ADE7880, Can't get ZXTOVA, ZXTOVB, ZXTOVC flag at STATUS1

Hello,

I am new to ADE7880.

I enable ZXTOVA, ZXTOVB, and ZXTOVC bit at MASK1 register. I expected to get ZXTO, ZXTOVB, ZXTOVB flag if I removed the voltage. I use the default value of ZXTOUT, didn't change the value. But I never got any one of those ZXTOVx flags. From the voltage RMS value readings, I can tell the changes on the voltage, but those flags on STATUS! register were never been set to 1. Bit 7:0 of STATUS1 register were always zero. Do I miss any setup here?

thank you very much for the help.

Karen