ANTISTATIC SHOES

HOW THEY WORK, MATERIALS, LIMITS, DRAWBACKS.

We interviewed for you Mr. Christoph Meyer, in charge of RICOTEST (Sandrà-Verona), a very important Institute with the longest national experience in footwear technology.

Q. What are the advantages you can expect from antistatic footwear?

A. In several cases these shoes can dissipate on the ground the electrostatic tension accumulated inside the body, avoiding in this way the unpleasant occurring of shocks and sparks. In some working environments, for instance where there is the risk of explosions, the need to have such dissipation becomes vital, therefore conductive shoes are used instead of those simply antistatic.

Q. How do you understand if a certain model is antistatic?

A. A distinction should be made between civil and safety footwear. For the former there aren’t any regulations, neither for marking, or for testing and certification: in this case the user must trust the declarations which are usually in the packing. The latter instead are ruled by the Reg. 89/686 EU for the Individual Protection Devices (DPI), approved in Italy by the Decree 475/92, which prescribes a Certification and the CE marking; almost always this is made in conformity with the regulations EN 344-347. Safety shoes have a marking showing an “A” which stands for Antistatic. Often you can find also one of the following codes, which guarantee antistatic characteristics: S1-S2-S3-S4-S5 (S=safety), P1-P2-P3-P4-P5 (P=protection) or O1-O2-O3-O4-O5 (O=occupation).

Q. What are the technologies used mostly to produce antistatic footwear?

A. The most common construction, and maybe the most simple, reliable and economic is the one using antistatic materials to make the bottom. This technology is used for most of the safety shoes. Besides this basic technique, there is a wide range of alternative constructions, often to be found in civil and patented shoes, which usually use metal inserts as conductive elements. Then, there are the mixed constructions, where the sole is made of antistatic material and the resistance of the non-antistatic insole is overcome with some kind of metal bridge, made of rivets, seams with conductive thread or something similar.

Q.  How do you define the antistaticity of shoes?

A. The presence and the level of antistaticity are defined through the electrical resistance, expressed in Ω or kΩ  or MΩ (Ohm, Kilo-Ohm, Mega-Ohm) of the entire construction of the bottom, including all the layers it is made of. This is logic, because the dissipation of the electrostatic tension must take place between the foot and the ground, through all the layers separating these “poles”. In order to guarantee only the “antistatic” effect, i.e. the dissipation of the electrostatic load, it should be enough to establish a maximum limit of resistance, which could be for instance of 100 or 200 MΩ ; in fact such a limit gets applied as recommendation for civil footwear. Instead, for safety shoes, the regulation EN344 rules a maximum limit of 1000 MΩ  and also a minimum limit of 100 KΩ (0,1 MΩ).  Such minimum resistance, which would have nothing to do with the real requirements connected with antistaticity, can be explained with an additional purpose to foresee at the same time also a certain safety against fulmination due to a contact with at least “normal” tensions of the house, i.e. of 220 or 360 V. Therefore the antistatic sphere defined by the regulation EN344 for safety shoes ranges from 0,1 to 1000 MΩ.

Q. And how can be distinguished the dielectric and the conductive footwear?

A. Dielectric footwear means – often ignored – insulating footwear, but at a much higher level compared to antistatic shoes. The tests are made with different methods, operating with tensions from 10 up to 50 KV (Kilovolt), including also the damp conditions or even the use of water as electrode for the test. Instead, the conductive shoes must have a low level of electrical resistance, well lower than the minimum resistance of 100 KΩ guaranteed by antistatic shoes. Unfortunately here there is a language  “trap” which may lead to some confusion: conductive footwear must be distinguished from antistatic footwear, even if  really its behaviour is more antistatic than antistatic shoes.

Q. What are the main tests methods to apply?

A.  As already mentioned, the method prescribed officially for safety shoes is the one according to EN344, which operates with a tension of 100 V (direct current) on the entire shoe, including eventually the drawable insole. Then you need two electrodes: the outside one is made of copper plate, which is the support. The inside one is made with a filling of 4 kg of steel spheres. The tension of 100 V.c.c. is applied between the two electrodes, and after 60 seconds you can read the result directly in Ω or in µA (to be transformed in MΩ). The conditions are important too. Half of the samples gets seasoned for a week in a drying climate (the electrical resistance goes up!), the other half (or the same samples afterwards) is conditioned for a week in a damp environment, and this makes the electrical resistance go down). As samples you need also big- and small-size shoes, besides medium-size ones.

Q. What are the most common problems met during the testing of antistatic shoes?

A.  Often the total electrical resistance of the bottom is too high, despite the certain antistaticity of the pure sole. The reason is due to the high resistance inside the additional layers of the intersole, the insole, etc., sometimes also in the insufficient contact surface of the antistatic sole with metal elements inserted in the insole as a conductive bridge.

Q. What are therefore the “sins” the manufacturer has to avoid?

A. He must not be deceived that one antistatic sole is enough to create an antistatic shoe. Beside the fact that one insulating layer in the bottom only ruins the result, also three antistatic layers, for example, accumulate a sum of different resistances so that the entire bottom becomes dielectric. In order to avoid this effect of accumulated resistances, we recommend to couple the antistatic sole with materials with conductive characteristics.

Q. Are there any contra-indications for the use of antistatic shoes?

A. As far as the civil, city etc. shoes are concerned I do not see any contra-indications, risks or disadvantages due to antistaticity. Instead, in professional and safety employments there could be some risks which imply a choice of different shoes, conductive or dielectric. In these cases the user must ask for a positive confirmation, able to guarantee the desired characteristics. Often and incorrectly is taken for granted that a shoe not marked “antistatic” must be in any case dielectric. Such assumption is absolutely not reliable. Very often the safety shoes with a simple construction, really are also antistatic, even if not with such a marking. The reason is simple to understand. The manufacturer decided to use the same materials, usually for logistic reasons.

Q. How can you protect yourself against shocks even without antistatic shoes?

A. A 1000 Dollars question! Pay me 1000 Dollars then, because there are some very effective “tricks”, actually. Before getting off the car, before putting your foot down, tighten your fist on the top edge of the door (or touch the sheeting of the bodywork in any other part); doing so, you will never receive a shock. Instead, when you get close to the car, to the lift or to any other  thing which might cause a shock, you can provoke a “controlled” electrostatic discharge,  bringing the suspect surface as first contact closer to a metal object (for instance the car keys) you keep tight between two fingers, but without glove of course. The spark will trip between the car and the key, you will see it but you won’t feel it. Once the spark is over, the difference in height of the electrostatic load will be compensated and therefore you will be able to open the car-door without fearing to receive further sparks.