Desco Europe Continuous or Constant Monitors pay for themselves by improving quality, productivity, eliminating wrist strap daily testing
and test result logging. EN 61340-5-1 specifies wrist strap standard ANSI/ESD S1.1 which states "The wrist strap system should be tested daily
to ensure proper electrical resistance. ... Daily testing may be omitted if continuous monitors are used."(1)
Companies manufacturing products containing ESD sensitive items need to ask themselves "how important is the reliability of our products"?
"Because wrist straps have a finite life, it is important to develop a test frequency that will guarantee integrity of the system. Typical
test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such
value that a guarantee of a continuous, reliable ground is needed then continuous monitoring should be considered or even required."(2)
"The wrist band will normally be worn for several hours at a time so it needs to be comfortable while making good contact with the skin. It is
a good idea to check the wrist strap every time it is applied. Constant on line monitors can be used so that any breaks will be immediately found."(2)
The ESD Association produced the ESD TR 12-01 technical report which is entitled "Survey of Constant (Continuous) Monitors for Wrist Straps"(3).
It contains useful information:
"Since people are one of the greatest sources of static electricity and ESD, proper grounding is paramount. One of the most common ways to ground
people is with a wrist strap. Ensuring that wrist straps are functional and are connected to people and ground is a continuous task." "While effective at
the time of testing, wrist strap checker use is periodic. The failure of a wrist strap between checks may expose products to damage from electrostatic charge.
If the wrist strap system is checked at the beginning of a shift and subsequently fails, then an entire shift's work could be suspect."
"Wrist strap checkers are usually placed in a central location for all to use. Wrist straps are stressed and flexed to their limits at a workstation.
While a wrist strap is being checked, it is not stressed, as it would be under working conditions. Opens in the wire at the coiled cord's strain relief
are sometimes only detected under stress."
"The goal remains consistent; electrical connections are tested between the ground point, coiled cord, wrist band, and body while the wearer performs
operations on static sensitive items."
"In many EPAs [ESD protected areas] constant monitor wrist straps are used. These fall into two categories, dual wrist band and single wrist band.
The dual wrist strap type using a split band is used with a two core cord. The dual wrist strap type normally works using the resistance bridge method.
The single wrist strap type uses a single strap for both grounding and monitor purposes and has an a.c. signal injected which is used for monitoring
purposes. This type has the advantage of using the simpler single wrist strap."(4)
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 [single wire]
wristband and coiled cord can be used."
Desco Europe 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.
Desco Europe has never received a skin irritation complaint using impedance wave distortion technology Continuous Monitors.
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."(3)
Desco Europe Dual Wire Continuous Monitors utilize a steady state DC signal and never have received a skin irritation complaint. Desco Europe Dual Wire
Wrist Straps passed the ANSI/ESD S1.1 flex test at over 1,000,000 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 S1.1 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, 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. The Desco Europe Zero Volt Monitor utilizes 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.
Working surface monitors
An option available with some continuous or constant monitors is the ability to monitor working surface ground connections. "Some continuous monitors
can monitor worksurface ground connections. A test signal is passed through the worksurface and ground connections. Discontinuity or over limit resistance
changes cause the monitor to alarm. Worksurface monitors test the electrical connection between the monitor, the worksurface, and the ground point. The
monitor however, will not detect insulative contamination on the worksurface."(3)
Most Desco Europe Continuous Monitors do monitor the working surface ground. Note: the working surface must have a conductive layer such as Dual Layer
Rubber or Dissipative 3-Layer Vinyl or Micastat® Dissipative Laminate with conductive buried layers. Desco Europe Continuous Monitors are not recommended
for use with homogeneous matting.
Selection Considerations
"When considering constant monitors, the equipment cost including the wrist strap, maintenance and training cost, labor time for performing wrist
strap tests, and the potential failures due to non-functional wrist straps should be considered. A broken wrist strap may expose products to ESD over an
entire shift if it is checked only at shift change with wrist strap checker. Constant monitors may reduce the time people spend testing wrist straps before
each shift."(3)
- ANSI/ESD S1.1 Annex A3 Frequency of System Testing
- User guide CLC/TR 61340-5-2 Wrist Strap clause 4.7.2.4.4 Test frequency
- ESD TR 12-01 Technical Report Survey of Constant (Continuous) Monitors for Wrist Straps
- IEC 61340-5-1 Electrostatics - Part 5-1: Protection of electronic devices from electrostatic phenomena - General requirements