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Electrical safety - verifying zero energy in large capacitors

Greetings:

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 the lasers.

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!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
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Reply to
Chris Carlen
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I think you should discuss your thoughts with the manufacturers !

Doesn't this equipment have safety interlocks that disconnect the supply and ground the caps if the case/door is opened ?

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Regards:
              Baron.
Reply to
Baron

So what's wrong with " allow x minutes before opening"?

So add a large value series resistor to the socket. Part of the test procedure would be to verify that the resistance is present. i.e.

1) check that voltage across terminals is 0 2) check that resistance between terminals is < x
Reply to
Mike Harrison

I once picked myself off the floor, about ten feet from the workbench where I had been working on a photoflash unit, wondering what the hell just happened.

What happened was the safety interlock, which should have grounded the caps when I removed the unit from its case, didn't work, and in a potent rebuke it taught me a lesson.

I've never since trusted an interlock, and I recommend that nobody should. Nothing beats the insurance of manually discharging HV caps.

Reply to
Don Bowey

I think there will always be some voltage over the capacitor. So a break can be detected as a true zero voltage. One could also send a small AC signal and measure it's attenuation.

Reply to
Sky465nm

Build your HV monitor with a divider network and micro. Have the micro monitor the HV continuously and activate/verify the decay prior to service work. If your safety committee raises any doubt (I believe this is the proper word here, as opposed to many questions elsewhere), tell them to check with your programming committee.

Reply to
linnix

It is an unacceptable risk that the bleeder resistor/circuit may have malfunctioned.

Yes, I suppose that works, since you'd be measuring the monitor connector resistor in series with the bleeder resistor.

--
Good day!

____________________________________
CRC
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Reply to
Chris Carlen

Egad. The mentality here is that if you can't see 2/0 gauge bare wire strapped across the capacitors, then it's not sure to be discharged.

I think this is overkill, and that it is possible for a carefully thought-out measurement procedure to both ensure safety and make it much more expedient to verify zero energy.

But I wouldn't trust my safety to a micro in this case. Plus, they would never understand it even if it were a ultra-high reliability type of system.

--
Good day!

____________________________________
CRC
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Reply to
Chris Carlen

"Chris Carlen" skrev i en meddelelse news: snipped-for-privacy@news1.newsguy.com...

No - Eventually someone will use a broken probe, read the meter wrong or forget about it.

Instead, I would use a HV relay for each capacitor (or -bank if the connections are sufficiently solid), with a low-resistance high-power resistor in series with it (maybe one of those resistors made from discs - "Ceramcarb"??).

The relays are powered by a safety circuit that runs through a series of switches on all access doors and hatches. When the loop current is interrupted, by turning off the supply or opening a door, the relays will drop and dump the energy in the capacitors. The access doors are naturally only released by using the captive safety interlock key that locks the main breaker open ;-).

Then one will hang an earthing stick off the capacitors. *Then* one will work on/near them.

If the capacitors are removed, one will place a short across the terminals and the housing because in my painful experience those suckers charge back up again due to charges migrated into the insulation. As long as the capacitors are in the equipment, the earthing rod should be sufficient.

For simple things like water filters one can, I.M.O., avoid the full earthing stick e.t.c. by having the "non-electrical" parts in separate compartments monitored by the safety loop.

So the supplies are Off & Grounded but not hardwired to earth.

Yeah: It *does* take time getting the breaker out, getting the key and getting access. So one must compensate by making the tuning easier, remote or superfluous! When I worked with high voltage we used to say that we did not want to do this work the rest of our lives ;-)

Reply to
Frithiof Andreas Jensen

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