Why are MOVs rated for mains voltages chosen for earth-to-signal protection in PoE PSE applications, as recommended in TI's SLUA818?

Question

I wondered why TI in their application note SLUA818 says MOVs rated for mains voltages must be used between signals and earth:

Therefore, based on the above, the MOV used for the earthed PSE must have an allowable operating voltage of at least 216 VRMS when installed in an area where the nominal AC mains is less than 130 V and at least 432 VRMS when installed in an area where the nominal AC mains is greater than 130 V. This report will assume that the PSE is installed in an area where the nominal AC mains are less than 130 V, which requires an MOV with an allowable operating voltage of at least 230 VRMS (standard value).

While researching surge protection in PoE PSE switches, I disassembled several PoE PSE ethernet switches and found that some use thyristor surge suppressors instead of MOVs. However, the thyristors suppressors were rated only slightly above the typical PoE voltage of 54V. Any answers are welcome!

Answer 1

The operative phrase in §6.1.2.1 Requirements is, "Surge suppressors that bridge the insulation shall have a minimum rated operating voltage Uop... of Uop = Upeak + ΔUsp + ΔUsa."

That means, as I understand it, a varistor or gas tube from mains line-to-line, or from a line-to-ground, must have that voltage rating, and a protection device from signal line-to-signal line could have a lower rating.

As a practical matter, the capacitance of a MOV would likely interfere with high-speed signals, though it might have been acceptable for POTS over twisted pairs.