Put a screw driver or large gauge wire between the A ,B,C,or D post and the U terminal and turn up the throttle. It should trip after a period of time. If the breaker is bad replace it with a suitable replacement. This is necessary to protect the transformer and will protect heavy bus wire but not thinner branch wires. For running modern engines add appropriate external protection,described here.
http://www.jcstudiosinc.com/BlogShowThread?id=486&categoryId=
Dale H