Electrical Safety in Medium Voltage

Medium Voltage Disconnectors and Safety
Are disconnectors "safety devices"?
How they are specified;
How they are used;
What are the actual stresses;
Unexpected field behaviors;
Safety is a system concern.
Didier FULCHIRON - France - Session 1- Paper 0048

Are disconnectors "safety devices"?
"safety device" is not a defined concept
"safety" is considered as a tolerable risk level
ISO / IEC Guide 104
disconnectors have little specific features
isolating distance on open gap (network requirement)
secured position indication (safety improvement)
criterion against leakage current (seen as a safety concern)

How they are specified
main feature is seen to be the open gap dielectric withstand, but...
with same acceptance criteria than for common value (2/15)
with no assessment of the failure mode
target application is open point between live networks; see IEC60071-1 about "longitudinal insulation"
Didier FULCHIRON - France - Session 1- Paper 0048 a.i.d. c.v. c.v. c.v. : common value a.i.d. : across isolating distance a.i.d.

How they are used
asumption of preferred phase-to-earth dielectric flash-over
mis-understanding of the purpose of the withstand values
most often with one side earthed
and specially when "safety" issues are concerned
expected to ensure the safety of the operator
some regulations even focus on that (visible break...)

specification means withstanding overvoltages on one side while other side is energised
common usage means withstanding phase to earth overvoltages
What are the actual stresses Didier FULCHIRON - France - Session 1- Paper 0048 a.i.d. a.i.d. : across isolating distance c.v. c.v. : common value

Unexpected field behaviors
double flash-over with remote earth
transmitted over-voltage with common earth

Safety is a system concern
as well as insulation coordination;
installation conditions are a key point;
safety involves:
identification of stresses
specification of components of the installation (system)
identification of operation procedures and failure modes
acceptability (tolerable risk level)?
Disconnectors alone are not the solution
let's specify and use them for what they are...

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Electrical Safety in Medium Voltage

by francois b on Aug 20, 2009

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