Dear Roger,
On top of the fact that I failed to verify the presence of HV on a set of
terminals believed to be quiescent is the fact that a reasonably new
component I spent hard-earned money to purchase failed soon after
its first energization.
However, all of us who deal with electronic componentry know that
the best chance for failure is within an hour or so after it first comes
up
on line, and we sort of expect to find a statistically-predictable amount
of
widgets that fail because of some reason or other, and that's how I felt
when discovering the various defects in the bleeder resistor network
assembly.
Once again, here's the final result of applying around 700 volts during
step-start
to the assembly:
Resistor One, 100K, 50W marked, now some 23 Meg-Ohms.
Resistor Two, 100K, 50W marked, now 101K-Ohms.
Resistor Three, 100K, 50W marked, now 101.89K-Ohms.
Resistor Four, 100K, 50W marked, now 51.4 Ohms.
The filter capacitors have no equalizer resistors and the measured
voltage
minutes after contact was 697 VDC. If the system was at let's say 750
VDC
just before I touched the beehive stud, and the resistors were still at
100K apiece,
it weould mean that there was 1.875 milliamps in the bleeder network, for
a
dissipation of 1.40625 W. With the #4 resistor failing at 51.4 Ohms just
then,
the amperage would have increased to 1.8752 mills for a dissipation of
1.4064 Watts.
These figures are what bothers me. I could well see a mechanical failure
at five times
the stress (which is about the full rating of the power plant), but not
at 750 volts.
There are 6 other 50W vitreous devices in the amplifier, and now I'm
worried.
Comments?
Thanks,
Hal
KA1XO
On Mon, 23 Aug 2004 14:37:57 +0100 (BST) =?iso-8859-1?q?Roger=20Parsons?=
<ve3zi@yahoo.com> writes:
> I am also quite suspicious of a wire-wound resistor
> failing to a low value.
>
> FWIW, my work includes certification of equipment as
> Intrinsically Safe for use in underground gaseous
> mines. This equipment has to remain safe with any two
> unrelated faults. All certification bodies of which I
> am aware will accept that a wire-wound resistor can
> only fail open or high resistance - most other types
> are considered to be capable of failure to a short. I
> would really hate there to be a documented failure of
> a wire-wound resistor to a low value, and for this to
> be noticed by the certification authorities, even
> though intrinsically safe equipment is always
> operating at dramatically lower voltages and currents
> than most amplifiers discussed on this reflector!
>
> If this happened my work would become very much
> harder, and huge amounts of equipment already
> operating in potentially explosive atmospheres would
> actually present an ignition hazard.
>
> 73 Roger
> VE3ZI
>
>
>
>
>
>
>
>
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