RS Continuous Monitors

Continuous Monitors: What is Better Single or Dual Wire?

Both single wire wave distortion impedance and dual wire resistance continuous monitors are based on sound reliable technologies.  Both have been used successfully for many years by leading companies to improve their quality and productivity, and immediately knowing when there is a break in grounding of operators and working surfaces.  User preference is typically the deciding element.

Per CLC/TR 61340-5-2:2008 User guide Annex B.1.3 “A properly grounded wrist strap will keep a person's body voltage to approximately + 10 V.  The main advantage to a constant [or continuous] monitor is the immediate indication that the employee receives if the wrist strap falls open. With an unmonitored system, the employee will not be aware of a wrist strap failure until the start of the next shift. This has reliability benefits for an ESD program as it might help reduce or eliminate ESD damage.  There are also other process benefits from using constant monitors such as the elimination of the need to maintain daily test logs and a reduction in the time for employees to make the daily test.”

The ESD Association produced the ESD TR 12-01 technical report which is entitled “Survey of Constant (Continuous) Monitors for Wrist Straps”.  It contains useful information that may be helpful:

 

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.” [ESD TR 12-01 Survey of Constant (Continuous) Monitors for Wrist Straps]

 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.  If the resistive component of the path to ground exceeds 18 megohms, the alarm will sound.

Any single wire wrist strap can be used which is less expensive that 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.

 

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.” [ESD TR 12-01 Survey of Constant (Continuous) Monitors for Wrist Straps]

 

Dual Wire Continuous Monitors sold by RS utilize a steady state DC signal.

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. Zero Volt Monitors 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.  For calibration downtime and sending the monitors to a calibration lab is not necessary as Calibration Units or Limit Comparator are typically available.  The EN 61340-5-1 required compliance verification can be performed at the workbench within a couple of minutes with no lose of production time.

Some hi-tech companies successfully use single wire wave distortion impedance continuous monitors.  Other companies successfully use dual wire resistance continuous monitors.  Both technologies are cost effective and help improve quality and productivity.  User preference is typically the deciding element.