Earthing and Safety

[dknguyen]

A question I have hijacked from this thread
https://www.electro-tech-online.com/threads/240v-ac-that-will-not-give-shock.107359/

With regards to ESD and earthing:

"He told a story about an ESD episode at NASA that killed an engineer. There was a solid rocket they were working on complete with ignition squib. To make things 'safe', an engineer tied the two leads to the squib together (should have stopped there) and then tied the leads to ground. When they lifted the plastic shroud off the rocket to work on it they generated enough charge to cause a spark to jump from the squib casing to the leads - igniting the squib which started the rocket motor at a bad time. The point of this story is that grounding everything with conductors is not the way to approach ESD control; you must stop the generation of charges if possible and SLOWLY bleed off any charges you can't prevent. "

What I don't understand about this one is why would the rocket ignite if the squib leads were tied together? I understand how grounding the squibs after they were tied to each other might cause a charge buildup on the case to jump to the squibs to reach ground. But if the squibs were tied together they would remain at the same potential anyways...what would cause them to ignite if there was no potential difference between the squibs?

 

[Brevor]

I doubt this is true, I have never heard of this incident. I dont think they would use a plastic shroud that wasn't antistatic anywhere near a rocket motor.

 

[MikeMl]

The spark jumped from the squib canister (which is connected to the rocket body) to either of the externally earthed wires inside the squib. The spark jumped inside the squib canister, igniting it. The rocket body was effectively earthed through the wires emanating from the squib.

The correct way to do this would have been to short the two squib connections with very short wires (THAT WERE BONDED TO THE ROCKET BODY ITSELF) within an inch or two of the squib canister. Then, the rocket body should have been separately bonded to earth, but not close to the squib.

That way, if the rocket body gets charged by triboelectric effects or a nearby lightening strike, the current flow is along the body of the rocket into the earth below it. The squib is in an equipotential zone on the rocket, with little or no potential gradient across the squib itself.

 

[dknguyen]

Any idea what to google so i have a better image of the structures of the squib and how it mounts onto the case?

 

[Reloadron]

Any idea what to google so i have a better image of the structures of the squib and how it mounts onto the case?

Squibs come in a variety of flavors, designed for different applications. If you are curious this article gives a pretty good overview and explenation of several types.

Ron