Mana sheildFollow

I have been practicing my AoE grinding in Hillsbrad (Im a lvl 29). I hit mana shield and run in to agro all of the melee mobs. But sometimes mana shield last allot longer and sometimes in the first few seconds I lose my shield.

My question is what makes me lose my mana shield. I know no mana will do it and after a minute will also but is there anything else that will drop my shield?

Ahh that makes sense thank you. O that brings up one more question, is that 210 cumulative damage or 210 in one shot?