PTCs on AC/mains line
Why is it that surface mount (SMT) PTC fuses can only be used on DC lines but through-hole PTC fuses can be used for both AC and DC?
  This came from a cautionary note on one of Bel Fuse's PTC application note documents.


This turns out to be a better question than it appeared to be at first :-).
The added reference to the Bel 0ZCM PPTC datasheet show that they do indicate that AC use is not appropriate for at least the 0ZCM series devices.

PTC or PPTC?

I have used the term PPTC below rather than PTC as PTC usually is taken to mean a device with positive temperature coefficient (which these devices DO have) but PPTC indicates a "polymeric" device whose action is based on the "melting" or softening of a polymer containing a matrix of conductive particles which are initially in close electrical contact but which lose contact with resultant very substantial increase of resistance upon heating. This mechanism is fundamental to an understanding of the various characteristics and limitations of the devices. 

AC/DC or DC only with SMT PPTC devices?


Bel Fuse says AC use is not a good idea
Mention of AC by other manufacturers is less common than mention of DC.  
Some manufacturers mention voltage specifications without AC / DC references.
Some (few) manufacturers specifically note suitability for DC or AC use.


Consideration of the method by which PPTCs operate and a (brief) resurvey of the various application documents does not suggest any fundamental mechanism which makes AC less applicable than DC. However, one can imagine various mechanisms which are potentially specific to a manufacturing process or device characteristic which may differ between AC and DC. For example only, the thermal time constants of PPTC devices as a whole device are large compared to AC mains cycle times. However, as current in a PPTC device will vary cyclically at AC frequency it is possible that micro-heating occurs at boundaries between conductive particles in the polymer matrix and that peak AC voltages thus may cause substantially more heating at the particle boundaries. 

As the datasheet is dated May 2019 it is likely that Bel would be happy for you to take up their invitation - " ... Use in AC voltage applications should be first discussed with Bel Fuse engineering." 

_______________________________________________

Tyco - AC acceptable:

THIS 13 page Tyco publication - "Fundamentals of PolySwitch Overcurrent
and Overtemperature Devices" shows images of TH (through hole) and SM (surface mount) parts, specifically mentions AC or DC use twice without other qualification (page 8), and in a selection guide (pages 11 &amp; 12) lists 3 families of Tyco SM PTCs with 250V max interruption voltage. While the table does NOT specify AC or DC for voltage the 250V rating and other information in the table "strongly suggests" that AC is the main focus in those 3 cases. ['Strong suggestion' is not the basis for good design - but can direct lines of probably useful inquiry]. 

Tyco say:


The test requires a power source (either AC or DC) capable of
supplying the maximum rms (root mean square) interrupt current specified for the device, at the maximum rms operating voltage specified for the test. The source voltage is controlled by the power supply; the source current is controlled by a load resistor. ... 


&amp;


... A single source may be used both to trip the device and to hold
it in the tripped state. Alternatively, one source may be used to
trip the device, and a second source to hold the device in the
tripped state. In either case, the source may be AC or DC.




Littlefuse - recommended use - "Linear AC/DC Adapter"

This 6 page Littlefuse POLYFUSE® PTC SELECTION GUIDE makes specific mention in the selection table on page 3 of SM PPTC AC/DC use. The phrase used is "Linear AC/DC Adapter" and it is just possible [tm] that they are only referring to use on the DC side of an AC to DC device but such a usage is "extremely unlikely" if it matters. [Again "extremely unlikely" is not a good basis for engineering design, but a guide to likely useful lines of inquiry]. 



Other

TE - 2 pages -  AC/DC not mentioned. PolySwitch Resettable Devices
Fundamentals

Littlefuse Fuse or PTC - no AC/DC mention