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.
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.
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:
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:
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.
Wave Distortion Technology continuous monitors feature:
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.
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
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 commo nly 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]
![]() | Basic Capacitance / Impedance Monitoring |
![]() | Resistance Monitoring |
![]() | Wave Distortion Capacitance / Impedance Monitoring |
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.
![]() | Basic Capacitance / Impedance Monitoring |
![]() | Resistance Monitoring |
![]() | Wave Distortion Capacitance / Impedance 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.
![]() | Basic Capacitance / Impedance Monitoring |
![]() | Resistance Monitoring |
![]() | Wave Distortion Capacitance / Impedance Monitoring |
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.
![]() | Basic Capacitance / Impedance Monitoring |
![]() | Resistance Monitoring |
![]() | Wave Distortion Capacitance / Impedance Monitoring |
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?
![]() | Basic Capacitance / Impedance Monitoring |
![]() | Resistance Monitoring |
![]() | Wave Distortion Capacitance / Impedance Monitoring |
We’ve created the below table for you to easier compare the different 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.