Explosive reactive armor is that brick-looking stuff that the Soviets loved covering their tanks in during the last days of the Cold War:
Kharkov Stronk!
I admit not knowing much about ERA, but a quick check around various sources has impressed the following things on me:
-ERA is, pound for pound, the most effective armor technology that exists. Nothing else even comes remotely close. First-generation Israeli Blazer ERA was estimated to have a mass efficiency (vs RHA) of 20-24 vs HEAT threats. That's insane. The very best modern steel