Fowler Associates Labs

Response from Jeremy Smallwood
www.static-sol.com

The thinking on limits for electrostatic fields and voltages in EPAs has been changing over the years.

European colleagues should note that the EN100015 has now been superceded by EN61340-5-1, which is identical to IEC61340-5-1. This recommends that ESDS should not be exposed to electrostatic fields greater than 10 kV/m, and gives a limit of 100V as an alternative. Note that these values are from 61340-5-2, the User Guide, and have the status of "Recommendations" rather than being mandatory.

EN61340-5-1 is more flexible than EN100015, and gives the ESD Coordinator the prerogative to set their own limits based on their judgement.Use this freedom with caution - it is easy to be caught out! In this case, one needs to assess the risk of damage, including factors such as device sensitivity, and likelihood that a device may be brought near the field source, or that some object, tool or device may be charged by induction in the field, and subsequently approach an ESDS to cause ESD.

To try to measure surface voltage is usually rather dubious in practice. The approach of the measuring instrument is likely to affect the surface voltage which we are trying to measure!

Note also that the connection between such surface voltages and device damage is unclear. The "sensitivity voltage" quoted for devices normally relates to a human body model test, where a 100pF capacitor is charged and discharged into the device via a 1500 ohm series resistor. This discharge has significantly different characteristics than an ESD event from a charged insulator. Beware of possible discharges from charged conductors, though - these can be much more damaging than the human body model ESD!

US colleagues should note that ANSI/ESD 20:20 merely demands that "insulators that have electrostatic fields that exceed 2000 Volts should be kept at a minimum distance of 12 inches from ESDS..."