How common is a floating 0v reference labeled 'GND'? Major trap! - Page 1
Forum section: electronics / testgear

author: rx8pilot
author_rank: Super Contributor
timestamp: May 27, 2017, 10:31:15 pm
content: I recently got an 80's vintage HP8800 mainframe with a pulse generator and a few other modules from the eBay. When I plugged it in - everything seemed to be working and I was getting some fast edge pulses just like I wanted. After about 30min or so - POP! followed by a plume of smoke. One of the input filter caps exploded. The repair was rather straightforward and I just got all the new caps in place. Here is the question part.....When I was checking the PSU outside of the mainframe I wanted to check all the outputs and the various test points to make sure it was still operating correctly. There are a number of glorious test points that make it quite easy to check out when you have the schematic and all the test data HP publishes. One of the test points is marked GND so I (for a moment) assumed it was actually grounded to the chassis and therefore the actual lab ground. After looking at the schematic for a moment, I notice the GND test point is actually floating, not grounded. It is floating at 160VDC! Not a trivial voltage if you connected a scope to it without a completely isolated transformer. Seems like a wicked trap to set for the bench technicians that will come across this unit.Is it common to mark something as GND when it is not connected to ground? These units were made by HP Germany, maybe the rules and common practice are a little different (at least in the 80's). I do not do many repairs of old test gear, but this was a surprise lesson to learn. Glad I did not blow anything up.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 28, 2017, 12:06:44 am
content: Ha! Another victim of the Rifa capsWas it definitely 160V above zero potential, or just floating? I would have made the same assumption as you, i.e. that ground meant something close to 0V, or chassis potential.

author: Monkeh
author_rank: Super Contributor
timestamp: May 28, 2017, 01:49:36 am
content: Never assume a circuit ground is connected to 'real' ground!Consider the case of a double insulated piece of equipment with no mains earth contact.

author: rx8pilot
author_rank: Super Contributor
timestamp: May 28, 2017, 01:58:57 am
content: Quote from: Monkeh on May 28, 2017, 01:49:36 amTotally. It just seems strange to label it that way instead of REF or similar. I was just barely dubious enough to look closer and measure it - it could have been a smokey dayThankful for the availability of the schematics.The good news is that it is all back together and tested. I have to say that this 8800 frame is quite a puzzle and easy to mess up. They also do things like riveting the ground wire to the chassis which makes it a pain to take apart. I ended up drilling the rivet so I could keep the wire and eye connector intact then used a normal screw-star washer-nut arrangement.

author: Monkeh
author_rank: Super Contributor
timestamp: May 28, 2017, 02:00:10 am
content: Riveted ground dates it, they don't get away with that any more.

author: tautech
author_rank: Super Contributor
timestamp: May 28, 2017, 07:36:43 am
content: Did you get to the bottom of this apparent 160V above chassis GND ?Is there continuity between that pin and chassis and/or mains earth with the power off ?Did you use a lowZ DMM for the measurement ?

author: dimkasta
author_rank: Regular Contributor
timestamp: May 30, 2017, 10:31:43 am
content: Is it 160V directly referenced to ground?Or 160V parasitics measured like what you get when you measure the chassis of a double insulated device?

author: rx8pilot
author_rank: Super Contributor
timestamp: May 30, 2017, 02:39:24 pm
content: Quote from: tautech on May 28, 2017, 07:36:43 amI did not investigate very far. Once I was able to measure all the outputs successfully, I reassembled the unit. There was no continuity from the GND test point to the chassis or the ground pin. The chassis and the ground pin are definitely connected and the 160v was measured from the chassis to the GND test point. I did not test to see how much current would flow and the measurement was with a HiZ DMM (Keithley 2700).