Why does mains LED driver have surge protection circuit?

[treez]

Hello,
Why is the surge protector, R26/Q4 in the schematic of page 7 of the below link (DER318)?

Page 9 of DER318 says its to stop damage due to line surges. -In this case, why haven't all the Mains LED drivers got such protection?
Is the R26/Q4 circuit put there to stop damage in case the customer accidentally connects it to a triac dimmer?
The powerint site wouldn't post my question to their forum

http://www.powerint.com/sites/default/files/PDFFiles/der318.pdf

 

[FvM]

It's said why - to handle IEC 61000-4-5 surge level 2 (1 kV).

 

[treez]

every PSU has to be able to handle that that's connected to mains...and not all of them have that damping resistor/fet.

 

[FvM]

Subject to detail analysis, most PSU (including PFC types) have larger input capacitors, so they absorb the surge pulses more easily. Switchers with discrete MOSFET can be also expected to have higher Vds breakdown voltages and avalanche capability to absorb pulses of low and medium energy.

 

[treez]

I see what you mean....incidentally the TVS, VR11, is P6KE400A
**broken link removed**
The higher breakdown voltage for this is 420V, which is above the ratings of C4 and C5, so those caps could get zapped by an overvoltage.
That is sure to seriously reduce the lifetime of these caps