Penn Elcom Rubber Feet

[dirkenstein]

OK, here's a little story for technicians with a serious side-note in why it's often unwise to re-purpose entirely innocuous objects for any kind of safety-related function.

 

I was fixing up an old piece of HP test gear. One of the PCBs had little rubber feet held on by screws used as spacers to keep it away from the adjacent earthed chassis. The rubber feet had partially perished, so I decided to use some small penn elcom rubber case feet to replace them. I attach the new feet after slicing a disc of rubber off the top to make them fit, and everthing assembles nicely, but the feet are still slightly wider than the originals. As everything fits mechanically, I assume this doesn't matter.

 

I power everything up, and it all seems to work OK until after 30s, a quiet sizzling noise is heard and the most atrocious stench emerges from the unit. I quickly turn things off and investigate, assuming I've managed to get the some component on the PCB to contact the chassis and let the magic smoke out.

 

I've fried a reasonable quantity of electronics in my time, but this smell is new and particularly offensive. Leaving a bit of time for any capacitors to discharge, I pull the newly-installled circuit board. It pongs something chronic, but no fried components or tracks are immediately visible. There is, however, a thin smear of carbon soot on the chassis behind the board.

 

I go and look at my newly-installed rubber feet. One of them looks entirely pristine, the other has a thin layer of soot on the bottom of it and its mounting screw. I assume the mounting screw has contacted the chassis. Nope- plenty of rubber below the screw, no contact possible. Although some of it seems to be mysteriously burnt, causing the atrocious smell. Even weirder, the mounting screw is connected to the ground plane on this board. I _really_ shouldn't be getting a significant PD between board and chassis earth. Bad things must be 'afoot'. High-voltage arcover? Has the CRT supply fed some ridiculous number of volts back onto the control board?

 

So I remove the newly-crispy rubber foot. Three discoveries are made:

 

a) There's a track with a little bit of conformal coating missing under the foot. It happens to be the 150v power rail.

b) Removing screws and washers from the rubber foot, there's a little tunnel burnt through the rubber.

c) The 250mA fuse for the 150v regulator has blown.

 

This rubber foot is about 3/8" thick. 150v really shouldn't be able to arc through that much rubber.

Time to investigate the HV supply.

 

And slowly, a somewhat bewildering realization dawns on me:

 

Just for giggles, I jab the rubber foot with my mutimeter. After all it is black, and there's an awful lot of carbon about after a tiny little burn.

 

Initial reading 240Kohms. I jab a little harder. Final reading: 3Kohms.

 

Oops.

 

Antistatic, coductive rubber feet.

 

Not labelled as such. The manufacturer just lists them as 'black rubber'.

 

Surprisingly conductive.

 

Too conductive to use a PCB spacers potentially in contact with any tracks on the PCB.

Dangerously conductive when in contact with a 150v rail.

 

Lesson learnt.

 

The next set of feet get bought with full materials specification. They're also slightly narrower, to stop contact with the 150v rail.

 

Consider the even more mysterious behaviours possible if these feet had been in contact with PCBs using lower voltages, where any failures are liely to be much more subtle.

 

The question is- would _you_ have tested the rubber feet for resistance before use?

[Ashley R]

I highly doubt it! Although I would have probably trimmed the sides down so they were the same size as the originals, I generally don't like anything touching the tracks/ components, only where its meant to touch.

[ramdram]

Of course, it's the very first thing I do...** laughs out loud **.

 

Thanks for heads up but from the moment you mentioned rubber feet then all began to become clear.

 

It might, ** laughs out loud **, be better to use proper standoffs with the white nylon spring clips that simply push into a hole on the board(s)?

 

This sort of thing?

 

http://www.keyelco.com/products/prod15.asp?SubCategoryID=101

[Richard P-W]

Reminds me of a moment in my youth when I powered up a mains-fed PCB (Linn Axis turntable for those who have encountered such a horror) only to realise a small antistatic bag was underneath. Oops!!

[Ashley R]

Of course, it's the very first thing I do...** laughs out loud **.

 

Thanks for heads up but from the moment you mentioned rubber feet then all began to become clear.

 

It might, ** laughs out loud **, be better to use proper standoffs with the white nylon spring clips that simply push into a hole on the board(s)?

 

This sort of thing?

 

http://www.keyelco.c...bCategoryID=101

 

ARGH I hate those things! Especially when one decides to fit 6 of them onto a relatively small board, makes it then a pain to get the board out, if you need to do something to it. I would go for the ones with holes in the ends for screws instead. Or just go with the proper rubber ones (may reduce vibrations or something?)

[ramdram]

Indeed you could AR and there are probably links off that first link, but, it might just be possible to lose the tiny screw inside the "unit".

 

Pros and cons to everything I suppose.