Reports until 21:19, Thursday 12 June 2014
H1 ISC
keita.kawabe@LIGO.ORG - posted 21:19, Thursday 12 June 2014 - last comment - 11:49, Friday 13 June 2014(12349)
OMC DCPD and OMC QPD problem

When OMC DCPD and OMC QPD in-air cables are both disconnected at the field rack (but connected to the in-vac components via the feed throughs), there is a continuity between DCPD signal ground (pin15, 16 and 19, 20 of the DCPD cable) and the shield of OMC QPD cable. Bad.

DCPD cable shield is not connected to the DCPD signal ground nor OMC QPD cable shield. (Good.)

This cannot happen anywhere except inside the OMC suspension cage, I think. I'm asking Rich Abbott for advice.

Comments related to this report
rich.abbott@LIGO.ORG - 10:24, Friday 13 June 2014 (12353)ISC
Link 

The case of the DCPD is at ground potential, and is attached to the incoming DCPD shield.  This is OK as the DCPD box is not grounded, and represents the same situation as used in the RFPDs.  I would have to assume it's a DCPD cable to QPD cable shield short developed when two cables are bundled in close proximity.  A whisker must be sticking out somewhere.  

It should be fairly easy to track down if you still have physical access to the cabling.  Otherwise, provided neither cable is directly shorted to chamber ground, the loop area for electromagnetic pickup in this scenario is far smaller than it would be in a true chamber ground loop.  Small consolation if any.
rich.abbott@LIGO.ORG - 11:49, Friday 13 June 2014 (12356)
Link 

Actually Koji reminded me that we had a similar situation at LLO.  See LLO log number 10121.  The cable bracket holding the QPD and DCPD cable Mighty Mouse Connectors bridges the shields between these two cables.  In the end, measurements confirmed that this small loop pickup problem is not a show stopper.  We did conclusively find that any chamber ground on the OMC shields was a big problem.