How would I test the continuity of an anti-static wrist band?

Question

I am looking to purchase some anti-static gear for when working on electronics(I'm assuming all electronics should be worked with on an anti-static pad, that's correctly grounded, correct?), and noticed in some reviews there are mention of not having "Continuity" in the strap they purchased.

One review mentions

WARNING FOR PURCHASERS AND ALL BRANDS OF A COMPUTER ANTI-STATIC WRIST STRAP – Before beginning my PC build, and looking at this $400 motherboard, from a past experience, I decided to verify that the Rosewill ESD Anti-Static wrist Strap will actually work, by testing it with my Multi meter. The Results were stunning - THERE IS NO CONTINUITY (CONNECTION) FROM THE ALLIGATOR CLIP TO THE METAL PAD ON THE WRIST STRAP – THIS NO STATIC PROTECTION!

So this brings a couple questions: 1) Is the bad connection just on the wrist strap I just bought, or on a whole bunch. 2) How many people buy any brand of anti-static wrist straps, put it on and think they are protected, then for their electrical part go bad thinking they were protected!!

Worse yet, most everything Nowadays is cheap China Stuff (CCS) and it is really hard for a company to make sure China/Asian products have quality.

I had randomly learned about continuity(right before reading this review), from a video of a water damaged iphone. The user used a multi-meter while connecting 1 side to something (couldn't see in the video), and would test both sides of the capacitor to see if it would beep. If both sides beeped, it seemed to be a bad capacitor, but if only one side beeped, that means it was good.

So I'm curious about,

1. If the information presented in the video is true, and if so what exactly is going on there with the beeps and why it should only beep on one side?

2. What exactly am I trying to test, and where, on the anti-static wrist band? The definition of continuity mentions "Continuous flow" so would I test the 2 ends, and multiple parts of the wire if something goes wrong, or...?

Any information about continuity testing, and any information about the subject in electronics, or some guidance to reading materials would be greatly appreciated, thanks.

EDIT: Review was from https://www.amazon.com/Rosewill-Anti-Static-Components-RTK-002-Yellow/product-reviews/B004N8ZQKY/ref=cm_cr_dp_synop?ie=UTF8&reviewerType=all_reviews&showViewpoints=0&sortBy=recent#R2LY6BRSK3A4UN

The segment is from 26 mins or so on this video https://www.youtube.com/watch?v=plmElKA6qKM&list=PLtjsWVbfWwk6cIQEN4aWjIiaP4VgXYuB0&index=3

Sorry share isn't working so I cannot get it directly at the timestamp.

Answer 1

Overview:

A good wrist strap system has a large resistance between the skin contact and ground. Typically this is 1 MΩ, which is relatively large. This large resistance is there for your safety, in case an error somewhere connects you to a hazardous voltage. Even this high resistance is sufficient for protecting your devices from ESD.

A typical multimeter should be able to measure this resistance. However, most continuity tests will probably report it as open. This is because many continuity functions are looking for resistances less than 100 Ω.

Verifying your wrist strap:

Here is how you measure your wrist strap system - check from the skin contact side of the strap (red meter lead) to the part that gets connected directly to ground (black meter lead). Make sure your meter is configured to measure resistances higher than 1 MΩ, as the resistor will not be a precision tolerance. Note that in this picture I am making sure I don't touch any of the metal surfaces, which would give a bad reading.

Measure the resistance from the inside of the wrist strap to the outside or the end of the cord. Make sure you're on the high resistance scale of your meter if it does not auto range.

In the labs I work in, the resistor is in the actual lead somewhere (the black wire). On the snap housing it is marked as 1 MΩ, and all the wrist straps are low resistance connections from the skin side to the snap connector. I have seen ESD straps with a 1 MΩ resistor built-in.

Don't use the continuity test:

For kicks and giggles, here's the result when you use the continuity check function on the whole thing - it reads as an open circuit (no continuity). With my setup, I would expect the meter to beep/show continuity on the wrist strap, but not on the wire lead.

Conclusions:

Without knowing if the reviewer just used a continuity test, or actually measured the resistance (or even where they measured), it's impossible to draw any conclusions from the review. If the reviewer knew how to properly measure the system, then I would consider that a defective product. If the reviewer tried to use a continuity check across a 1 MΩ resistor, then the reviewer is not being helpful at all. The wording of the review makes me suspect the latter, but it is impossible to know from what was written.

Answer 2

The built-in resistor might be well above 10M ohms, and a cheap DMM meter would see this as an open circuit. (But many wrist bands use 1M resistors, easily measured.)

Wrist straps are not supposed to short your body directly to ground. Doing so creates an electrocution hazard, especially if you accidentally bump against live AC connections with your other hand, the one lacking the wrist strap. (This would direct the current-path of line-voltage through your chest and out your grounded wrist.)

To avoid the danger, wrist straps must contain a built-in resistor in the Megohm range. Their purpose after all isn't to make you touch a questionable power-tool while standing barefoot in grounded salt water. Instead, the goal is to remove any DC body-voltage by draining you (perhaps kilovolts worth!) of any surface-charge.

To perform this task, what is the max value of series resistance appropriate? The resistor must significantly discharge your personal capacitance in roughly one second. 0.1sec might be better, so let's use that value. A human body has approximately 100pF capacitance to ground. But worse-case, you might be lying on a concrete floor with grounded steel inside it. This produces a body-capacitor to Earth having a much higher value. Let's say that max human capacitance is 3x higher: 300pF.

The crude time-constant calculation is simple: T(sec) = R(ohms) x C(farads). Or, R=T/C. So, R = 0.1sec / 300x10^-12F = 300Megohms max. A much larger resistance would still work fine, but any kilovoltage built up on your body would drain in a few seconds, rather than semi-instantly.

Also, what's the minimum appropriate resistance? If you poke your finger into 120VAC, you REALLY REALLY want any resulting current to stay well below 1mA. Let's say 0.1mA for a fairly safe value of through-chest current. And, lets say 240VAC for the line-voltage hazard. By Ohm's law this gives a wrist-strap series resistor value of R = V/I = 240V/.0001A = 2.4 Megohms min.

Various manufacturers may err on the side of caution, and their wrist strap resistor may be closer to 20M than to 2M. If so, a typical DMM would see it as an open circuit. Yet your high-volt body-charge sees the same resistor as nearly a short circuit, and will be discharged almost instantly. "Frictional" electrostatics is weird that way: typical supply-impedances of the charge-pumps are above 1M, and can even be up in the giga-ohms. A normal voltmeter cannot measure human body voltages without applying their "short circuit" of 10Megs. To deal with such HV measurements, your voltmeter needs a Zin of 10G-ohms or much higher, not the typical 10M-ohms. Such voltmeters are called "electrometers" or "moving-vane voltmeters," and can have a Zinp of 10^15 or higher, ten thousand gigohms (the leakage across clean plastic during humid conditions.)

Answer 3

Short Answer: the consensus is measurement error. Strap is OK, so Trust, but Verify. ( Test Engineer's creed. )

https://www.amazon.com/product-reviews/B004N8ZQKY/ref=acr_dpx_hist_5?ie=UTF8&filterByStar=five_star&reviewerType=avp_only_reviews&showViewpoints=0

At least one reviewer correctly measures leakage resistance at 1002 Kohm

The others must have been using continuity which measures voltage drop with 10uA or 100uA which is overscale and actually reads in Volts on a xxxx scale using my Fluke75, intended for Vf diode drop . I get 2.095V max on my tongue which also measures at 0.4M so it is just our of range for EOS straps ( the proper name ) This means continuity applies a constant current ( AC) of 2V/0.4M= 20uA roughly

If you only had a continuity meter scale ( like mine, not all the same uA) it can still be measured with a 500K then strap in parallel drops 33% or 100K and 1M strap drops roughly 9%

1M is the defacto std for Wrist , heal, toe and matt straps. Matts are measured in Ohms per square of any size around 10e10 to 10e11, no more and no less.

Answer 4

If you don't own a wrist strap you can still hook an alligator clip to your pinky and the other side to chassis ground. Disconnect if pinky shows discoloration.