14 years ago
We have numerous flashlamp pumped lasers with one or more 60-80uF 2kV
capacitors inside. They operate at about 1.5kV peak. We are possibly
moving toward a requirement that the caps must be grounded and strapped
before doing work on the power supplies (including non-electrical work
such as changing water filters). This will be extremely cumbersome
since the caps were intentionally located in a difficult to access
location in the power supply by the manufacturer.
Additionally, this grounding work would have to be performed with
arc-flash clothing/face protection to the voltage/energy levels
expected, and appropriately rated insulating gloves. Yet the use of
this equipment makes the task even more difficult, and potentially more
prone to error.
The manufacturer's circuitry includes bleeder resistors for the caps.
I propose that we use SHV or similar, rated HV connectors connectors to
provide a safe direct monitoring port to the capacitor terminals. Then
by simply plugging in a high-voltage probe + DMM, we can verify that the
caps are at zero volts.
After that, one can proceed with work without concern. Such a rapid
means of verifying a safely discharged condition is essential for we
frequently need to iterate many times to troubleshoot or perform mods to
There is a concern though about a direct wire to the caps. If anyone
managed to short the SHV connector while the cap was charged, this would
create a large arc-flash hazard. Originally I had proposed using a
resistive voltage divider to monitor the caps, and argued that one could
actually watch the RC decay on a scope. Barring any step
discontinuities in the resulting trace or large deviation in time
constant from that expected, one could then conclude with a high degree
of condifence that zero energy was verified. I also provided detailed
analysis of the failure possibilities of the divider in this scheme.
The SHV connector idea was a simplification of the divider idea,
eliminating the failure modes and the fact that one cannot verify after
zero energy is read, that the divider hadn't suddenly broken (though I
argue that you DO know that, by looking at the scope trace).
Alternately, several parallel resistors could be placed in series with
the SHV connector, to limit current, while reducing the probability of
resistor failure causing a faulty reading to
1 / ((probability of a single failure)^(the number of resistors))
which would be pretty close to zero for 3-4 resistors.
This is all too logical and our safety folks want to hear none of it.
Even regarding the directly wired monitoring port they say "but there's
no way to know that the wire didn't break the moment after you measured
normal cap voltage, and now measure zero."
Would you EEs think it is sufficiently safe to verify zero energy via
such a monitor connector (with or without intervening signal
conditioning circuitry) wired to the caps, and perform work without
visibly grounding and strapping the cap terminals?
-- Good day! ________________________________________