We repeated the test (moving the chiller's temperature by half degree), having a look at others signals: the effect (100Hz sidebands moving) is well visible in:
 

• CEB environmental magnetometers

• local magnetic probes (the one currently below SPRB)

• the electric ground noise probe (sensing the voltage drop between metallic towers body and racks in INJ  EE_room)

• the UPS "sniffer" (a sample of the UPS voltage that supply the whole Virgo experimental area in CEB)

Worth noticing that:
 

• moving the temperature setpoint of both chillers affect the temperature in a different way (different time constant), allowing a better understand of who is who (which sidebands is related to which chiller)

• both chillers sidebands are visible in magnetic sensors, as well as in the ground electric noise probe

• only ONE of the two chillers sideband is visible in UPS sniffer

• nothing relevant is visible in IPS sniffer

Conclusions:

No definitive conclusion yet, we know where the problem begin (TCS chillers) and where it comes (DARM), but the path is unclear; could be a magnetic coupling, could be an "electrical" coupling (voltage drops un UPS line). For sure having this kind of signal (100Hz sidebands) well visible in a distant room (EE_room) as "voltage drop" between ground is not a good sign. We plan to repeat this test after all EE_room racks have been grounded (they are not, for the time being).

Many other unwanted signals seems present as sidebands around Mains harmonics, and these could have origin from other powerful devices (e.g. the beam-dump chiller) in EE_room, affecting DARM all togheter.