FAQ on Wrist and Foot Straps

 

 

Q:    What type of persons’ footwear is recommended?                                                           

 A:    Special ESD protective footwear can only be effective if a dissipative or conductive flooring is available. If a conductive floor is in use then antistatic shoes are recommended, particularly if there is mains danger. However if the flooring is dissipative then both, the dissipative or conductive type of footwear can be used. Footwear can be a heel-strap or a toe-strap.

 

Q:    How do I maintain my wrist strap clean and effective?

A:    Wash with a mild detergent periodically, keep any metallic parts very clean and dried as avoid corrosion.

 

 
Q:    What types of wrist straps are available?                                                                      

A:    There is a metallic one an elastic and a hook and loop adjustable ones. In tests it has been found that the metallic type gives more consistently a better performance, more of the time is working ok. It is understood that the human body, say people, are the main cause of ESC accumulation and the wrist strap operation is most vital. Some wrist straps are provided with double cabling, one to earth and the other one to a local monitoring station. 

Also there are the cordless wrist straps, they are useful to use by supervisors and managers that must walk the floor but don't need to handle delicate electronics as often (most also allow for an earthing cord connection if deemed necessary, a worker for example needing to leave a workstation, hence disconnecting the earth cord, will be somewhat more protected). The cordless wrist straps reduce charges slowly in the human body, but has been found  to be unlikely to reach more than 500V, they work on a combination of factors eg skin effect, point discharging, corona effect, air neutralisation and potential differences as to slowly dissipate charges.    

 

Q:    Wrist and foot straps need grounding, could you elaborate?                                      

A:    A grounding cord connects a wrist strap to a common ground point and a conductive ribbon tail placed under the heel, inside the shoe grounds the foot. The idea is to achieve a same potential in between your body and the ground so there would be no current flows in between. The terms soft and hard groundings come to mind.  The hard ground is seldom used for human body straps as it would be a hard connection to ground and hence dangerous where mains are nearby. Standards recommend a soft grounding, say through a 1MΩ resistor (1 or 2MΩ in foot-straps) is provided in order to limit the current flowing though the user to <250uA @250Vrms AC. However antistatic bench tops or floor mats may be hard grounded. Never daisy chain or serially make a connection to ground, for example it is bad technique to connect a wrist strap's ground cord, to a workstation mat, which in turn is connected to ground

 

Q:    Could you describe the parts forming part of a heel-strap?                                            

A:    Typically a heel-strap has two bands about 2.5cm wide forming a square angle, so one side, usually elastic,  is placed over the back of the heel and the other goes under the heel of the shoe, the latter part makes contact with the floor. The bands on the inside are of a soft non blemishing material and resistant to tearing, the band will not stain a shoe surface with carbon marks while the outside part of the band is of a stronger conductive or dissipating material (resistance under 3.5MΩ) which is also a triboelectric inhibitor eg rubber on a mix with conductive carbon. Also, as to maintain the heel strap bands described above in place there is an elastic/adjustable type ribbon strap which goes over the top of the foot. Finally there is a tab made of conductive material (usually a mix of polyester with conductive carbon fibres) which is electrically bonded to the conductive part of the heel bands and which it must be placed under the sock of the foot, inside the shoe's heel, this may also carry in-line a 1MΩ or 2MΩ safety resistor. Measuring the resistance of the end of the tab to the bottom of the heel band, depending of length of the tab (say around 40cm long) should be in the 1MΩ to 10MΩ range when applying a 100V scale.

Q:    Could you describe the parts forming part of a toe strap?                                             

A:    The materials used to manufacture a toe strap are not different to the ones used in the making of a heel strap. The design structure is however different. There is a harnessing collar stretching band which is made of the same material in the inner and outer as described on the heel-strap question. This is placed right into the tip of the shoe, two lead straps are connected to this collar band running sideway to the shoe and which are fastened at the back of the heel, hence providing support to the front collar. It also has a connecting tab, similar to the tab in the heel-strap, which must be placed under the heel and sock inside the shoe to facilitate the discharge process. These straps are recommended for ladies where the heel-strap will not work because of the small area provided by a high heel shoe. 

Q:    Are wrist straps unnecessary for people working with electronics equipment where an antistatic flooring is provided?

A:    No. Wrist straps are to be used at all times when handling electronics’ equipment. There is a possibility of the strapped foot (or both)  to be in the air while walking, accumulating charges. 

 

 
Q:    That 1MΩ resistor imbedded in wrist straps  and most foot straps how precise the value should be?

A:    The resistor, a 1MΩ 1/4 watt carbon film is intended to provide safety for the wearer in areas where mains are provided, however a maximum limit for its usage is given by some manufacturers </= 250V AC. A tolerance fo variation when measuring the resistor would be ok to +/-20%.

 

 
Q:    Are there any written standards as to how often test of wrist straps must be effected?

A:    No really. How often a wrist strap should be tested is very much dependant as to how sensitive, delicate or expensive is the electronic equipment being handled. As a general rule each staff should be wearing his/her own wrist strap and which should not be removed during working hours and should be at least tested once a day where a close visual inspection must be applied, this is preferable at work start. In addition the ESD site co-ordinator should organise and implement his own test to all wrist straps in the site for at least once every six months. 

 

 
Q:    How do I measure my antistatic wrist strap?                                                                

A:    By use of an adequate tester (range 100KΩ to some 10MΩ) measure the wrist strap continuity to earth. An acceptable value is anywhere between 1MΩ and 10MΩ, however should the value exceed 10MΩ then check for open circuit or resistor, alternatively, should the value be under 1MΩ then check for a short circuit. It is common to find, while doing tests, that some straps' resistance to ground goes up to 20MΩ or 30MΩ where conductive lotions can bring this down to 2-4MΩ, it is also found that metallic type wrist straps show better grounding more consistently, usually under 6MΩ. A wrist tester should not be used to measure footwear, different ranges.

Q:    What is a sole grounder?
A:    These provide better contact to ground than heel or toe straps separately and they have more surface contact as the dissipating material wraps around a shoe in some 50% and about half of the sole. They still use the polyester/carbon mix in the tab for better skin contact.  
 

Q:    We have inherited a variety of snap plug and socket types, so how we go about using our wrist straps which are provided with a banana plug type of connector?

A:    I think there is a product available which you can fit to a banana plug terminal providing the two ends, say the snaps plug and socket in a back-to-back arrangement, they would be a universal 10mm.

 

 

Q:    How the antistatic shoe an the boot-strap are connected to a person's skin?

A:    The two elements use the principle of high humidity which is on average collected after some 15mins of wearing a shoe over a sock. The sock become quite conductive (in the dissipating region) and bridges the skin to the shoe or to the tongue of the foot-strap, which is located under the heel for a heel strap and under the foot plant for the toe strap, however the best foot straps cover both the heel and the toe. Some people that may have a very dry skin may need to use a dissipating lotion on the sock's contact area.

 

 

Q:    What is in an elastic band wrist strap?

A:    An elastic wrist strap band is about 3.5cm wide and they can be found of different lengths say from about 12cm to 25cm and in colours (JewelTM), it has a square part where the connection to a grounding cord is made, these upper and lower square parts snap together over the fabric as to produce a joint. In the centre of this square we find a stainless steel (perhaps machined) snap stud (found on a few standard sizes) to allow for a wrist strap cord snap-on connection. This square part is of a dissipating material in the outer part and of stainless steel on the inside, for better contact to the skin. The strap band itself is also of a ESD dissipating material on the outside but carries conductive fibres interwoven with the fabric in the inside as to facilitate conductivity, these fibres do not touch the outer material.

These elastic band can also be found adjustable, this is similar in manufacturing to the above strap, however has a stainless steel cam locking mechanism on top of the square where the snap stud is, allowing for the band length adjustment, this is required to do only once and use as an elastic band thereafter, the excess band material needs to be cut off short enough to cover the end with the snap lid as to avoid fraying of the end. Should the elastic band become damaged or it is too won out then you can buy only the elastic band and replace yourself. 

 

 

Q:    What are the components of a grounding cord for a wrist strap?

A:    One end is a spring loaded banana plug to which is connected a cable cord (different lengths) and available in colours (JewelTM), the cord inside is multi stranded and on a mix of copper/polyester to allow for flexibility (different manufacturers may have their own designs) and is jacketed with a PVC material.  A strain relief mechanism is found at both ends, usually a thermoplastic material. On the other end there is a spring loaded socket type connector (different sizes) with a 1MΩ resistor in series with the multi strand wire cable for safety.

 

 
Q:    What is a metallic wrist strap?                                                                                            

A:    The band is made of metal configured as retracting spring loaded links similarly to those found in wrist watches. It has a metal snap stud to receive a grounding cord terminal. The length of the band can be adjusted by sliding it in and out of the steel cap, should this adjustment be not enough for a very thin wrist then excess links can be cut off. There are a few manufacturer varieties of the type.

Q:    What is involved in a disposable wrist strap?                                                              

A:    This is a strip of dissipative vinyl or similar material of around 60cm to 70cm in length and provided with a wrist attachment mechanism at one end and an alligator clip on the other. Is designed for occasions where electronics equipment is to be handled at remote locations or for site visitors, once used is disposed off.