I have measured my heartrate at over 240 bpm. Doctors checked out my heart, one of my valves is weird, and my EKG looks like I'm having a heart attack. My doctor actually told me to carry a reduced size copy of my EKG so I'm not treated for a heart attack if I get knocked out.
However, at 240+ bpm, you know something is wrong. I feel quesy, light headed, have no strength, can barely see, and it's all I can do not to topple over. And that's if I'm just walking around. On the bike I have to stop. It happens spontaneously every now and then. And in a beat or two everything goes back to normal.
If you don't have those symptoms it's some errant signal, whether from your chest band or from something else.
cdr