ESD mat is earthed without a formal earth connection

We have an ESD mat on a long worktop with a plastic formica type surface. The worktop surface is 1 metre by 5 metres, and the ESD mat covering it is the same size.

The ESD mat is one of those with the black conductive underside, and light blue static dissipative topside. If I measure resistance from the black underside to earth ground, then its just a few Megohms, even though the ESD mat is not actually connected to earth anywhere (non earth poppers). How is this?

1) what in the world does “non earth poppers” mean? Poppers from outer space? What’s a popper?
2) youve either got a defective meter, a conductive formica table, or your meter is measuring the ac field picked up by the large antenna that is your mat, not resistance.

Ohmmeter might be fooled by AC on the
mat or "ground", it just reads voltage really
(imposing a small current, very small on
megohms range). Pickup from overhead
fluorescents' hum? Pickup on the "ground"
point from co-routed AC lines?

treez said:

If I measure resistance from the black underside to earth ground, then its just a few Megohms, even though the ESD mat is not actually connected to earth anywhere (non earth poppers). How is this?

Click to expand...

Bulk resistivity of common plastics is around 10^10 (ohm-meter) and the surface resistivity can be similar (10^10 to 10^13) but can be much lower if the humidity is high.

The notion that it is not actually connected to the mother earth is not correct; I suspect that the humidity level is above 80% in the region and you do not need ESD mats at all.

The initial resistance reading of a few M-ohms is also possible due to charging currents. You are charging a capacitor via a large resistor and the initial current values can be misleading. How did you measure?

To do a good test: Take few (as many as you can) strong glass drinking glasses and clean them thoroughly and put them in the oven for 1 hour at 120C (not Fahrenheit) and put them (inverted) under the legs of the table. Wear plastic gloves while taking measurements. See how the readings slowly keep on increasing with time.