  In the past years, it has been observed at several beam test campaigns that
irradiated LGAD sensors break with a typical star shaped burn mark when
operated at voltages much lower than those at which they were safely operated
during laboratory tests. The study presented in this paper was designed to
determine the safe operating voltage that these sensors can withstand. Many
irradiated sensors from various producers were tested in two test beam
facilities, DESY (Hamburg) and CERN-SPS (Geneva), as part of ATLAS High
Granularity Timing Detector (HGTD) beam tests. The samples were placed in the
beam and kept under bias over a long period of time in order to reach a high
number of particles crossing each sensor. Both beam tests lead to a similar
conclusion, that these destructive events begin to occur when the average
electric field in the sensor becomes larger than 12 Volts per micrometre.
