industrial wrist strap bonding

[karl riley]

Tom, since you have installed and used the wrist straps used by the electronics assemblers, I have a question. Since they are bonded to the GEC (by way of different pathways), what do the workers do during thunderstorms? They are protected by a 1 megohm resistor at the strap, but to an intrusive lightning bolt, that's nothing.

They would have to detach from ground if they follow the usual cautions, and that could cost the company a lot of money for no productivity. So What do they actually do?

 

[bphgravity]

Let me preface by stating I am in no way an ESD expert. However, my research into lightning and grounding has exposed me to this sector of the "grounding and bonding world."

If a facility is one that requires employees to have wirst /shoe ESD bonding, there are assumptions that can be made.

1. The facility is familiar with and complies with the dozen or so ANSI/ESD standards. So that being said, I would assume that the building, electrical system, and grounding / lightning protection systems are not typical or standard.

AND

2. The facility has a storm safety plan that limits its employees and operations to the risks of lightning and other surging event hazards.

But to answer your question directly, detaching from the ESD is really not necessary even during a lightning event. The structural elements of the building in combination with a properly designed electrical and grounding system, along with a purpose designed surge and lightning protection system will "shield" the workers from any potential hazard.

While the rise and fall of voltage on the equipment and even maybe the workers could be imense from a remote point, no potential should be present on any metal part, building componet, or materials and equipment in contact with the workers.

It would be the same principle that protects explosives and the like in military facilites.

 

[Speedskater]

@Bryan, I believe that there are many, many small shops that are not familiar with ANSI/ESD standards.

 

[bphgravity]

Than they probably shouldn't be using ESD devices, right?

I am not certain, but OSHA might also play a role when these devices are used by workers. Then again, small shops tend to not obey OSHA either.

Again, I am not an ESD expert. Just stating what I do know.

 

[dereckbc]

Karl in those facilities the workers are bonded to the nearest structural steel and usually on ESD mats. The wrist straps with the tether wire only allows for about 10 feet of movement in any one direction away from the bond point There are so many bonds and loops forming an integrated ground plane, it would be almost impossible for enough potential to develop.

Most of the buildings in question use a ground ring around the building perimeter, and any nearby strikes would have the fault current shunted around the building in effect forming a Faraday Cage.

Only place I have ever seen lightning been a problem is in high rise buildings, where a technical facility uses an Isolated Ground Plane in one of the mid level floors and the GEC passed down through all the floors to the basement without being bonded to the buildings structural steel at each floor is passes through. When the building was struck, the building itself formed a voltage divider along its height, with the mid level floor being referenced to the ground level, and the Isolated Ground Plane, several thousand volt potential was developed burning up all equipment.

 

[karl riley]

Bryan, that makes sense, since if the lightning raises the potential of all grounded surfaces at the same time, there is no other surface they could touch to find a different potential. Just like the linesman who helicopters in to work live on a HV line. He uses a wand to touch the line and equalizes his potential; then sits on the line and does his work.

Still, lightning is an 8000 pound gorilla, so I would think that workers in a small shop would disconnect during a storm. Unless it's a third-world swaet shop?