Incorporation of 10 wt% 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) in polybutylene succinate (PBS) showed a significant improvement of fire behavior at the UL-94 test (3.2 mm): (i) V-0 rating and no ignition of the bar for PBS/DOPO with dripping but with no ignition of the cotton whereas (ii) no classification for neat PBS with dripping and with ignition of the cotton. To fully understand these two different phenomena, an optimized UL-94 instrumentation was developed. Thermocouples embedded in the bar measures heat gradient and infrared (IR) camera equipped with a specific filter permits estimating heat gradient in the bar and surface temperature of the falling drop respectively. The main advantage of this optimized UL-94 instrumentation is to get measurement in situ and in dynamic. This instrumentation has successfully captured the temperature evolution of a falling drop during a UL-94 test. It was shown that the drops of neat PBS exhibits a constant temperature of 453 ± 10 °C when detaching the bar. However the drops' temperature of PBS/DOPO varied during the UL-94 test when they detach the bar: the drops formed at around 403 ± 10 °C at the end of the first ignition, whereas at the end of second ignition they detach the bar at about 313 ± 15 °C. Neat PBS exhibited temperature at least 50 °C higher than PBS/DOPO when the drops touched the cotton: 435 ± 5 °C for neat PBS at the end of first ignition vs. 385 ± 10 °C for the PBS/DOPO at the end of first ignition vs. 295 ± 15 °C for the PBS/DOPO at the end of second ignition. This can explain why the drops of PBS can ignite the cotton whereas those of PBS/DOPO cannot. Additionally, IR camera images indicate the change of rheology by incorporation of DOPO into PBS. A measurement of viscosity by the rheometer showed that PBS/DOPO has lower viscosity than neat PBS.