I'm hoping you could give me some information about why static electricity increases in the cold weather?
The reason static is more problematic in cold weather is because of low humidity of the air. "Relative humidity" is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. So 50% rh means the air has only half the amount of moisture it could hold. It turns out that static is promoted if rh drops below about 30%.
Cold air can hold less moisture than warm air. So cold outside air at say 0oC and 100% rh is taken into a building and heated up to make it comfortable. The relative humidity drops by one half for every 10oC rise in temperature - so if no moisture is added, the air will be 50%rh at 10oC and 25%rh if heated to 20oC. 25% is certainly dry enough to promote static electricity!.
The main factors conributing to static electricity indoors are floor covering and shoe sole materials, and furniture covers materials, and dry air conditions.
Tuesday, February 21, 2006
Thursday, February 02, 2006
Grounding through ESD footwear and flooring
We have a very peculiar problem while using the ESD ( Static Dissipative ) Shoes. The the pass band - lower limit on the tester is 0.75M Ohm and the upper limit is 35M Ohm) some shoes pass and some fail. Are the limits set on the footwear tester ok or should we increase the upper limit?
The limits on your tester are correct for measuring resistance from the person's body to ground via the footwear and flooring, for a person who is handling ESD susceptible parts. This is recommended by IEC 61340-5-1 to be between 750 k ohm and 35 M ohm.
However the limits might not be correct for testing footwear as worn, to a metal plate electrode. Under the IEC 61340-5-1 standard the range limit for this is between 100k ohm per shoe, or 50 k ohm testing both shoes together, and 10^8 ohms. Footwear can be within this range (and so compliant with the standard) and yet fail your test.
For primary grounding of personel (in other words, the main means of grounding people who are handling ESD susceptible parts) the resistance for shoes needs to be lower than the maximum required resistance for grounding through footwear and flooring. There must be some margin for the resistance of the floor. Also, the floor resistance needs to be low enough to achieve a resistance to ground less than 35 M ohm - this will require floor resistance considerably less than 10^9 ohm allowed by the standard. In this case the best procedure in my view is to measure the resistance from person's body to ground while they stand on the floor - this is recommended to be between 35 M ohm and 750 k ohm. This measurement, and compliance with this resistance range, is not specifically required by the standard.
In summary:
1) 61340-5-1 allows footwear and floors to have higher resistance than would be needed to achieve 35 M ohm resistance from a person's body to ground
2) In order to achieve 35 M ohm resistance from body to ground you would have to choose footwear and flooring types that would reliably give low enough resistance
3) It is recommended, but not required, by 61340-5-1 that the resistance from a persons body to ground should be between 750 k ohm and 35 M ohm when grounded by footwear and flooring - if they are relying on this as the primary means of grounding while they are handling ESD susceptible components.
The limits on your tester are correct for measuring resistance from the person's body to ground via the footwear and flooring, for a person who is handling ESD susceptible parts. This is recommended by IEC 61340-5-1 to be between 750 k ohm and 35 M ohm.
However the limits might not be correct for testing footwear as worn, to a metal plate electrode. Under the IEC 61340-5-1 standard the range limit for this is between 100k ohm per shoe, or 50 k ohm testing both shoes together, and 10^8 ohms. Footwear can be within this range (and so compliant with the standard) and yet fail your test.
For primary grounding of personel (in other words, the main means of grounding people who are handling ESD susceptible parts) the resistance for shoes needs to be lower than the maximum required resistance for grounding through footwear and flooring. There must be some margin for the resistance of the floor. Also, the floor resistance needs to be low enough to achieve a resistance to ground less than 35 M ohm - this will require floor resistance considerably less than 10^9 ohm allowed by the standard. In this case the best procedure in my view is to measure the resistance from person's body to ground while they stand on the floor - this is recommended to be between 35 M ohm and 750 k ohm. This measurement, and compliance with this resistance range, is not specifically required by the standard.
In summary:
1) 61340-5-1 allows footwear and floors to have higher resistance than would be needed to achieve 35 M ohm resistance from a person's body to ground
2) In order to achieve 35 M ohm resistance from body to ground you would have to choose footwear and flooring types that would reliably give low enough resistance
3) It is recommended, but not required, by 61340-5-1 that the resistance from a persons body to ground should be between 750 k ohm and 35 M ohm when grounded by footwear and flooring - if they are relying on this as the primary means of grounding while they are handling ESD susceptible components.
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