Under Voltage and Over Voltage Fluctuation

Use a servo controlled voltage stabiliser of adequate size.

The additional equipment needed is the telephone, and the job it is needed for is to complain to the utility company about over- and under-voltage fluctuations until such time as it has done something about it and cured the problem.

Well, it also depends from which country the poster is posting from, it some countries the main line voltage can fluctuate a lot.

"A customer who complains is doing you a favour" - Anon.

One common fault that occurs in a single phase service that uses a center tapped transformer from the utility is loss of connection to the center tap. In the US, most of the loads operate on 120 VAC from either side of the tap. Together, they provide 240 VAC. But if the neutral is missing, some circuits will "brown out" or experience "over voltage" depending on what else is on at the time to balance the resultant voltage divider for the 240 VAC feed.

A good solid connection to the center tap is essential.

check this

Under voltage Collapse Condition