Ashok:  Regarding the type of resistor needed.  Typically a one-megohm,
one-quarter watt 20% carbon resistor will suffice.  Using a resistor in
series with the table surface and the floor mat are desirable, but depend on
the type of materials that are used for each.  Remember that the resistor in
wrist straps, table tops and floor mats has two purposes.
- Purpose 1 is to protect the person from electrical shock by acting as a
current limiter.  The human body (mostly fluid tissue) will react with
perception, surprise and reflex action at currents up to twenty-one
milliamps, between 21 and 40 milliamps the person will experience muscular
inhibition (can't let go of the voltage source), from 40 to 100 milliamps
respiratory block occurs and current levels over 100 milliamps are usually
fatal. [Current values based on MIL-STD-454, canceled by DOD].  Standing a
person on a floormat that goes directly to ground (without a current limiting
resistor) may be a bad practice if the floormat has a low intrinsic
resistance value.  If however the floormat has a high resistance value it
would be OK to install without a current limiting resistor.  Same goes for
table tops with static dissipative or conductive laminate.  See ASTM D257 for
test methods for various materials.
- Purpose 2 is to protect the equipment.  Recall that the human body has (on
average) a mean capacitance of 100 picofarad (based on human body model
analysis) and the value of resistance selected determines the time constant
of the RC circuit that you are discharging.  When considering the discharge
rate you should include the effect of the person (with their one-megohm wrist
strap) standing on a mat that is connected to ground and being in
electrically intimate contact with a grounded table surface.  If wrist strap,
floor mat and table surface all have one megohm resistors, then the effective
circuit resistance would be 0.3 megohm.  If the table top and floor mat have
substantially lower resistance values the circuit resistance, and hence
discharge time and current density for a given voltage, would be lower.  The
maximum current density of devices that you are trying to protect may be in
the nanocoulomb range and limiting current density would be a significant
consideration.
- Bottom line, use an inexpensive carbon resistor to ground floor mats and
table tops.
Hope the above helps, sorry about the length of the reply (hate for ESD
problems to be taken for granted).
Regards, Jim Moffitt, Technical Director, Electronics Training Advantage

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