Beginning of june ,the supply dropped to almost zero volts and then came up to 100volts ,after a bit it went down to 80 volts and then almost zero.It was about an hour before it came back in stages to 100volts and a bit later back up to 230 volts.I have a (rescued from a scrapyard!) voltmeter permanently on and it has almost always been 230 volts

 So what could have caused this?It only seemed to affect half of our street so looks like a phase problem?Certain it could not be tap changing problems!

         My house is 12k north of Pattaya.

Lost HV phase would give you a voltage around 100-130V, could have been a cable down and flapping around on the ground until the breaker went out. Many HV breakers are auto-reclose in the hope of blowing the fault (snake, monkey) out.