All EX I/O chassis fans appear in DARM, and I have been using them as injections, along with other injections to investigate the voltage noise coupling to DARM. I have left for a couple of weeks so I wanted to enumerate changes that have not yet been undone (or made permanent).
1) Temporary ESD power supplies, HV, 48 and 18, have been set up just beside the field rack and grounded to both the rack and the beam tube.
2) The temporary power supply ground connection was moved from the field rack ground at the rear bottom of the field rack to the field rack chassis ground that is attached to the ground bar on the cable tray. This reduced the resistance between the ESD chassis and the power supply ground.
3) All IO chassis were grounded to the racks. This reduced the resistance between the IO chassis and the rack from very high values to less than 1 ohm.
Moving the ESD power supplies out to the field rack and grounding them in various ways did not change the coupling to DARM of the SEI, ISC, and SUS fans as well as temporary injections that affect the grounds. Also, I have not yet been able to find voltage differences, such as between the beam tube and the ESD shield ground, that have consistent coupling ratios to DARM for each of the different peaks (see figure) and so I cant yet predict the contribution of the broad band V noise to DARM.
Tagging @DetChar -- Derek's been pointing fingers at the 2023-era 283.91 Hz PCALX calibration line (documented in LHO:66268) in trying to investigate some low-level constant glitches that have appeared in recent summary page plots, eg 2023-02-08 Omicron Triggers. However, we were questioning this given that the PCALX is a single mono-chromatic excitation. Following other rabbit holes today, I re-discovered this aLOG covering IO chassis at the end-station -- and in particular to the h1susex and h1susauzex IO chassis fans cause lines visible in DARM. I would would be much more easily convinced that these off-the-shelf fans are mixing with each other, varying their speed, and glitching. There's already an FRS ticket in about this, FRS Ticket 12399 (which is how I found it), but I think it might be worth escalating this FRS ticket to be an IIET ticket so that we actually spend some money to fix these fans -- or at the very least re-dedicate person power to performing this investigation again with our new sensitivity and tying the investigation closer to the CBC group's glitch metrics to get rid of it. Also, all these IO chassis were upgraded to a -v5 version of the IO chassis design in 2021 -- see LHO:60058. So -- maybe the character / impact of these fans is now different?