Desco Europe ESD - Questions And Answers

Dear Sir / Madam
I have a question regarding ESD safety when working with AC voltages (240V AC).
We are an electronics company and obviously we have all ESD pre-cautions to safe guard our products. We started buying such products directly from you and I require your help on the following issue. Apart from the standard products that we produce using 5V DC, 9V DC and 12V DC, we have a specified model that works with 240V AC and therefor needs to be tested with such supply input.
Our operators all wear a heel strap / wrist strap with a 1 MOhm resistance. My questions are:
1. Is it safe for the operator to wear such heel strap when working on 240V AC?
2. Is the operator safe if he is wear such heel strap and accidentally touches the AC connection and takes an electric shock?
3. Would such heel strap effects positively or negatively when taking the shock?
Kindly send me the required information, preferably with some technical info as soon as possible.
Best Regards & Thanks for your support.

When working with voltages over 250 VAC, ESD personnel grounding should not be used, rather ionization may be required to neutralize electrostatic charges. 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 has to make the determination if the worker has any risk of being exposed to the high voltage. 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. If operators are being shocked the source should be identified and eliminated. If exposure is to elect rical line voltage, a licensed electrician should perform this work.

Per ESD-S1.1 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.” This should cover your 240 volt application.

However, when working with voltages over 250 VAC, ESD personnel grounding should not be used, rather ionization may be required to neutralize electrostatic charges. Per ESD Handbook TR 20.20 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 can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air.”

ESD Systems.com Wrist Straps [and many Foot Grounders] are Underwriter Laboratories listed. UL cautions that these products are not recommended for use on equipment with operating voltage exceeding 250 volts recommending that the electrical current that the operator be exposed to be limited to 0.70 milliamp. At 250 volts, a 1 megohm [1,000,000 ohms] resistor limits current to 0.25 milliamp.

UL Marking for Wrist Straps and Foot Grounders:
CAUTIONARY MARKING
This product is not recommended for use on equipment with operating voltage exceeding 250 volts.A one megohm resistor is molded into Wrist Strap Coil Cords. DO NOT REMOVE. If it becomes damaged, replace Coil Cord immediately. These products are not to be used in areas where the individual may come in contact with exposed electrical circuitry exceeding 250 volts. 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 wrist straps, 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 Wrist Strap.
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, ¼ 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.
Per ESD Handbook TR 20.20 section 5.3.13.6 Other Considerations “For personnel safety, static control garments should not be worn in situations where there is exposure to high voltage.”When both Foot Grounders are in contact with the ESD Floor, the resistance is in parallel or halved; you should consider using Foot Grounders that have 2 Megohm resistor.

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