AMC1200: ESD Failure

Part Number:AMC1200

Hi team,

  My customer has been using AMC1200 in central air conditioner system for bus voltage sensing. Currently they find that AMC1200 is easily damaged when doing -8KV SED testing, while the interface where ESD is injected is far away from AMC1200. The only electrical connection is Ground. AMC1200 is found to have VCC and GND pin shorted togehter. 

  I'm not familier with ESD testing and wonders in order to pinpoint the root cause where I can start from? Thanks.

ESD is injected as below(PWM drive signal interface)

Regards,

Patrick