Static Beat | Install Static Dissipators So They Work Effectively

Kelly Robinson offers 3 key points on how dissipators work so your system is installed properly.

Static sparks can ignite fires, shock operators, and damage thin coatings such as release coatings. Insulating webs accumulate static from a number of different sources including corona treaters, high traction drive rollers, and tension control nips. Installing static dissipaters on web spans exiting these sources of charging can effectively prevent sparks and avoid the problems from static sparking.

Knowing how static dissipators work helps you install them so that they will work properly. The tinsel in Figure 1 is installed near the insulating web. Tinsel, just like all passive dissipators, must be connected to ground to function. The negative static charges on the insulating web induce electric fields to the nearest grounded object. Just by convention, electric field always point toward negative charges. When the electric fields exceed a threshold of about 1.5 KV/cm (4 KV/in), a low energy corona discharge forms at the sharp tips of the tinsel. The corona discharge generates both positive and negative corona ions. The positive corona ions are attracted toward the negative static charges on the web. The positive corona ions deposit on the web neutralizing the negative static charges.

Figure 1. Low energy corona discharges at the sharp tinsel tips form corona ions. The positive ions are attracted toward negative static charges on the web.

The 3 key points are:

 

  • Static dissipators always add more charges to the web.
  • Static dissipators generate corona ions having a polarity opposite to the web charges.
  • The electric fields from the web charges attract the opposite polarity the corona ions.

 

Static dissipators must be installed so that they “feel” the electric field from the web charges. So, the static dissipator must be the closest grounded object to the web. To check this, I do the following “mental exercise.” Suppose we want to install tinsel on the web span in Figure 2 between idler roller 1 and idler roller 2. We are considering 2 locations: Tinsel A and Tinsel B.

 

Figure 2. In this web span, is Tinsel A or Tinsel B the best location?

 

In your mind’s eye, draw a pencil line across the web under the static dissipator. The pencil line becomes the center of a cylinder that expands outward like a balloon. As the cylinder grows larger, it must touch the static dissipator before it touches any other conductor.

Figure 3. Will the active static bar effectively dissipate the charges on the incoming web so that the outgoing web is charge-free?

I expanded the cylinder in Figure 2 until it touches the two idler rollers. Clearly, tinsel location A is outside the cylinder so installation here would be ineffective. And, of course, tinsel location B is inside the cylinder, so tinsel B in Figure 2 would work well.

 

Let’s take a look at another example.

The charged incoming web in Figure 3 wraps around a metal idler roller. The active static bar is installed 2 in. from the web surface as recommended by the vendor.

Will the active static bar effectively dissipate the charges on the incoming web so that the outgoing web is charge-free?

 

Figure 4. Draw a pencil line across the web and expand the cylinder until it touches the static bar. In this location, the metal idler roller is inside the cylinder and the static bar is ineffective.

To answer this question, let’s do the “mental exercise.”

In your mind’s eye, draw a pencil line across the web in Figure 4 just above the active static bar. Expand the cylinder until it touches the static bar.

Since the metal idler roller is inside the cylinder, the idler is closer to the web and the static bar is ineffective. The outgoing web will be charged.

The mental exercise shows that static dissipators should be installed on web spans away from idler rollers.

Passive dissipaters work better when they are closer to the web. Active dissipators are more forgiving and can be positioned farther away from the web typically in the range 2 in. – 6 in.

 

I invite you to ask questions about this column and to suggest future topics. My email address is: This email address is being protected from spambots. You need JavaScript enabled to view it..

 

Static control expert Dr. Kelly Robinson, president of Electrostatic Answers, has 27+ years of experience in problem-solving and consulting. Kelly writes PFFC's Static Beat column and the Kelly on Static blog. Contact him at 585-425-8158; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.electrostaticanswers.com.

 

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