Breaker filler plate, what is that? This is a common question I receive on home inspections...and for good reason. I find open knockouts and breaker slots on main panels and remote panels (sub-panels) on a very regular basis. Simply put a knockout or breaker filler plate is meant to block an opening within an elecrical panel.
Openings can exist for a number of reasons but one way or another a knockout was removed accidentally, or a change in the panel was made over its life, whether it be from a remodel, new wiring, moving circuits around, or a different change. Nonetheless, when a change has been made in the panel and an opening still exists it is always recommended a filler plate be installed.
The reasons for this are many. First, you don't want someone accidentally placing a finger inside a panel, this could lead to a painful shock and serious bodily harm. Second, you don't want animals entering the panel and potenially chewing away at wires, seems like it wouldn't be possible with some of the very small holes present in a panel, but I've seen it, and on more than one occassion; let's just say mice aren't very big. Lastly, you don't want insects creating a nice nest in your panel. This also another scenario I have seen on many occassions. It is not overly uncommon for me to open a panel and find a nice nest of bees waiting to sting me. So, to make a short point, if you have breaker slots or knockouts in your panel, FILL THEM! Filler plates can be found at most harware and big-box retailers and are very inexpensive, some costing as little as $2-$4.00 for a small pack of 5-6.