Mat Grounding

Q:

WHY DO WE RECOMMEND TO GROUND ESD ROLL MAT MATERIAL AT FREQUENT INTERVALS?

A:

THE ANSWER IS BASED ON DISCHARGE TIMES.

EXAMPLE

LETS TAKE A 28"x24' (5 SECTIONS) OF TYPE "A" MAT MATERIAL (NOTE: HOMOGENEOUS MATERIAL)

RESISTANCE IS THE ELECTRICAL PROPERTY DESCRIBING THE ABILITY TO RESITRICT

THE FLOW OF CURRENT (OR LIMIT THE NUMBER OF FREE ELECTRONS).

RTT = 6.3E+04 OHMS

(RESISTANCE FROM POINT TO POINT WITH A 10" SEPARATION AND @ 10 VOLTS)

RTT = 4.5E+05 OHMS

(RESISTANCE FROM POINT TO POINT WITH A 6' SEPARATION AND @ 10 VOLTS)

RTT = 1.8E+06 OHMS

(RESISTANCE FROM POINT TO POINT WITH A 24' SEPARATION AND @ 10 VOLTS)

THE VOLTAGE DISCHARGES AT A RATE OF:

V=V(ESD)e^(-t/RC)

WHERE t IS THE TIME, RC IS THE TME CONSTANT (RESISTANCE x CAPACITANCE)

INITIAL VOLTAGE (ESD VOLTAGE):5.0E+03 VOLTS

RESISTANCE @10" (RTT): R1 =6.3E+04 OHMS

RESISTANCE @6' (RTT): R2 =4.5E+05 OHMS

RESISTANCE @24' (RTT):R3 =1.8E+06 OHMS

CAPACITANCE: C =3.1E-10 FARADS

TIME CONSTANT: (R1)(C) =2.0E-05 SECONDS

NOTE: CAPACITANCE IS ESTIMATED ON PREVIOUS TEST ON TYPE G2 MATERIAL

YOU CAN SEE THAT A VOLTAGE OF 5000 VOLTS 6 FEET FROM A GROUND CORD WILL DISCHARGE 80 % OF THE ESD VOLTAGE (OVER 4,000 VOLTS) IN LESS THAN 0.2 MILLISECONDS. THIS IS QUITE QUICK, MAYBE TOO QUICK, BUT DOES NOT NEED TO GROUNDED EVERY 6 FEET. FOR EXAMPLE AT 24 FEET, THE DISCHARGE TIME WILL BE 2.6 MILLISECONDS AND THEREFOR NOT NEED AN ADDITIONAL GROUND CORD. WE WOULD RECOMMEND KEEPING THE DISCHARGE TIMES BELOW 100 MILLISECONDS WHICH IS WELL BELOW HUMAN MOVEMENT SPEEDS.

FOR CONDUCTIVE HOMOGENEOUS MAT MATERIALS, TYPICALLY ONE GROUND CORD IS SUFFICIENT. FOR TYPE A MAT MATERIAL, YOU WOULD NEED A SECOND GROUND CORD AFTER 355 LINEAR FEET BUT THEN

ANOTHER FACTOR COMES INTO PLAY. ANY ESD FLOOR SURFACE THAT IS OVER 60 SQUARE FEET, WE RECOMMEND 100 SQAURE FEET, WILL ACT AS A HUGE CAPACITOR WITH THE FLOOR, AND SUCK THE CHARGE OFF YOUR BODY (ASSUMING YOUR USING A PS FOOTGROUNDER OR EQUIV.).

CONSIDERING THE CAPACITANCE COUPLING OF A FLOOR SYSTEM,

AND THE CASE OF TYPE A AT 355 FEET LONG BY 28" WIDTH (800 SQUARE FEET),

WOULD THEN NOT REQUIRE A SECOND GROUND CORD.