Reposted from Our Friends at SCS-Static-Control Solutions
Dual wire ESD monitors and ESD wrist straps are the most common personnel grounding device to ground operators. In today’s blog post, we will talk about the difference between dual-wire and single-wire wrist straps and continuous monitors.
What is an ESD Wrist Strap?
A wrist strap is arguably the best way to provide a safe workstation ground connection for the operator to dissipate accumulated static charges and 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.
- On-demand testers require a dedicated action by the wearer of the wrist strap to make the test.
- 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 ESD Wrist Straps
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and 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.
Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground.
Dual Wire ESD Monitors vs Single Wire 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 the 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 Continuous ESD Monitors
1. AC Capacitance Monitors
The first continuous ESD 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.
This technology is still around today and is purchased by some because of its low cost. A big plus of this technology is the ability to use any standard single-wire wrist strap.
Example of a single-wire capacitance monitor – more information
2. Wave Distortion ESD Monitors
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 monitor 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.
Dual-Wire ESD Continuous Monitoring
A number of issues can come up when using single-wire monitors, such as:
- 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.
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 to measure the DC resistance of the circuit. The monitor will alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted).
- 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 if a wrist strap fails.
Example of a dual-wire pulsed monitor – more information
How to Install a Dual-Wire ESD Wrist Strap Monitor
- Determine the mounting location of the monitor. The front panel should be visible to a supervisor.
- Determine the mounting location of the operator remotes. The white remote is for operator #1 and the black remote is for operator #2. Make sure that each remote is located so that the remote leads reach the monitor but are convenient for the operator.
- Attach the operator remotes to the bench or other surface using the provided screws.
- Attach the tinned wire ends of the mat wires to the appropriate screw terminal connection on the rear of the unit. (See Figure 2). The white wire is for operator #1 and the black wire is for operator #2.
- If not already done, attach the work surface to workstation common point ground using the pictured ground wires. For mats, attach the grounding point snaps at the ends of the ground monitor cords to the grounding point snaps on the work surface. These ground monitor cords are used for sensing whether or not the unit is properly grounded. Refer to figure 3 for snap-plate fitting diagram and for laminates or their hard surfaces with a buried conductive layer.
- Attach the tinned wire end of the black ground-reference wire to the center position of the screw terminal block on the rear of the unit. Attach the ring terminal end to an
alternate ground point. It is important that this ground wire is attached to a separate ground point other than the work surface ground cords from the previous step to ensure proper independent monitoring. The faceplate screw of a grounded AC wall outlet may provide a convenient connection point.
Does a Constant Voltage ESD Wrist Strap Cause Skin Irritation?
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.
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. Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards.
- Dual-wire continuous wrist strap monitors ensure that the wrist strap is worn properly at all times. These units monitor proper connection of the operator to ground and alarm should this connection fail.
- In critical applications, dual-wire systems have redundancy built-in to have a backup if the primary option fails. Two-wire monitors require two wires to create redundancy – 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.
- A two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement while eliminating the shortcomings of the AC capacitance monitors
For applications where sensitive components are being handled, the ability to guarantee that the wrist strap provides proper dissipation of charges on the operator is critical.