The Intermatic AG3000 Surge Arrestor / Surge Suppressor device wired into my main breaker panel had a green indicator light that indicates correct operation. But the green light disappeared, indicating that the device was no longer providing protection, so I replaced it.
Afterwards, I opened up the failed device to see if there was any evidence of a surge (all the electronics in my house still appeared to be working) and to see what was inside it.
The surge suppression functionality is provided entirely by a pair of Mersen 150TPMOVSL (150 volt thermally protected MOV with “short leads” for the internal micro-switch) They are in series between the two 240v hot leads, with the neutral lead in the middle. (You can also use this device on a 120v circuit by connecting both black wires to the single live/hot wire, putting both MOV’s in parallel).
The microswitches are normally open, and the spec sheet says they close when the metal oxide disk inside fails. [This particular model does not include a visual pop-out indicator.] The indicator circuit board (normally soldered to the top of the MOV’s) is extremely simple, and is powered from neutral and one of the hot lines (120v AC) via two large ohm value resistors (I measured 63 VAC after the resistors) through a single diode. The microswitches are placed in parallel to the indicator light (which may be a LED of some type, it used a LED symbol next to a 93V-0 marking on the PCB) so that they short it out if they close.
Once I removed the indicator circuit board, the MOV micro-switches were open (non-conducting), so it is possible that the indicator light failed shorted and the MOV’s are still providing proper protection.