[Kokeyama and Ge]
As a part of the ground hunting campaign, we started for IMC. The attached picture shows what we figured out today.
The first concern is that the BNC shells of the 3-axis piezo driver (used for the WFS pointing steerings) are all connected. Also the PZT mirror mounts connected to this driver is short to the optical table. They create small multiple GND loops (there are two PZT mirrors/mounts).
The second concern is that the two WFSs and one RFPD are almost connected to the optical table too. Because the black paint of the bottom part of them are faded away (see the second attached picture), the PD box and the table are almost attached. We tried to disconnect using plastic white tapes to separate the box from the table, but it didn't work very much. In the beggining, WFS2 was completely short to the table first and with the tape, it is still somewhat short (the multimeter shows 300 ohm, but it varies depending on where of the PD box. Some area shows 0 ohm to the table). The other WFS and the RFPD is also connected with 300ohm to the table. We must disconncet them from the table, otherwise we create large multiple GND loops.
With clues from Tomura-san and Miyakawa-san, I figured out the grounding cuase for RFPDs and WFSs.
Note that this is related to all RFPDs and WFSs.
In the design (LIGO-D1101177), the PD box and the base are insulated with an aluminum plate. However, as noted in the main post, WFS2 box and the base was electrically connected in a somewhat sattle way. It was because there were not insulate washers for the screws attaching the base and the box (see the yellow circle of the attached screenshot). If the black coat (anodized surface) of the base is fade away, the base and the box are electrically connected through the screws. So, depending on the condition of the base surface, in the unfortunate case, the box are connected to the optical table, and it possibly make large ground loops.
To make sure the box doesn't electrically connected, we must use the insulated washer for those screws. Once the fiscal year of 2019 start, I will order bunch of the insulated washers and we will do the PD insulating campaign.
For WFS2, as a temporary treatment, I made two insulated washers with kapton tapes. Now the box is disconnected from the optical table (but they should be replaced in near future). The PDs on the IMC REFL table should be fine now.
Also, I quickly checked the PDs on the IFO REFL. The WFSs seemed fine but the RFPDs are electrically connected to the optical table.
[Ge, Kokeyama]
Insulating washwers were inserted for IMC REFL WFSs, IMC REFL RFPD, PMC RFPD, RefCav RFPD. The latter three weren't short to the optical tables but we did it anyway just to make sure not to short even when the surface coats are deteorated.