High Voltage and ESD Control

When working with voltages over 250 VAC, ESD personnel grounding should not be used, rather ionization may be required to neutralize electrostatic charges. “Wrist straps should not be worn by personnel where they could come into contact with voltage over 250 V.” [CLC/TR 61340-5-2:2008 User guide wrist strap clause 4.7.2.6]

Per CLC/TR 61340-5-2:2008 User guide wrist strap clause 4.7.2.5 “For personnel safety, wrist straps incorporating a 1.0 x 10^6 ohm resistor should not be used in situations where there is an exposed electrical circuit of 250 V or higher. Some additional options for high voltage operations are as follows:

• Select a wrist strap system that utilizes a higher value resistor
• Intentionally isolate the operator from ground through the use of high resistance floor mats or gloves.

NOTE: Several publications exist 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.”

Per CLC TR 61340-5-2 2008 Ionization clause 4.7.6.1 “The primary method of static charge control is direct connection to ground for conductors, static dissipative materials and personnel. However, a complete static control programme must also deal with isolated conductors that cannot be grounded as well as insulating materials (e.g., most common plastics). … Air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment, this will be neutralized by attracting opposite polarity charges from the air.”

As important as ESD control is, it is of secondary importance compared to employee safety. Questions regarding safety should be answered by the facility’s safety officer. The safety officer and ESD Coordinator should be aware of this information

EN 61340-5-1 clause 4 “Personnel safety, The procedures and equipment described in this standard may expose personnel to hazardous electrical conditions. Users of this standard are responsible for selecting equipment that complies with applicable laws, regulatory codes and both external and internal policy. Users are cautioned that this standard cannot replace or supersede any requirements for personnel safety.
Electrical hazard reduction practices should be exercised and proper grounding instructions for equipment must be followed.”

Per EN 61340-5-1 wrist strap product qualification is to be tested per ANSI/ESD S1.1 which includes “Current-Limiting Resistance, A resistance value incorporated in series with the wrist strap’s electrical path to ground. This resistance limits electrical current that could pass through the ground cord in the event of inadvertent user contact with electrical potential.” [Section 3 Definition of terms]

When equipment voltage exceeds 250 volts, guidance should be obtained from the appropriate Safety Officer if inadvertent user contact is possible. If so, Wrist Straps and ESD footwear, as well as Static Control Garments, should not be used. If the Safety Officer determines that the high voltage is adequately insulated and isolated, then personnel grounding, which is more effective and less expensive, can be used.

Vermason Wrist Straps [and many Foot Grounders] include a 1 megohm current limiting resistor (or resistance). At 250 volts, a 1 megohm [1,000,000 ohms] resistor limits current to 0.25 milliamp. These products are not recommended for use on equipment with operating voltage exceeding 250 volts. Do not remove the resistor. If it becomes damaged, replace the product immediately.

These products are for ElectroStatic control. They will not reduce or increase your risk of receiving electrical shock when using or working on electrical equipment. Follow the same precautions you would use without ESD grounding devices, including:

• Make certain that equipment having a grounding type plug is properly grounded.
• Make certain that you are not in contact with grounded objects other than through the ESD grounding device.

Per CLC/TR 61340-5-2:2008 User guide garment clause 4.7.7.5 Other considerations “Personnel safety should be considered before allowing static control garments to be worn where there is exposure to high voltage.”

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Measurements To Ensure Effective Static Protection

D. M. Taylor,
School of Electronic Engineering Science,
University College of North Wales,
Dean Street, Bangor, Gwynedd LL57 1UT

1. INTRODUCTION
It is now well established that electronic devices and systems can be damaged by exposure to high electric fields as well as by direct electrostatic discharges to the pins. While good circuit layout and on-board protection may reduce the risk of damage by such events, the only safe action at present is to ensure that devices are not exposed to levels of static electricity above a critical threshold. In BS 5783: 1984 a safe level is deemed to be a potential of 50V adjacent to the device. This low level of static can be achieved by instituting a Static Control Programme which usually involves setting up a Special Handling Area (SHA) in which personnel are correctly earthed and all materials e. g. flooring, bench tops etc. meet the specifications laid down for conductive and static-dissipative materials. However, setting up.a SHA does not of itself guarantee a low static environment. Production procedures may change, new materials may be introduced, the performance of older materials may degrade and so on.

To ensure the effectiveness of any static control programme it is important that regular measurements should be carried out:

(a) to determine the sensitivity to ESD of the devices being produced or handled.

(b) to confirm that static levels are lower than the critical level, and that new or modified work practices have not introduced high static levels.

(c) to ensure that both new and existing materials in the SHA meet the necessary specifications for conductive and static dissipative materials. Only after an ‘operational baseline’ has been established by regular auditing will it become possible to identify the origin of unexpected problems arising from the presence of static.

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