Our customers tell us that one of the most frustrating things in selecting a wrist strap monitor is comparing competing manufacturer's claims and representations because you can't compare them. Representation is routinely made that are designed to impress or disparage, whether they're accurate or not.

Our company policy is simple. We make an excellent product. Our product performance is presented in a straightforward, no-nonsense manor. Every ESD SYSTEMS product is backed by a 1-year customer satisfaction warranty.

The following explanation of how the ESD SYSTEMS monitors work and also how other types of monitors work, together with a Head-to-Head comparison sheet, was created to debunk the myths and sort through the common misconceptions due to the claims, counter-claims and misrepresentations that are heard all too often.

First, to answer some basic questions:

1. Q: Why ground people for ESD protection?

A: To keep their body voltage as close to 0 volts as possible at all times, since people tribocharge. Tribocharging causes voltage buildup, and rapid voltage discharge can damage sensitive electronic equipment. (Tribocharging is an act of nature)

2. Q: Why monitor them?

A: To verify that their wrist strap grounding system is constantly doing their jobs

3. Q: How many types of monitoring systems are there?

A: Three. There are one-wire, two-wire, and three-wire monitors.

4. Q: How do they work?

A: AC theory, capacitance bridge, DC theory? We'll get into that in just a moment.

5. Q: What are their relative merits and/or shortcomings?

A: One-Wire Monitors: Accuracy, reliability, simplicity, economy

Two-Wire Monitors: Old technology, easily fooled, no lower limit

Three-Wire Monitors: More wiring required, complexity, special wrist strap plugs and jacks required, wrist straps do not meet EOS/ESD S1 safety pull off test, higher life cycle cost.

We've answered the first two questions. As for the third question, namely the three types of monitoring systems:

ESD SYSTEMS model 41130, 41101, 41102, 41125, 41115, 41135, and 41140 are one-wire systems. They work with any conventional single-conductor wrist strap and coil cord, grounds itself through the AC outlet, and will automatically tests the outlet's connection to ground.

These are AC systems, however they are distinctly different from the earlier so-called capacitance systems, since they do not measure capacitance. What they do are, "analyze the distortion of the coil cords currents waveform as a consequence of the impedance presented by the load." This is how they work! They do not work by measuring the attenuation of a high frequency signal sent down a standard conductor into the wrist strap and the user who wears it. Nor, do they work by detecting the presence or absence of a level of capacitance on the end of the coil cord. You might say that this is a "distortion analyzer" type of system. It is a unique instrument. The coil cord is the only wire.

One hears this commonly referred to as a capacitance system, sometimes an impedance system. These systems also work with any conventional single-conductor coil cord. They are based upon what is termed a Barkhuisen circuit. It is essentially a capacitance bridge. The operators' capacitance, nominally 100pF, often somewhat more, balances the bridge. If the operators' contact with the monitor is broken, the bridge becomes unbalanced causing the monitor to alarm.

Unfortunately, these monitors cannot tell the difference between the capacitance of a person and the capacitance of a coil cord laying on a grounded static dissipative work surface. Most of these types of monitors do not include a low resistance limit; hence they do not verify the presence of the standard one-megohm current limiting resistor in the coil cord. They also must be connected to ground, or what is presumed to be ground, or at least to a substantial capacitance, with a second wire.

These use special dual-wire coil cords and special wrist straps, which have two electrodes. They also require a third wire to connect them to ground, hopefully a proven ground. They work by passing a current through the loop formed by the dual coil cord wires and the operator. The current flows through the first wire and the wrist strap, then through the skin of the operators wrist between the wrist straps' two electrodes and finally back to the monitor via the coil cords' second wire. The monitors measure the loop resistance to ascertain whether it is within acceptable limits.

It is essentially an Ohmmeter. It is a DC system and is easily understood, for DC theory is more widely understood than AC theory, since it's less complex. There are only real terms in it, whereas in AC theory there are also imaginary terms, mathematically speaking.

All of the above systems have their various bells and whistles, i.e., lights and audio alarms, plus the circuitry necessary to drive them. Some are more and some are less immune to external interference. Their features vary. Attached is a comparison chart for the ESD SYSTEMS units vs. the three wire resistive type monitors. Remember that the objective is to keep the operator's voltage down by grounding them.

Other monitors may insure that the operator is connected to the monitor, but only ESD SYSTEMS units automatically ensure that the monitor is connected to earth ground. No other monitor automatically ensures that the user is actually grounded. And remember also that the ESD SYSTEMS units are not capacitance monitors. It is a waveform distortion analyzer. To use it there is only one-wire to connect, a conventional, single conductor coil cord.