During last week we've performed some test in order to disentangle as much as possible about the origin of the 50Hz peak in Virgo Dark Fringe.
Basically what we've done is to selectively supply buildings (NEB, WEB, CEB+MCB) using a DIESEL generator instead of standard ENEL IPS mains supply; this because the diesel generator is not able to maintain the exact 50Hz frequency, and we can thus appreciate the contribution of each building in the 50Hz peak visible in Dark Fringe.
Test was possible for NEB and WEB, but failed for CEB and has to be repeated (Diesel failure).
Here the relevant (Local) Timing:
NEB (see plot 1)
start Sep07,17-11:16:49 -> GPS: 1188811025
50Hz move to 50.7Hz
ENV_NEB_MAG sees mainly 50.7Hz (about 10nT), but also 50Hz (1/10 - why so high? to be checked)
stop Sep07,17-11:39:30 -> GPS: 1188812386
WEB (see plot 2)
start Sep07,17-11:49:36 -> GPS: 1188812992
50Hz move to 50.2Hz
ENV_WEB_MAG sees mainly 50.2Hz (about 10nT), but also 50Hz (1/10 - why so high? to be checked)
stop Sep07,17-12:04:09 -> GPS: 1188813865
CEB
due to diesel generator failure, test has been done plugging Emerson UPS from "rete_2" to "rete_1" (that is: from the old and noisy Merlin Gerin UPS to the standard ENEL IPS)
from about 11:13:35
to about 11:55:00 (ITF unlock)
Temporary conclusion: we have seen a new 50.x Hz peak growing in Dark Fringe during the first two tests, but the original 50Hz peak height remain at the same level.
More test are needed