman’s) shoes and through a static conductive floor to electrical/earth ground is about 7.0E5 Ω.  My combined resistance from my body through my Static Dissipative C4341 shoes and through a static conductive floor to electrical/earth ground is about 1.6E6 Ω.

I hope this answers your questions.  Please comment.

Thank you very much, Pat

Static Conductive shoe C4327 Resistance per ANSI/ESD STM97.1-2006

Static Conductive shoe C4327 Resistance per ANSI/ESD STM97.1-2006

0708090842

0708090845

Static Dissipative shoe C4341

Static Dissipative shoe C4341

18 Jun

We don't need no stinking wrist straps, do we?

Q: I have read the White Paper 1: A Case for Lowering Component Level HBM/MM ESD Specifications and Requirements and found the ESD Control Programs and Resulting Data (Chapter 1, Page 20-23) particularly interesting.

Assuming a production environment with ESD flooring, footwear (and clothing), by the time a person walks to a workstation and sits down, the voltage of this persons should not exceed 500V (or even 100V as seen in Figure 3). That would mean even a seated operator in this case would not need to wear wrist strap, that theory would be correct right? After sitting down and this person sits on a stool (feet off the floor) with resistance to floor < 1.0x10exp9ohms, any HBM risk would be further reduced wouldn’t it?

A: Hello ****.  Nice try.  Even if you have an ESD flooring system and even if you have ESD footwear and even if you have an ESD task chair with ESD casters or an ordinary task chair with an ESD chair cover (very effective as well), ESD smock on… you STILL have to wear the wrist strap when seated at an ESD workstation.

The only time, per ANSI/ESD S20.20-2007 page 4, 8.2 Personnel Grounding, that personnel in the EPA (ESD Protected Area) should be without a wrist strap is when doing standing or walking about operations, and then two conditions must be met;
·         “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is less than 3.5E7 Ω…”
·         “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is greater than 3.5E7 Ω and less than 1.0E9 Ω and the BVG is less than 100 v per 97.2…”

This is what is said about seated personnel:

“When personnel are seated at ESD protective workstations, they shall be connected to the grounding / Equipotential bonding system via a wrist strap system.”

Hope this helps.   I guess you could say redundancy is good in the realm of ESD.  It’s the weak link in the chain that will cause an ESD event.  If someone lifts their ESD footwear from the ESD flooring system while seated, they can tribocharge to above 100 volts.  It takes only 0.3 seconds of charge time to exceed 20.20 requirements.  If personnel is seated and getting up to go to break, it seems best to stand up, remove the wrist strap from the wrist, carefully set it down and walk away from the ESD workstation.  Worst case is to take the wrist strap off while still seated, set it down, put your hand on the ESD workstation and near ESDS devices, then stand up out of the task chair before leaving the work station.  Under proper conditions and with good bench mats, clean ESD floors, ESD task chairs, etc. in place, no ESD event.  The problem with ESD events is that we cannot see, hear, feel them.

The only alternative to not wearing a wrist strap while seated may be the used of a smock with a grounding coil cord attached to it.  You can see the footnotes on the 20.20 document at the bottom of page 4 for further details.

 We adhere to and meet or exceed requirements put forth in ANSI/ESD S20.20-2007 or IEC 61340-5-1, which assumes a target HBM of 100 volts and less.

16 Jun

How do we test ESD conductive or dissipative gloves?

Q: How do we test ESD conductive or dissipative gloves?

A: The glove industry offers gloves for the protection of ESD sensitive items by using materials that will provide specific measurable “intrinsic electrical resistance of gloves and finger cots” as per ANSI/ESD SP15.1-2005.

Some materials are being used which reduce the amount of charge generation “and/or have static dissipative properties to reduce charge accumulation”, such as Nitrile or vinyl.  I would image cotton could be effective based on the layer of sweat on our skin.  But if you require ESD gloves in the Static Conductive range, those would need to be specifically made for that purpose.  I’m currently working on nailing down an exact value of what these gloves should read and how that affects the ESD testing of it and the closest I could find comes from a test fixture from Prostat called the CAFÉ, or Constant Area & Force Electrode.  They recommend using 1.5 to 10 volts when the measurement of glove in combination with personnel through a wrist strap assembly without the 1 meg Ω resistor is less than 1 meg ohm.  They use 10 volts between 1.0E6 Ω and 1.0E7 Ω.  Then they use  100 volts for above that.  This is fairly easy to do using a sophisticated megger like the 801 in manual mode, otherwise the mere testing of the glove per 15.1 could be a challenge.

Here’s what confuses about ANSI/ESD S20.20-2007 and -1999 …

 What’s the range of the glove and finger cots?  Only in 20.20-2007 Tables 1, 2, and 3 final column does it give us “Required Limits” to measure up against.  So then what?  Go to manufacturing specs.  Some list a value, some don’t.  Be careful how they’re categorized; anti-static (describes that it’s low charging but doesn’t really quantify a resistance range unless you’re talking about packaging), static dissipative (1.0E6 Ω to 1.0E9 Ω ??), and static conductive (less than 1.0E6 Ω but greater than what??  1.0E4 Ω rings a bell, but I’d hope it’s not less than that.).

Ok, so for our Static Conductive or black finger cots, they measure between 1.0E6 Ω and 1.0E8 Ω per ASTM D257 and meet the static decay specs per MIL-STD-81705B from 5000 to less than 100 volts in less than 0.01 seconds.

So here’s the upshot;   My improvisation in measuring ESD gloves and finger cots involves using the PFA-861-H Handle (see attached), a DUT (esd glove), and a wrist strap without the 1 meg ohm resistor for measurements known to be below about 1.0E7 Ω  , I hook that up to my meg ohmmeter and see what I get (see attached photos).

wand wand-and-sd-glove wand-and-sc-glove

 

This ESD TR20.20-Handbook has a wealth, a plethora of information about ESD gloves and finger cots, such as referring to yet other standards such as ANSI/ESD STM11.11 Surface Resistance Measurement of Static Dissipative Planar Material , and let’s not forget ANSI/ESD STM11.12 Volume Resistance Measurement of Static Dissipative Planar Materials, oh, and of course ANSI/ESD STM11.13 Two-Point Resistance Measurement of Static Dissipative and Insulative (what the??) Material, then it goes on to tell us to use the CAFÉ method, which is specifically designed for resistance measurements at the thumb and fingertips, which can yield much lower results than those obtained by the above test BECAUSE THEY INVOLVE A REAL LIVE PERSON, THE WAY THEY ARE ACTUALLY USED IN PRACTICE!  Oh, and they say you can only measure once due to a “person’s skin emissions”.  Fair enough.  Time to reorder?

So…  If this info helps anybody, let me know and send over a comment.

05 May

Why 3.5E7 Ohms limit for flooring/footwear?

Q: Does anybody know the reason behind the upper limit resistance (3,5×10E7Ohms)of a grounding system (personnel+conductive shoes+conductive flooring)? Why not 1×10E8Ohms?
We have tried many waxes and all of them either give an overall reading for the system that is barely, when it is, within the limits above (IEC 61340-5-1 Table 1 – Note 2.

A: That reading is for ANSI/ESD STM97.1-2006 Floor Materials and Footwear- Resistance Measurement in Combination with a person.

So make sure you’re measuring a clean spot on the floor, someone wearing good clean heel grounders, sole grounders, or static dissipative shoes with one probe from a megger in the palm of their hand to earth or machine ground and the voltage on the meter set for 100 volts, as the resistance is greater than 1.0E6 ohms. Now if they fail this test and are less than 1.0E9 ohms, then they pass if they generate less than 100 volts as per ANSI/ESD STM97.2-2006 Floor Materials and Footwear- Voltage Measurement in Combination with a person.

Sorry so long for the response time.

Q2: Many thanks for you help.
What you are actually saying, if I understand it correctly, is that “if the combined resistance of an operator wearing whatever shoes over a a conductive flooring is greater than 1 x 3,5E7Ohms he will generate more than 100 Volts” and
currently in many electronic plants static generation above 100 Volts is not tolerated.

A2: No, that’s not what I’m saying.  I’m saying, as per ANSI/ESD S20.20-2007, that if you fail the < 3.5E7 ohms test, you may pass the less than 100 volts test and still be compliant to 20.20
 
Look on table 2 of page 4 of 20.20 and you’ll see what I mean.
 
Let me know if that helps.

 

ADD: I guess what needs to be understood with 20.20-2007 is that the < 100 volts and the < 1.0E9 Ohms still stands as well.  But if you’re testing per 97.1 and you get >3.5E7 ohms, then you can still pass 20.20-2007 if you have < 1.0E9 ohms per 97.1 AND < 100 volts per 97.2.

If you go to the table 2 chart on page 4 of 20.20-2007, it makes more sense.