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Question My questions relate to working on a computer and using only a wrist strap:
1) Should the computer be plugged in while servicing?
2) If not, can you just clip the alligator clip to the pc chassis and have the other end connected to the wrist strap around my wrist and begin working?
3) In this situation, if the power is not connected to the power supply and it is just me the pc and the wrist strap working inside a room, how is grounding taking place?
4) Does the pc chassis have to be on the floor while working?
5) I read from your website that the resistor in the wrist strap is to protect the human working on the pc, is that to say you can work on a computer that has voltage going to it!???
6) I do not understand where the electrons in my charged body go when i am connected to the chassis using the wrist strap...The electrons most likely travel to the chassis... but then me and the chassis are at the same potential, but what does that have to do with me and the motherboard. Me and the chassis are sharing the same potential now, but when i touch the motherboard for example, what is stopping electrons from going from my skin to the motherboard?
7) Is the motherboard supposed to be in contact with the chassis?
8) After you take a card out of the motherboard is it still safe to touch when you are connected to the chassis with the wrist strap?
Answer

1) Should the computer be plugged in while servicing?

     There is no ESD control benefit or detriment to having the computer plugged in. Safety issues should be addressed to your company’s safety officer.

2) If not, can you just clip the alligator clip to the pc chassis and have the other end connected to the wrist strap around my wrist and begin working?

     The Wrist Strap is the first line of ESD control. Using the Wrist Strap to electrically connect the computer chassis and the operator will keep them at the same electrostatic potential so no ESD event or discharge can occur between the chassis and the operator.

Typically people who open a computer and replace or repair circuit boards, electrically connect the computer chassis with the operator using a Wrist Strap connected to a Field Service Kit mat with its ground cord attached to the computer chassis or ground such as a 3 wire AC outlet ground. Although there many be an ElectroStatic charge, it will be the same potential on each item. At equipotential, no ESD event can occur.

3) In this situation, if the power is not connected to the power supply and it is just me the pc and the wrist strap working inside a room, how is grounding taking place?     Electrostatic charges on the operator are not being removed to ground. The charge on the chassis and the operator are brought into balance or equalized; having the same charge on each creates equipotential where an ESD event cannot occur. Per the ESD Association Glossary Electrostatic Discharge (ESD) is “The rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field. Note: Usually, the charge flows through a spark between two bodies at different electrostatic potentials as they approach one another. Details of such processes, such as the rate of the charge transfer, are described in specific electrostatic discharge models.” (ESD ADV1.0)

4) Does the pc chassis have to be on the floor while working?

     There is no ESD control benefit or detriment to having the PC chassis on the floor.

5) I read from your website that the resistor in the wrist strap is to protect the human working on the pc, is that to say you can work on a computer that has voltage going to it!???

     Safety issues should be addressed to your company’s safety officer. Per ESD Handbook TR20.20 section 5.3.2.6 Current Limiting “Most wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 volts. Resistors limit current as defined by Ohm's Law, which states the current is equal to the voltage divided by the resistance. In a practical application, the maximum amount of current through a wrist strap ground cord if it was placed across a 250 Volt source is 250 microamps or 0.25 milliamps. This amount of current is well below the 0.7 milliamps that Underwriters Laboratories uses as the peak current in a LIMITED CURRENT CIRCUIT. See document UL 1950. For personnel safety, wrist straps should not be used in situations where there is an exposed electrical circuit of 250 volts or higher. Note: Several publications exist (beyond UL) that provide varying limits of allowable current for personnel safety. For example, most power supplies are current limited to 5 mA. For this reason, the user should check with local safety personnel to determine requirements for their particular area. Refer to the Personnel safety section of this handbook for further guidance.”

6) I do not understand where the electrons in my charged body go when i am connected to the chassis using the wrist strap...The electrons most likely travel to the chassis... but then me and the chassis are at the same potential, but what does that have to do with me and the motherboard. Me and the chassis are sharing the same potential now, but when i touch the motherboard for example, what is stopping electrons from going from my skin to the motherboard?

     Per ESD Handbook TR20.20 section 5.1.3 "When neither equipment or auxiliary grounds are available an equipotential bonding system may need to be used. In this situation, all of the items in the system are bonded together so that the charge that resides on the elements will be shared equally and therefore there will be no potential difference between the items. Once this step has been completed it is safe to handle ESD sensitive parts without inducing damage. A real life example of this is often observed in office equipment field service operations. For safety reasons the service technician will often disconnect the AC power cord which detaches the equipment from ground. In order to safely install ESD sensitive products into the equipment it is necessary to electrically connect or bond together the service technician, the equipment frame and the ESD sensitive product. Once bonded together an ESD event will NOT occur when the technician handles the product or installs it in the office equipment."

7) Is the motherboard supposed to be in contact with the chassis?

     The circuit board should be in the computer chassis rack, held by the operator attached using the Wrist Strap, or placed on the Field Service Kit mat. People often make the mistake of using an ESD bag as a worksurface or handling ESD sensitive items. This is improper, ESD bags should be used for packaging, not as a worksurface.

8) After you take a card out of the motherboard is it still safe to touch when you are connected to the chassis with the wrist strap?

     That is the correct procedure, ESD sensitive items can be handed by a grounded operator or one at equipotential. However, only conductors can be grounded. Insulators or non-conductors cannot be grounded by definition. In addition, the motherboard or most any circuit board may have isolated conductors not able to be connected to ground.

Per ANSI/ESD S20.20 Section 6.2.3.1. Protected Areas Requirement “Ionization or other charge mitigating techniques shall be used at the workstation to neutralize electrostatic fields on all process essential insulators if the electrostatic field is considered a threat.”

Per ESD Handbook TR20.20 Ionization, Paragraph 5.3.6.1 Introduction and Purpose / General Information “The primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. A complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g., most common plastics), and moving personnel who cannot use wrist or heel straps or ESD control flooring and footwear. Air ionization is not a replacement for grounding methods. It is one component of a complete static control program. Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods.”

 
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