It wasn’t Dragnet, but an old time radio program I heard last night.
It centered on a clock that was somehow used to determine an alibi (I missed the beginning)
Anyhow, the detective knew it was planted - because the victim lived in a area of New York that used DC and the clock was an AC clock.
I knew DC was used a long time ago, but did not know it was used as late as the 1950s. The last DC customer in NYC was in 2007:
http://www.coned.com/newsroom/news/pr20071115.asp
Brian