Help Identifying Inverted Fuse Symbol
That is the symbol for a "dummy fuse" used in a disconnecting fuse holder for the PTs.  Like these:



When a circuit is grounded (i.e. the "Neutral" in this case), you cannot have a fuse in it, but because this is for a meter, you want to use disconnecting style fuse holders so that the PT can be easily isolated for testing. So to avoid having the fuse in there to make it work, you use a "Neutral Dummy Fuse", like this: