From: tethys-request@mbari.org on behalf of Kieft, Brian [bkieft@mbari.org]
Sent: Friday, March 04, 2011 1:00 PM
To: tethys@mbari.org
Subject: [tethys] tethys ground fault

Hi there,

 

 Another ground fault was discovered on the Tethys vehicle yesterday. It was caused by a short on backplane 1 (or “B”). Some of the leads on the backplane board were not trimmed and the U4 pin 12 side of JMP4 was contacting the bracket through the protective tape. This part was providing 3.3V to the entire chassis (including end bells) and could source up to 100mA of current. It was on continuously to the best of my knowledge. The return path was the board ground dropweight connection that sits in seawater on the aft end of the vehicle. All backplanes have since been trimmed to prevent this problem from happening again in the future. If this problem was happening prior to the Freewave/NAL bracket being isolated, it would have been masked by this less resistive path to ground and the 330 mW would have been dissipated by U4 (I believe it can dissipate up to 500mW). 

 

Next we will find out if this is enough to cause the damage we saw over the past 7 days. In the mean time, work is being done to get the GF detection circuit fixed to the point where it can detect a fault from one of the power supplies to seawater. 

 

Any suggestions, corrections, ideas, etc? 

 

 

Brian