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Question I have just started studying of constant wrist strap monitors. I see there is a “capacitive type” and I think this measures the AC impedance of the wrist strap to the person. That means that if there is no DC path to the person but their wrist strap is on the wrist that it is deemed OK, right? If that is the case, it really is not testing continuity and a loose wrist strap that does not make good contact to the person will still pass. I suppose it is a really good thing to confirm that the person is wearing the wrist strap and that in concert with daily checks is a good solution. What are your opinions of the capacitive type and do I understand the capacitive type correctly?
Answer

Capacitance (or single wire) constant monitors

“The monitor circuit sends out a signal through the wrist strap that changes because of the person to ground capacitance. The monitor then detects this change to determine that a person and ground are present. If the electrical connection to the person (wrist strap), or to ground (grounding wire) should open (become disconnected), the circuit will go into the alarm state.”

Capacitance continuous monitors are typically the least expensive but do not monitor a lower limit, verifying the presence of a current limiting resistor, can be fooled, and often have to be tuned to a specific installation and operator.


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 wristband and coiled cord can be used.”

Impedance Wave Distortion Technology utilizes 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 any brand single wire wrist strap can be used which is less expensive than dual-wire wrist straps. The single wire wrist strap is also more durable having a longer life; over the life of the continuous monitor system, the wrist strap cost savings can be considerable. In addition, ESD Systems.com wrist strap coil cord utilizes a patented contoured spring supported banana plug. The dual wire wrist strap uses a much less robust mini-plug, similar to those used in home audio equipment.


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.”

Dual Wire Continuous Monitors utilize a steady state DC signal and never have received a skin irritation complaint. Dual Wire Wrist Straps passed the ANSI/ESD S1.1 flex test at over 11,000,000 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 S6.1 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, are 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.  Volt Monitor and Ultra Low Volt Monitor utilize 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.

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