This pretty much is the definition of the WTF tag. Why would a sewer grate be charged to 480 volts? That seems like a bad idea, imo. (I took this photo on 7/11/2010 in Minneapolis off of 3rd Avenue downtown.)
If you work for the city of Minneapolis and have any idea what this is about, please leave me a message in comments.
Here is a good response from Reddit user mcki0154
Ok….Here to save the day…As a resident of Minneapolis I can answer this easily. The storm sewer has a heating coil wrapped around it in order to keep the sewer from bursting due to a cold snap or heavy freeze during the winter months. As xoites said, the sign is there to warn engineers and workers of potential dangers b/c only some pipes (particularly storm sewers with less turbidity) are prone to freezing while others (high traffic) do not freeze b/c of constant rushing water (like how a river does not completely freeze)