Thread: Solving static electricity problems by grounding the frame
View Single Post
  #17   Spotlight this post!  
Unread 12-02-2009, 11:10
Russ Beavis Russ Beavis is offline
Registered User
no team
  
Join Date: Nov 2005
Location: Manchester, NH - DEKA R&D Corp.
Posts: 341
Russ Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond reputeRuss Beavis has a reputation beyond repute
Re: Solving static electricity problems by grounding the frame
The cRIO isn't ESD sensitive - the Driver Station is. Have you ever caused the cRIO to "fail" with an ESD event?

The auto industry is ALWAYS doing safety vs. cost analyses. There are plenty of single-point failures that can result in dangerous situations but they've done everything "reasonable" to ensure that such events are unlikely. Imagine, for example, that your steering rack snaps or your brake pedal becomes disconnected. Automobiles are NOT fail-safe under every circumstance.

I don't agree with the rationale provided in Section 8 regarding the need for chassis isolation. Collisions would need to have multi-point/multi-differential voltage contact to get really exciting. For example, birds don't seem to mind sitting on high tension wires. Current needs to have a return path in order to flow.

I do, however, agree with the FIRST isolation rule for this very specific reason - if the chassis is grounded and ANY cable touches it, current will flow.

The chassis current will be limited by the breakers in most such failures but there are arcs that can be struck and maintained at < 40 amps. I've personally experienced this in non-FIRST applications. This is one of the primary reasons why UL often limits low-voltage devices to 8 amps.

The most dangerous instance, by far, is when the battery shakes loose and its positive rail touches the chassis. Now THAT'S exciting - a dead short on a 12V/18AHr lead acid battery!! The 120A main breaker won't help you then.

The second most exciting failure is when the output from the 120A main breaker is shorted to the chassis. That breaker takes a while (relatively speaking) to open since the short circuit current for the battery is "only" a few hundred amps. That's a lot of damage before the breaker opens.

The auto industry and PC industry have obviously weighed the pros and cons. I would NEVER consider shorting my car battery's positive output to the chassis to test for isolation and breaker operation but, if you care to try this on your own car, please post the video on Youtube for everyone to watch.

Russ