During the day I'm a .gov contractor but I've been a volunteer EMT since May 2001. My department works in conjunction with our PD and have several guys on our squad who also have a Tac Med cert (myself included) and have consequently been given the green light to accompany PD into "warm zones" provided we are wearing armor.
My personal kit is a First Spear Strandhogg with hybrid ceramic/poly 3++ plates that stop all 5.56 rounds (IMO the ideal protection level for all CONUS operations) but that was purchased out of pocket. We are currently in the planning stages of grant writing to get funding and soliciting RFIs for a department wide armor acquisition.
I know this is crazy talk, but AR500 plates are honestly not a terrible idea for fire/EMS. They are indestructible and they are cost effective. If your department is anything like mine, gear gets used and abused pretty heavily. We run about 4500 calls/year out of a single HQ. I know I take excellent care of my own plates, but I would not trust the other ~70 members to take the same care. You can get level IV ceramic Hesco plates for about $170 each. They are excellent plates for the money and were it not for their heavy weight, they would honestly be great even for going to war. But without in place protocols for annual xray inspection, I think leaving them to bounce around on rigs would open the Department up to liability. Steel doesn't require anything other than visual inspection to my knowledge. Also from a procurement perspective, I know that we are going to be buying everything with a 1 time Federal grant. I know that we would not be able to likely get another grant to replace ceramic plates in 3 to 5 years. So steel wins again in that respect.
Seeing as EMTs should never be coming under direct fire anyway unless something has gone horribly wrong, I actually am not terribly opposed to equipping rigs with AR 500. But it should be a conscious decision with informed participants.
Final caveat: If you are equipping EMTs to join entry teams, then this advice changes completely. Everything I said above applies to bog standard Street EMT's who will be staging several blocks away (at least) from any active shooter situation.