Measuring charge transfer
Objects made of insulating materials can become electrostatically charged through rubbing or as a result of operational procedures such as emptying or filling canisters. This can lead to brush discharges or propagating brush discharges.
To avoid dangerous charges, insulating objects or equipment may only be used in hazardous areas in Zone 0, Zone 1 or Zone 2. (TRGS 727, chapter 3.2)
In addition to limiting the surface area of such insulating objects, it is also possible to verify that the insulating object cannot be dangerously charged or that a critical electrostatic discharge will not occur. This verification can be provided experimentally, for example, by determining the charge transfer.
The charge transfer measurement is carried out in accordance with the test methods in standard DIN EN 60079-32-2 – Electrostatics hazards.
To determine the transferable charge, the insulating object is placed on a grounded metal plate. The object is then selectively charged with a needle electrode as well as with a Fakir electrode (multiple-needle electrode) with e.g. 40 kV and 60 kV (charge duration 15 – 60 seconds).
An HS 70kV (negative) high-voltage generator is used. Then it is determined which charge can be picked up from the charged object. The transferable charge is measured by means of a portable coulomb counter HMG 11/02 (measuring range: 5 – 200 nC). To observe the precision (repeatability and comparability) of the measuring method, at least 10 individual tests are carried out. The result with the highest charge transfer is used to classify or assess the test object.
In accordance with TRGS 727, the maximum load transferred by an object in Zones 1 and 2 must not exceed the following values:
- For explosion group IIA: 60 nC,
- For explosion group IIB: 25 nC,
- For explosion group IIC: 10 nC.
In Zone 0, the following values must not be exceeded:
- For explosion group IIA: 25 nC,
- For explosion group IIB: 10 nC,
- For explosion group IIC: No discharges may be detectable.