Static

[iaov]

I was talking today to a rep from an industrial hose company. The WE energies plant in Oak Creek had a coal dust explosion yesterday. The company that does the cleaning of these plants wants to make certain that thier equipment could never cause an explosive event to occur. They have hired me to take a look at the grounding and bonding of thier equipment. While talking with this hose rep today he told me that the hose currently in use is "static dissapating". When I asked him to explain the physics of this to me, he could not. He informed me that the hose was made with carbon black in the rubbber however it was not conductive. They make hose with a ground wire woven into the hose that is conductive. I know that some plants require this type of hose be used and it is probably what I will recomend that they switch to. My question is does anybody know what static dissapating could possibly mean. Without a path to ground to bleed off a charge I do not see how this could work.

 

[charlie]

If I am not mistaken, a gasoline dispensing hose is built along those lines (this is a guess on my part).

 

[don_resqcapt19]

It doesn't take much to prevent the build up of a static charge. A hose with carbon black may very well be conductive enough to prevent the static charge. There is a statement in the IEEE Green Book that says a path with a resistance of 1,000,000 ohms or less will prevent the build up of static charges.

 

[iaov]

Thanks Don & Charlie. You are right about the gasoline hose. Thery have used this stuff for years without problems. I think they have just switched recently to a low resistence hose not because of problems but rather because of liability concerns.

 

[don_resqcapt19]

I should have noted that NFPA 77: Recommended Practice on Static Electricity, calls for a bonding/grounding path of 10 ohms or less in many cases.

 

[iaov]

I would think the 10 ohms or less rule would be better. I'm meeting with thier engineer on Monday and I'll see what they want, but I believe I will recomend to my client that they go with a bonded low resistence system.

 

[don_resqcapt19]

You can also look at systems to monitor the bonding path and wire the monitor relay to prevent the operation of the transfer system if the bonding path is lost.

 

[McGowdog]

My question is does anybody know what static dissapating could possibly mean. Without a path to ground to bleed off a charge I do not see how this could work.

Static Dissipating with regards to ESD flooring means that the resistance from the surface of the floor or Point to Point reading and/or the volume resistance or Resistance to Ground of the floor is in the range of 1.0E6 ohms to 1.0E9 ohms.

The Static Conductive flooring range is 2.5E4 ohms to 1.0E6 ohms. Either floor will typically generate less than 100 volts when personnel using SD footwear stand, move and walk upon it.

The only term that I know of that no longer describes an actual resistance range is "anti-static" which is typically used in packaging and literally means "resists tribocharge".

The first thing I would do is measure a point on the hose to its groundable point. If it's not less than 1.0E9 ohms, I'd be very concerned. If it's around 1.0E6 ohms or less, I'd wonder why they call it SD and not SC.

I'd like to learn more about this topic myself, but thought I could maybe add my experience with the terminology to this discussion.

There is a statement in the IEEE Green Book that says a path with a resistance of 1,000,000 ohms or less will prevent the build up of static charges.

Well, so does a path of 1 gig ohm or less, but it's all a matter of how much charge how fast.

Here's the basic laws of ESD Control that I've learned;

1. 
   You can can dissipate charges from a conductor by grounding it.

• You can't dissipate charges from an insulator by grounding it.

• You can dissipate charges on insulators and isolated conductors via +/- balanced Ionization

 

[McGowdog]

Sorry, I'd added a bunch of stuff and lost it in the attempted edit; went over 10 minutes.

I agree with Don Resqcapt about the 1 meg or less as a good rule for explosive mixtures and powders as per DoD 4145.26-M Chapter 6. This may correlate to 0.1 J or less, but that's the specs for Conductive flooring and other ESD technical elements such as that.

I know nothing about industrial hoses and such. It would seem the manufacture of such products should make available the measurable resistance of that product and instructions for verification/audit out in the field.

Does anyone know where to get this specific info?

 

[sgunsel]

You must contact the manufacturer, as there are many techniques employed. I used to work for an electrostatic spray equipment manufacturer and our equipment operated at up to 120,000 volts (but not many amps!). Another employer involved tanker trucks. In both cases, we preferred conductive hose materials over bonding wires embedded in a flexible hose. Embedded wires frequently break due to flexing or other mechanical abuse. A broken wire can act as a capacitor and actually increase the potential for a spark. If all the wires break, also not uncommon, no more bonding! One megohm (10e6) to ground is generally accepted as adequate for static dissipation in most circumstances and easily achievable. Higher resistances can be effective. Where metallic conductors are used, anything over 10 ohms indicates that something is wrong, such as deteriorating connections.

 

[petersonra]

I was talking today to a rep from an industrial hose company. The WE energies plant in Oak Creek had a coal dust explosion yesterday. The company that does the cleaning of these plants wants to make certain that thier equipment could never cause an explosive event to occur. They have hired me to take a look at the grounding and bonding of thier equipment. While talking with this hose rep today he told me that the hose currently in use is "static dissapating". When I asked him to explain the physics of this to me, he could not. He informed me that the hose was made with carbon black in the rubbber however it was not conductive. They make hose with a ground wire woven into the hose that is conductive. I know that some plants require this type of hose be used and it is probably what I will recomend that they switch to. My question is does anybody know what static dissapating could possibly mean. Without a path to ground to bleed off a charge I do not see how this could work.

Do you any special expertise in this area? if not, it might be best to pass.

A path to ground does not magically dissipate static. By bonding all the metal conductive parts together, you make it hard to get enough of a difference in voltage between any of the metal parts for a static spark to be created.

By connecting the metal parts to ground, you make it difficult for enough voltage to exist between earth and the metal parts for a spark to be generated.

Those parts are pretty easy.

Static electricity is pretty common on the surface of electrical insulators. Its common to have some kind of static dissipating material used in lieu of an insulator. It does not take much conductance to dissipate static electricity. Again, it does need to be bonded to all the other things in the area it might spark to. In the case of a conductive hose, it is bonded to everything else typically through some kind of metal fitting.

 

[petersonra]

Where metallic conductors are used, anything over 10 ohms indicates that something is wrong, such as deteriorating connections.

I have seen the 10 Ohm figure mentioned for such applications in the past. I always wondered where it came from. I suspect you are right that it is more a way of determining if the bonding is still OK as it is a performance standard.