sentences:  Two current collectors are required in order to supply and return current, because the return current cannot pass to the ground (as is done by streetcars on rails) since trolleytrucks use tires that are insulators.Lower powered trucks, such as might be seen on the streets of a city, tend to use trolley poles for current collection.Higher powered trucks, such as those used for large construction or mining projects, may exceed the power capacity of trolley poles and have to use pantographs instead.