Category Archives: Single-Wire Continuous Monitors

Why Wave Distortion Technology is superior

We talked in the past about the benefits of continuous monitors and also introduced the different types (single-wire vs. dual-wire) to you. The focus of today’s post is the technology behind continuous monitors – how they work and how they compare to each other. So, let’s jump right in.

Introduction to Continuous Monitors
While wrist straps are the first and best line of defence against ElectroStatic Discharge (ESD), they must be tested to ensure that they are installed and working properly. On-demand or “touch” testers have become the most common testing method; they complete a circuit when the wrist strap wearer touches a contact plate.
One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk. Continuous monitors eliminate the possibility of a component being exposed to ESD during the time that the wrist strap was not working properly.
If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.

Technologies used for Continuous Monitors
There are three types of wrist strap monitoring on the market today:
1. Basic Capacitance / Impedance Monitoring,
2. Resistance Monitoring and
3. Wave Distortion Capacitance / Impedance Monitoring.

So, let’s look at all 3 types in a bit more detail:

1. Basic Capacitance / Impedance Monitoring
This single-wire technology makes use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit.
A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor) resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm.

AC capacitance monitors have a few drawbacks:

  1. They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
  2. Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
  3. Since the capacitance and therefore the impedance of the circuit will also vary with such things as the persons size, clothing, shoe soles, conductance of the floor, chair, table mat, the person’s positions (standing or sitting), etc., these monitors often have to be “tuned” to a specific installation and operator.

2. Resistance Monitoring
Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. Here again the concept is simple. By providing a second path to ground (without relying on the capacitor above) we can apply a tiny DC current. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.

Two-wire monitors require two wires to work. This means that the wearer must wear a dual wire two-conductor wrist strap / coil cord which are more expensive than standard single wire wrist straps.
There have been some reports that a constant DC voltage applied to the wristband causes skin irritations.

3. Wave Distortion Capacitance / Impedance Monitoring
Wave Distortion Technology continuous monitors feature:

  • low test voltage,
  • a low monitor range for 1 megohm of resistance in the operator’s wrist strap and
  • instantaneous detection of an intermittent or failure of the path-to-ground of the operator or work surface that other monitors / technologies miss.

Continuous monitors using wave distortion technology apply a continuous test voltage (1.2 volts peak- “Wave Distortion” or vector impedance works by applying a continuous test voltage of 1.2 volts peak-to-peak at 1 to 2 microamperes (0.000002 amperes) to the wrist strap that is connected to the continuous or constant monitor. The test voltage creates a sine wave that the monitor circuit compares to established patterns. By monitoring the “distortions”, or shape of the sine wave, Wave Distortion Technology determines if the monitored circuit is complete – the operator is in the circuit and the total equivalent DC resistance is within specifications. Wave Distortion Technology produces a very fast alarm time (<50 milliseconds) and minimal false alarms.

Comparing Continuous Monitor Technologies
We’ll compare the three different technologies using the following parameters:
1. Safety Resistor Monitoring
2. Test Voltage
3. Banana Jack & 10mm Socket Monitoring
4. Response Time
5. In-Use Verification

1. Safety Resistor Monitoring
The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. “Wrist straps have a current limiting resistor moulded into the ground cord head on the end that connects to the band. The resistor most commonly used is a 1 x 106W, 1/4 watt with a working voltage rating of 250 V.” [IEC TR 61340-5-2 User Guide, Clause 4.7.2.5 Current limiting]

Neutral Basic Capacitance / Impedance Monitoring
Happy Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

2. Test Voltage
We’ve mentioned further above that some people have reported skin irritations when using resistance monitors which apply a constant DC voltage to the wristband. The problem is that the test voltages of resistance monitors is quite high (up to 16V). You have a similar issue with basic capacitance/impedance monitors (3.5V). Another thing to remember is that higher test voltages increase the risk of damage when handling ESD susceptible devices. Luckily for you, wave distortion monitors only use a test voltage of 1.2 – way below the other two technologies.

Neutral Basic Capacitance / Impedance Monitoring
Happy Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

3. Banana Jack & 10mm Socket Monitoring
Coiled cords with banana jack and 10mm sockets are commonly used in the electronics industry. Unfortunately, these cannot be used with dual-wire resistance monitors. As mentioned further above, special dual-conductor wrist straps need to be purchased.

Happy Basic Capacitance / Impedance Monitoring
Sad Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

4. Response Time
Detecting intermittent or complete failures in the path-to-ground of the operator or working surface is the job of a continuous monitor – but, it’s also important to look at how long it takes the monitor to report the issue. What’s the point of using a continuous monitor, if it takes the monitor 5 minutes to tell you there is an issue? All the sensitive devices you handled in the last 5 minutes may have been damaged. An instantaneous detection/alarm is therefore crucial. The slower the response time, the higher the potential impact on sensitive items. Response times for basic capacitance/impedance and resistance monitors is ~1s and ≤ 2s respectively. Wave distortion monitors on the other hand have a response time of <50ms.

Neutral Basic Capacitance / Impedance Monitoring
Neutral Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

5. In-Use Verification
So, imagine this scenario: you received a new constant monitor, you found a nice new home for it, you install it and use it. 12 months down the line, it’s time to verify/calibrate the monitor. You have to remove the monitor from its cosy place, complete the calibration and put it back. What a pain, right? The good news is: the test limits of wave distortion monitors can be verified without removing them from the workstation. Sound like a dream, right?

Sad Basic Capacitance / Impedance Monitoring
Neutral Resistance Monitoring
Happy Wave Distortion Capacitance / Impedance Monitoring

We’ve created the below table for you to easier compare the different technologies:

Comparison of Continuous Monitors Technologies

As you can see, the latest Wave Distortion Technology provides the most reliable and stable confirmation of an operator’s continuous path-to-ground to ensure ESD sensitive product is protected at all times.
Shop our range of Wave Distortion Monitors here.

Single-Wire vs. Dual-Wire Monitors

A wrist strap is arguably the best way to provide a safe ground connection to the operator in order to dissipate accumulated static charges with the purpose to prevent dangerous ESD exposure to sensitive ESD components.

Wrist straps must be tested to ensure that they are installed and working properly. On-demand or “touch” testers have become the most common testing method. On-demand testers complete a circuit when the wrist strap wearer touches a contact plate. One drawback with on-demand type testers is that they require a dedicated action by the wearer of the wrist strap to make the test. Also, knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk. Continuous monitors eliminate the possibility of a component being exposed to ESD during the period that the wrist strap was not working properly.

Types of Wrist Straps
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, by itself is connected to a known ground point at a workbench or a tool. While a wrist strap does not prevent generation of charges, its purpose is to dissipate these charges to ground as quickly as possible. A single-wire wrist strap is comprised of one conductive surface contacting the wrist of an operator and providing one electrical connection to ground. A dual-wire wrist strap has two electrically-separate parts and two separate electrical connections to ground combined in one cord.

Wrist StrapA Wrist Strap

Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground. A single-wire wrist strap is undoubtedly less expensive than its dual counterpart. However, for applications where sensitive components are being handled, the share of dual-wire wrist straps is growing rapidly. The reason for this is its ability to guarantee that the wrist strap indeed provides proper dissipation of charges on the operator. The way to ensure that the wrist strap is worn properly at all times is to utilise a continuous wrist strap monitor. These units monitor proper connection of the operator to ground and alarm should this connection fail. If you want to learn more about the benefits of continuous monitoring, we recommend you read this post.

Wrist Strap Monitors
Monitoring of single-wire and dual-wire wrist straps is fundamentally different:

  • Single-wire wrist strap monitors do not have a return signal path; the only physical parameter they can rely on is parasitic capacitance of operator’s body to ground.
  • Dual-wire wrist strap monitors measure the resistance of the operator’s wrist between the two halves of the wrist strap.

Single-Wire Wrist Strap Monitoring
1. AC Capacitance Monitors
The first constant monitors developed made use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit. A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor) resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm. AC capacitance monitors have a few drawbacks:

  • They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
  • Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
  • Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat, the person’s positions (standing or sitting), etc., these monitors often have to be “tuned” to a specific installation and operator.

This technology is still around today and is purchased by some because of its low cost and a lack of knowledge by the End-User. A big plus of this technology is the ability to use any standard single-wire wrist strap.

2. Wave Distortion Monitors
Many of the short comings of the capacitance and other earlier monitors have been overcome with the development of AC monitors that use the concept of the wrist strap wearer as a capacitor, but in a different way. The concept of the wrist strap and wearer as an RC circuit is not wrong but it is an over simplification. The total circuit actually contains resistance, capacitance and inductance (RCL). Each component value will vary with the environment, size of wearer, and the other factors that affect the accuracy of the AC capacitance monitor. What the wave form distortion monitor looks at is not the impedance level, but at the waveform generated by the circuit. Current will lead voltage at various points due to the combinations of resistance and capacitive reactance. (There is a negligible amount of inductive reactance from the coil cord.) By monitoring these distortions” or phase shifts the WDM will determine if the circuit is complete i.e.; the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. Essentially, the unit will monitor the operator by sending a “signature” signal down the coil cord to the operator’s wrist. The operator acts as a load and will reflect that signal back to the monitor with a different signature. The monitor will then compare the reflected signature to its factory pre-set signatures. If the signal is within the “good” range, the operator passes and the monitor will continue its work. If the signature is “not” good, the monitor will go into an alarm-state to warn the operator to stop working and fix the problem.

Using ESD shielding bagsExample of a single-wire wave distortion monitor

Wave distortion monitors solves many of the problems of the other types:

  • It allows the use of any brand of single-wire wrist strap
  • It cannot be tricked like the AC capacitance units
  • It provides a warning if the lower (safety) resistance limits are compromised
  • The tiny amount of current required to generate the waveform has never caused reported skin irritation.

As an added bonus, wave distortion monitors will also detect an open circuit or bad ground all the way back to the building ground point. This is a fundamental advantage of this kind of monitor. Other monitors may insure that the operator is connected to the monitor. No other monitor automatically ensures that the user is actually grounded.

Dual-Wire Wrist Strap Monitoring
Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. By providing a second path to ground (without relying on the capacitor above) we can apply a tiny DC current. It is then simple to measure the DC resistance of the circuit and alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted). Thus, a two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement. The shortcomings with the AC capacitance monitor are eliminated.
Two-wire monitors require two wires to work. This means that the wearer must wear a dual-wire two-conductor wrist strap / coil cord which are more expensive than standard single-wire wrist straps.

Example of a Dual-Wire Continuous MonitorExample of a dual-wire monitor

There have been some reports that a constant DC voltage applied to the wristband causes skin irritations. This has been addressed in some models by pulsing the test current and in others by lowering the test voltage.

Conclusion
Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire systems are used to create redundancy. In critical applications, you build-in redundancy to have a backup if your primary option fails. With dual-wire wrist straps the redundancy is there as a protection rather than an alternative. If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm. You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails. The wrist strap would still need to be replaced immediately. So, while both single-wire and dual-wire wrist strap monitors help to dissipate accumulated charges on an operator, only dual-wire wrist strap solutions provide assurance of a proper dissipative path from operator to ground.

Continuous Monitors and ESD Control

Most of you have probably heard of continuous monitors before but do you know:

  • what exactly they do or
  • why you should be using a continuous monitoring system?

If your response to one (or both) of the above questions is ‘no’, you’re lucky because we’ll be answering them here today. If you already know the answers, why not read on anyway? You never know: you might be learning something new…

What are continuous monitors?

When talking about wrist straps a couple of weeks ago, we mentioned the need for periodic testing. Every day before each use, wrist straps need to be checked (while worn) to ensure they ground the operator correctly.

So imagine this scenario: you come to work in the morning, you test your wrist strap, it passes and you get to working on your ESD sensitive devices. 3 hours later, when you come back from your tea break, you test your wrist strap again and it fails. What to do? You don’t know if the wrist strap only just failed or if it failed right after your first test in the morning. How do you know if the devices you worked on all morning have been damaged? You don’t – after all latent defects are not visible and failures may only occur at a later time. That’s where continuous monitors come into play.

Continuous monitors provide operators with instant feedback on the status and functionality of their wrist strap. They detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components.
Continuous monitors come in different styles and sizes but are intended to be kept on your workstation. Some units just ‘sit’ on your bench; others are attached to your working surface matting; some can even be attached underneath the workbench so they don’t take away valuable workspace. Operators connect their wrist strap to the unit to allow for real-time continuous monitoring. If the wrist strap fails, the unit will alarm. Many continuous monitors also feature a parking stud providing a means for the operator to disconnect when leaving their workstation.

In our scenario above, had our operator used continuous monitoring while working on those ESD sensitive devices, they would have been alerted as soon as their wrist strap failed. The faulty wrist strap could have been replaced with a brand spanking new model from stock and everyone would have been happy – no ESD sensitive devices damaged and no unhappy customers.

Using continuous monitorsUsing continuous monitor 222603

Some continuous monitors even provide the ability to monitor working surface ground connections. “Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.1

These days you can even find a number of ‘smart’ monitors on the market. They are connected to a computer and software records and displays failures to notify supervisors or engineers when a device needs attention (see the EMIT software as an example).

When and why you should be using continuous monitors

If your company manufactures products containing ESD sensitive items, you need to ask yourself “how important is the reliability of our products”? Sooner or later a wrist strap is going to fail. If your products are of such high value that you need to be 100% sure your operators are grounded at all times, then you should consider a continuous monitoring system.

The ESD Association produced the ESD TR 12-01 technical report which is entitled “Survey of Constant (Continuous) Monitors for Wrist Straps“. It contains some useful information: “Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.” “Wrist strap checkers are usually placed in a central location for all to use.  Wrist straps are stressed and flexed to their limits at a workstation.  While a wrist strap is being checked, it is not stressed, as it would be under working conditions.  Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.1

Types of continuous monitors

The impedance (or single-wire) constant monitor “… uses a detection circuit designed to reduce false alarms and eliminate adjustments. [It] use[s] the phase difference between current and voltage to detect changes in impedance of the cord, band and person. A very low AC voltage is used for constant sensing. Any standard [single wire] wristband and coiled cord can be used.”1

Single-wire monitorsExamples of single-wire monitors

Single-wire monitoring allows the use of any standard, single-wire wrist strap and coil cord. The monitor / wrist strap system life-cycle costs are significantly lower than dual-wire systems. While they would not be suitable for the most critical applications, single-wire continuous monitors are an economical way to monitor both the operator’s wrist strap and/or workstation surface.

Resistance (or dual-wire) constant monitors are “… used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire.  If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC.  Pulsed DC signals were developed because of concerns about skin irritation.  However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.1

Examples of dual-wire monitorsExamples of dual-wire monitors

Dual-polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards. Dual-wire continuous monitors provide redundancy; even if one dual-wire wrist strap conductor is severed, the operator still has a reliable path-to-ground with the other conductor.

 

Need help choosing the correct continuous monitor for your application? Make sure you check our continuous monitor selection chart.

 

 

1 ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps

Which Single-Wire Monitor is Best for your Application?

Continuous Monitors provide operators with instant feedback on the status and functionality of their wrist strap and/or workstation. They detect split-second failures when the wrist strap is still in the “intermittent” stage. This is prior to a permanent “open” which could result in damage to ESD sensitive components. Continuous Monitors verify the ground integrity of both the operator and ESD workstation and eliminate the need for periodic testing (and record keeping).

Single-Wire Monitoring allows the use of any standard, single-wire wrist strap and coil cord. The monitor / wrist strap system life-cycle costs are significantly lower than dual-wire systems. While they would not be suitable for the most critical applications, Single-Wire Continuous Monitors are an economical way to monitor both the operator’s wrist strap and workstation surface.

222608
The Multi-Mount Monitor continuously monitors:
• One operator wearing a wrist strap
• One ESD worksurface

Its small package and mounting tabs with holes make it highly suitable to install on most equipment or work bench surface.222603

 

The Jewel® Mini Monitor continuously monitors:
• One operator wearing a wrist strap
• One ESD worksurface

This unit is ideal for individual workstations for assembly, test, packaging or other applications.

222744

 

The Dual-Operator Monitor continuously monitors:
• Two operators wearing a wrist strap
• Two ESD worksurfaces

This unit is ideal for workstations that are shared by two operators.

 

To determine the correct monitor for your application, make sure you check our Continuous Monitor Selection Chart.

Vermason Continuous Monitors and ESD Control

Vermason Continuous or Constant Monitors pay for themselves by improving quality, productivity, eliminating wrist strap daily testing and test result logging. EN 61340-5-1 specifies wrist strap standard ANSI/ESD S1.1 which states “The wrist strap system should be tested daily to ensure proper electrical resistance. … Daily testing may be omitted if continuous monitors are used.“(1)

Companies manufacturing products containing ESD sensitive items need to ask themselves “how important is the reliability of our products”?
Because wrist straps have a finite life, it is important to develop a test frequency that will guarantee integrity of the system. Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that a guarantee of a continuous, reliable ground is needed then continuous monitoring should be considered or even required.“(2)

The wrist band will normally be worn for several hours at a time so it needs to be comfortable while making good contact with the skin. It is a good idea to check the wrist strap every time it is applied. Constant on line monitors can be used so that any breaks will be immediately found.“(2)

The ESD Association produced the ESD TR 12-01 technical report which is entitled “Survey of Constant (Continuous) Monitors for Wrist Straps”(3). It contains useful information:
Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task.” “While effective at the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge. If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift’s work could be suspect.

Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation. While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord’s strain relief are sometimes only detected under stress.

The goal remains consistent; electrical connections are tested between the ground point, coiled cord, wrist band, and body while the wearer performs operations on static sensitive items.

In many EPAs [ESD protected areas] constant monitor wrist straps are used. These fall into two categories, dual wrist band and single wrist band. The dual wrist strap type using a split band is used with a two core cord. The dual wrist strap type normally works using the resistance bridge method. The single wrist strap type uses a single strap for both grounding and monitor purposes and has an a.c. signal injected which is used for monitoring purposes. This type has the advantage of using the simpler single wrist strap.“(4)

Impedance (or single wire) constant monitors:
The impedance monitor uses a detection circuit designed to reduce false alarms and eliminate adjustments. [It] use[s] the phase difference between current and voltage to detect changes in impedance of the cord, band and person. A very low AC voltage is used for constant sensing. Any standard [single wire] wristband and coiled cord can be used.

Impedance Wave Distortion Technology used in Vermason Single Wire Continuous Monitors utilises AC analysis to determine if the correct conditions exist. A steady state 1 volt AC signal is sent out and the impedance (combination of resistance and capacitance) of the system will distort the signal wave. The acceptable distorted wave consists of a 1 megohm resistor and the impedance ranging from a 90 lb., 5 foot tall person on the low end and a 250 lb., 6 foot, 5 inch tall person on the high end. If the 1-megohm resistor is not there, the alarm will sound. If the resistive component of the path to ground exceeds 18 megohms, the alarm will sound. Vermason has never received a skin irritation complaint using impedance wave distortion technology Continuous Monitors.

Resistance (or Dual Wire) constant monitors:
This type of monitor is used with a two wire (dual) wrist strap. When a person is wearing a wrist strap, the monitor observes the resistance of the loop, consisting of a wire, a person, a wristband, and a second wire. If any part of the loop should open (become disconnected or have out of limit resistance), the circuit will go into the alarm state.” “While the continuity of the loop is monitored, the connection of the wrist strap to ground is not monitored.” “There are two types of signals used by resistance based constant monitors; steady state DC and pulsed DC. Pulsed DC signals were developed because of concerns about skin irritation. However, pulse DC units introduce periods of off time (seconds) when the system is not being monitored.“(3)

Vermason Dual Wire Continuous Monitors utilise a steady state DC signal and never have received a skin irritation complaint. Vermason Dual Wire Wrist Straps passed the ANSI/ESD S1.1 flex test at over 1,000,000 cycles vs. the 16,000 requirement while the top competitor has touted their dual wire wrist strap life at only 200,000 cycles. Some brand dual wire wrist straps do not meet the ANSI/ESD S1.1 Breakaway Force requirement of less than five pounds but greater than one pound force.

Resistance Dual Wire continuous monitors, using the same technology as on demand touch testers, is easily understood. An important feature of the Dual Wire Wrist Strap is that even if one conductor is severed, the operator has reliable path-to-ground with other wire. The electrical signal does place that amount of charge on the operator. Vermason Zero Volt Monitor utilises a steady state DC dual polarity signal, with a plus signal sent via one wire and a minus signal sent via the other wire balancing and leaving virtually zero voltage on the operator. A Programmer is available to quickly and accurately set the upper and lower resistance levels to be monitored while the Continuous Monitor is installed at the workstation.

Working surface monitors:
An option available with some continuous or constant monitors is the ability to monitor working surface ground connections. “Some continuous monitors can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The monitor however, will not detect insulative contamination on the worksurface.“(3)

Most Vermason Continuous Monitors do monitor the working surface ground. Note: the working surface must have a conductive layer such as Dual Layer Rubber or Dissipative 3-Layer Vinyl or Micastat® Dissipative Laminate with conductive buried layers. Vermason Continuous Monitors are not recommended for use with homogeneous matting.

Selection Considerations:
When considering constant monitors, the equipment cost including the wrist strap, maintenance and training cost, labor time for performing wrist strap tests, and the potential failures due to non-functional wrist straps should be considered. A broken wrist strap may expose products to ESD over an entire shift if it is checked only at shift change with wrist strap checker. Constant monitors may reduce the time people spend testing wrist straps before each shift.“(3)

(1) ANSI/ESD S1.1 Annex A3 Frequency of System Testing
(2) User guide CLC/TR 61340-5-2 Wrist Strap clause 4.7.2.4.4 Test frequency
(3) ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps

(4) IEC 61340-5-1 Electrostatics – Part 5-1: Protection of electronic devices from electrostatic phenomena – General requirements

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