Brian,
Historically it is well known that solid material based VDRs exhibit
a leakage current that grows with time; the end point is a large current
that destroys the device and (hopefully) blows the fuse protecting the
circuit.
Because of this increasing leakage current phenomena, it has been
common practice to put a Gas Discharge Tube in series with the VDT. The
GDT, of curse, does not conduct under normal operation thus stopping the
normal VDR leakage current which eliminates this long-term failure of the
VDR. When the GDT fires because of an impulse the VDR limits the current
appropriately, as it is designed to do - else the GDT acts as an ongoing
short circuit & blows the fuse.
The GDT needs to be sized to not fire under the test conditions to
be applied; the VDR needs to be sized to handle the impuse fault current
then return the circuit to normal operation.
There has been long-term controvosy over this belt& suspenders (belt
& braces to our English friend) approach but it seems to work quite well.
Users are happier when the equipment continues to run normally rather than
have the fuse blow or the device die under, seemingly, unseen & unknown
conditions.
br, Pete
Peter E Perkins, PE
Principal Product Safety & Regulatory Consultant
Tigard, ORe 97281-3427
503/452-1201 fone/fax
[email protected]
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