1. Isolation Voltage: The 4N29 offers 5300 Vrms, while the 4N29M provides 4170 Vrms. This represents an approximately 21% reduction in electrical isolation strength for the 4N29M. In high‑voltage applications or those with stringent safety isolation requirements—such as medical equipment or industrial power supplies—direct substitution could reduce the system’s safety margin. It is necessary to reassess whether the insulation safety standards are still satisfied. 2. Operating Temperature Range: The 4N29 is rated for -55 °C to 100 °C, whereas the 4N29M is specified for -40 °C to 100 °C. This indicates that the low‑temperature performance of the 4N29M is limited, making it unsuitable for severe cold environments or high‑reliability military and aerospace applications. In general industrial or consumer electronics, this difference is usually acceptable.