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Answer: The easy answer to give here is “less than 10 ohms”. This value is a practical value that is often used as a base case when undertaking electrostatic hazards resistance checks. In fact, much higher resistances – hundreds or even thousands of ohms – can still be safe in many situations since currents generated by static electricity are generally tiny and ohms law tells us that the voltage that will be generated on a component is a product of the electrostatic charging current and the component’s resistance to ground. A current of 1mA to a nozzle handle, for example, with a resistance of 1k Ohm would only generate 1 volt on the nozzle; not an ignition hazard.

If we look at static dissipative footwear, for example, we see that an acceptable resistance range for footwear is usually between 106 to 108 ohms. It is not zero since such footwear would provide no protection for the wearer who might be unfortunate enough to encounter mains electricity. In fact, footwear resistance-to-ground can be as high as 108 ohms because however fast and hard an operator works, he is only ever going to be able to generate limited charging currents through his actions.

[For interest, your works electrician is unlikely to be happy with a ground connection giving a reading of 1000 ohms, nor with footwear that is not highly insulating. But that’s another story!]

NFPA 77, Recommended Practice on Static Electricity, provides examples of acceptable resistance values in a variety of different plant situations.

We would contend that there are 4 critical material properties that contribute to evaluation of electrostatic hazards in a powder handling processes. Three of them are independent of each other. These properties are:

  • powder Minimum Ignition Energy (MIE)
  • powder Volume (or bulk) Resistivity (related to charge relaxation time)
  • powder Chargeability

The dust cloud Minimum Ignition Energy (MIE) tells us how easily (or indeed, if at all) the dust can be ignited by electrostatic sparks.  It is a measure of the lowest (capacitive) spark energy capable of igniting an optimal dust cloud concentration.

The powder Volume Resistivity tells us if charge that builds up on a powder in a grounded, conductive container will lose much charge by conduction. It is related to charge relaxation time (or charge decay time) which is a direct measure of rate of charge loss.

Powder Chargeability will indicate if the powder will charge, and to what level, when moving against various plant surfaces (plastics, stainless steel …., pneumatic conveying, mixing, sieving …)

It’s important to note that the chargeability of a powder is completely independent of the Volume Resistivity/Charge Relaxation Rate.  A very conductive powder may become highly charged (high chargeability) and a highly insulating powder may or may not charge much, for example.

Although bonding and grounding is an essential component of electrostatic hazard control, often there are other very important measures that one must consider.

The control of static electricity requires specialist focus since it is not always apparent where charge can be generated and indeed if the generation of charge is likely to cause a discharge/ignition hazard or a processing/handling problem.  In addition to metal plant one also needs to consider electrostatic hazards from people (facility employees), insulating (plastic) materials, non-conductive liquids, and powders.

No. Electrostatic sparks involve ionization of the air, which is the breaking apart of the air molecules. Impact sparks are glowing fragments of material resulting from impact. These two types of sparks have very different incendivity (different igniting capability).

The term electrostatic spark has two meanings. On the one hand it refers to a particular type of electrostatic discharge – from charged metal to grounded metal. Other types of electrostatic discharge are brush, cone (from surfaces of bulking powder), propagating-brush, and corona. In popular usage the term “spark can also refer to any type of electrostatic discharge but this is potentially confusing and should be avoided when discussing electrostatic hazards. Different types of electrostatic discharge have different igniting powers.

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