XIX Sympozjum Srodowiskowe PTZE - materialy.pdf

XIX Sympozjum PTZE, Worliny 2009
LIMITATION OF USAGE OF RELAXATION TIME
OF LOW-CONDUCTING MATERIALS FOR ESTIMATION
OF ELECTROSTATIC CHARGE DISSIPATION TIME
Introduction
Zygmunt J. Grabarczyk
Central Institute for Labour Protection – National Research Institute
Laboratory of Electrostatics
Electrostatic discharges (ESD) constitute serious problems in the plants and other
organization, in which the explosive atmospheres or explosives are used or can appear
unexpectedly. The energy of ESDs can excess the ignition energy of the explosive media.
In case of ESD the available amount of the electric charge (cumulated on the surface of
electrified solid body or inside the volume of bulk or liquid materials) and the energy
cumulated in the electrostatic field is finite. Energy is usually limited up to the order of 1 J or
less. For that ESDs are short current pulses in the thin plasma channels. The plasma can be
generated only if the electric field intensity E is high enough to start the air ionization. The
ESD in the normal atmospheric conditions can occur if the electric field intensity E locally
achieves at least (at the surface of one or both electrodes) approximate value 3 MV/m. The
most of electrostatic discharges like spark, brush, cone or propagating brush, are able to ignite
almost all mixture of flammable gases and vapours and, excluding brush – a wide range of
dusts. For that reason, the possibility of fast dissipation of electrostatic charge is of a great
importance in the explosion prevention.
A lot of guidance and technical standards (e.g. Britton, PN-E-05204) demand to let electrified
bulk or liquid materials for charge relaxation during a time not less than a few relaxation time
constants. As it was shown in the paper, the real time of charge dissipation can be
significantly longer than simple material relaxation time constant.
Method
It is well known charge decay rate relation:
d Q
Q = − ,
dt τ
(1)
where Q = Q(t) is a electrostatic charge and τ is a relaxation time constant of the material,
usually defined as τ = ε/γ, where ε = ε0 εr and γ is the volume conductivity of material.
It must be take under consideration, that the real time constant of the charge decay in real
system equals to material relaxation time constant τ only if all electric flux vector (D) from
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