When selecting over-voltage protection (OVP) devices (most commonly an MOV or TVS diode) for use with standard AC utility power, it’s critical to choose a device with a suitable voltage rating; parts with too high a rating won’t provide effective protection, and parts with too low a rating will promptly burn out and/or become a fire hazard.
AC line power is most commonly discussed in terms of RMS values, rather than peak values. The standard 120VAC power in the US actually has a peak amplitude of about 170 volts-the conversion factor between RMS and peak for a sine wave is approximately 1.414 (the square root of 2).
Additionally, the RMS values of standard utility service are nominal and come with a tolerance, typically taken to be +/- 10% in places with well-developed infrastructure. Thus, in order to avoid unwanted conduction at the peaks of the AC waveform under routine high-line conditions, OVP devices should be chosen such that they will not activate at voltages lower than 1.56 times the nominal AC line value. For a nominal 120V service in North America, this translates to about 188 volts, and a slightly higher value is advisable.
Since OVP devices are characterized in differing ways, it’s vitally important to understand whether a given specification refers to a peak or an RMS value; if the parameter doesn’t say “AC,” assume it’s referring to a DC or peak value, and base voltage selections on the 1.56*RMS rule. For example, if choosing a TVS diode for use on a typical 120V line, pick one with a reverse voltage standoff no less than 188V. A slightly higher value is suggested, as insurance against unnecessary failure.
Regarding MOV-based devices, a more thorough discussion of selection criteria can be found in this post.
Regarding OVP devices in general, a few general principles are worth bearing in mind:
- They’re sloppy and imprecise: A significant guard band between the nominal OVP actuation value and the poof voltage of the protected systems needs to be allowed for.
- They have a bad habit of failing short-circuit, so be sure to provide over-current protection upstream, to avoid burning the place down when that happens.
- Temperature and threshold voltages tend to be inversely related, the warmer a device gets, the lower the voltage at which it actuates, which means that it will tend to get hotter still… Put differently, you really, really, really do not want to pick an OVP device with too low a nominal value.