I recently purchase a used Tripp-Lite IS250 isolation transformer off of eBay. Here is the datasheet. It's a fairly open secret that these are not true isolation transformers suitable for a tech bench, in that they bond neutral to ground off the secondary winding and also run the mains ground straight through to the output ground pin. These are typically marketed as "power conditioners" that remove noise (via a Faraday shield & the mentioned neutral-to-ground bonding) and protect sensitive equipment (typically medical) against surges.
I am modifying mine into a tech-bench isolation transformer that provides true ground isolation by removing the neutral-to-ground bonding, installing an isolated ground receptacle, and adding a switch to flip on/off the mains ground to output ground depending on my use case. Upon opening the device up, I noticed that there were two capacitor and a MOV installed across the back of the outlet receptacle (one in the top outlet and another in the bottom). The capacitors are MPP film rated at 2 µF 400V, while the MOV is a 130VAC varistor. What function are these serving, and how are they going about that? Photos below.
It seems fairly obvious to me that the varistor is there for surge suppression/protection, but wasn't 100% sure why, as it seems to be in addition to a 2-amp circuit breaker installed in the case. Why would they need both? Regarding the capacitors, I'm much less sure. I'm assuming they're for some kind of filtering - but of what? It was my understanding that AC "passes through" capacitors. Are these to prevent some sort of DC bias in the AC signal, or filtering out noise on the AC signal itself? I thought the noise would be taken care of by the Faraday shield.
Any response that explains what & how these capacitors are filtering, and what is causing the signal they are filtering would be greatly appreciated. I've not been able to find much on the topic besides generic answers. Thanks.