Today we have done at NEB the same UPS sequential switch-off done yesterday at WEB.

Timing is:

around 10:10:00 Paolo-san deactivated the 48.3Hz NEB crossbar resonance damper, in order to see again the line in Hrec

before 10:33:00 UTC both UPS are ON, as usual

10:33:00   UPS DX OFF
13:28:00   UPS DX ON
13:35:00   UPS SX OFF
15:50:00   UPS SX ON

after 16:50:00 both UPS are again ON, as usual.

The effect in Hrec seems not relevant as the one seen yesterday at WEB, while we see in ENV_NEB_UPS_VOLT sensors the same "cleaning" effect.

Also, in one magnetometer (ENV_NEB_MAG_V, that is close to the tower) we see that two lines that were around 52 and 54Hz (50Hz mains sidebands?) have a jump in frequency. This effect has been noticed at WEB too, with a group of lines that are just above 50Hz. It seems related to the real change in mains amplitude (about 0.5Vrms), that happens while changing from using one UPS only or both in parallel.

Attached some plots