Found some really old extension cables (+20 years). Is it safe to use them? I was worried that the plastic could deteriorate with time, but I have never seen a power cable with a best-by date.
No, no 'Best By' date codes LOL. However, your instincts are on track. As the plastic jacket ages it becomes brittle and can crack allowing in moisture and presenting an obvious hazard. However, if with inspection the jacket appears to be without defect; if it feels relatively flexible and supple then it may be ok. The cord ends need to be inspected as well. Any signs of heat, burning or serious abuse chuck em. Thanks for being safety conscious. P.
That wouldn't make any sense. Cords that are well kept and stored could be good for a very long time, whereas ones exposed to UV, weather, stretching, abrasion, chewing etc. could be unsafe after a very short time.
Ultimately, you should do exactly what they say you shouldn't do with child car seats: physically inspect the cable for discoloration, abrasion or cracking, flexiblity, etc.