Questions And Answers

Hi

I have built up quite a large selection of ESD wrist straps, work mats, connecting cables and mains plug earth bonding points over the years.

Only recently have I been thinking about the theory behind the use of them, leading me to question the way I currently use them.

All but one of my connecting cables have no indication of built-in resistance, so I've measured them and found that some have 1M ohms, some 2M ohms, and some 4M ohms.

The earth bonding plug I use has three 10mm studs, each stud connected to mains earth via a 1M ohms resistor.

I currently connect my wrist strap to one of the earth studs on the mains plug via a 2M ohms cable, giving a 3M ohms total series resistance to earth.

I connect my work mat in an identical way, so this also has a series resistance of 3M ohms to earth.

I then connect the chassis of the piece of equipment I'm working on (e.g. a PC) to the third stud on the mains plug using what happens to be a 1M ohms cable, giving a series resistance to earth of 2M ohms.

The mains supply in my country is 240VAC.

Does it matter that the series resistance is 3M ohms for my wrist strap and mat to earth? Does this mean that it takes too long for any acquired charge to bleed away?

Is this star earthing arrangement best, even with the high overall series resistance of each leg to earth?

I can't find any 10mm stud cables that don't have at least 1M ohms built in, so the series resistance of patch cords etc. quickly seems to add up.

Many thanks for your help.

Peter
London, UK.

The current limiting resistors are not for ESD control purposes. A Wrist Strap coiled cord almost always has a minimum 1 megohm resistor in the cuff end of the cord. Per ESD Association Wrist Strap standard ANSI/ESD S1.1 ANNEX B “1. Current-Limiting Resistance, A resistance of sufficient resistance to limit current to less than 0.0005 amps (0.5 mA), at the highest voltage that may be encountered, should be incorporated into the wrist strap. Nominally, 800,000 ohms (800 kilohms) are sufficient for voltages of up to 240 volts alternating current (AC). The value of 1 megohm is specified because it is a standard value discrete resistor.
Special situations may dictate the use of values above or below the 1 megohm value. Wrist straps with nominal resistances other than 1 megohm should be marked in accordance with paragraph 5.9.
Discrete current-limiting resistors should be located near the connection between the ground cord and the cuff.”

ESD Systems.com has its wrist strap Underwriters’ Laboratories listed. The UL warning is: Note: This product is not recommended for use on equipment with operating voltage exceeding 250 VAC.
CAUTION: The ESD Series is for electrostatic control. It will not reduce or increase your risk of receiving electric 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 ESD Series.

The ESD Association Grounding standard ANSI/ESD S6.1 recommends hard ground with no resistor for mats, etc, however a resistor is permissible. “The grounding conductors (wires) from wrist straps, worksurfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.
Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.” [Grounding ANSI/ESD S6.1 section 5.3.3 ESD Technical Element Conductors]

There may be practical reasons not to use a current limiting resistor for grounding a mat, etc. 1) It might provide a false sense of safety and encourage unsafe practices around line voltage, 2) It may provide misleading RTG resistance to ground measurements. If one attaches a ground cord with a 1 megohm resistor to a very conductive worksurface, say stainless steel, the RTG would be 1 x10E6 ohms.

I am not aware of any negative impact in the speed of the ground cord removing the electrostatic charge, but dissipative worksurfaces are recommended to slow the discharge. When a charged ESD sensitive item makes contact with a grounded worksurface an ESD event occurs as the charge is removed to ground. The dissipative surface slows the discharge; if the worksurface is very conductive the discharge may occur quickly with resulting increase in current and may damage the ESD sensitive item.

The ESD Association Worksurface standard ANSI/ESD S4.1 says in section 8 Resistance Guidelines “Resistance-to-groundable point 1 x 10^6 to 1 x 10^9 ohms. Resistance from point-to-point greater than 1 megohm. These guidelines represent a range of resistance that has generally been proven to provide protection in the manufacturing environment.”