Insane AC wiring in NIM PSU

[ChristofferB]

Ok, this had me stumped for a while. I purchased a(nother) NIM (Nuclear Instrumentation) PSU from ebay, Ortec 402A. https://www.ebay.com/itm/184846344993

I already had a NIM connector backplane to mount on it from a Berthold NIM PSU that was FUBAR.

The NIM standard is very rigid, and specifies mechanical layout and connectors too, so I thought all was fine. Put the backplane (with on/off switch and neon lamps on a small front panel) on the PSU, and it turned on fine. DC voltage test points were within 10 mV. Fine.

Adding in a NIM module made the bin blow the house GFCI breaker...

The module drew +-12V, +-24V and 115VAC.

Another module, drawing no 115VAC was fine. (the psu downregulates mains 230V to 115)

After a lot of cursing, and reverse engineering the transplanted backplane to the PSU, turns out the 115V Neutral is just connected to the chassis, along with DC gnd. See attached schematic.

Here is the PSU manual with schematics, and an extracted schematic of how the backplane is "supposed" to be wired.

What is going on here? Am I missing something?

Thanks!

https://groups.nscl.msu.edu/nscl_library/manuals/eggortec/401-402A.pdf

[amyk]

Perhaps someone wanted a live chassis for some reason...? Having AC neutral connected to chassis was common in early equipment.

[ChristofferB]

I'd estimate the Berthold backplane-donor PSU no older than early 1970's, but maybe.

Oh, I forgot the most important question:
How to correct this? Can I just separate the 117V N from the chassis and call it good?

Thanks!

[TheMG]

My best guess is in its previous life it was powered through an isolation transformer, and they didn't want the neutral floating. Though it would have made more sense to make that connection at the isolation transformer rather than in the chassis.

Could be just a pure wiring mistake too?

Also in North America where you bought it from, that neutral-gnd connection would have gone completely unnoticed, as GFCIs are typically only installed on individual circuits for receptacles in locations where water is involved (kitchens, bathrooms, garages, outdoors), not for the whole building, especially in commercial/industrial locations where this likely was last used!

[ChristofferB]

That makes sense, thanks!

It looks like just disconnecting the N from the chassis connection would pretty much solve the issue.

[miken]

Having worked in the field as a student, I can say that physicists sometimes are in that "know enough to be dangerous" category when it comes to electricity. I'd forgotten that NIM actually has provisions for a line voltage on the bus... that's kind of nuts, but in an experiment you'd have much higher voltages running around for the photomultiplier tubes, and all sorts of fun dangerous things. The presence of an isolation transformer is a good guess, and we still have old radios and things floating around that would've been strictly two-conductor AC with a handy schematic to show how it was done. Or whoever owned last it ran it in a house without grounded sockets (plenty of those still around).

I'm horrified, of course, but not that surprised. People will get desperate to cure a ground loop, that sort of thing.