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Check your 34401A multimeter
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BennoG:
My 1997 model has the same problem.
Put some nice isolation between it.
with some glue so it wont shift due to vibration.

Benno
Sensorcat:
Same here, Agilent model from the early 2000s. Zero clearance.

Does anyone know more about the root cause? Does it start with vibration, which causes the insulation to be damaged, and only then the high voltage creates the failure? Or is high voltage enough to make it fail because of the lack of clearance?

And if it's vibration, transport or operation?

I ask because it is not yet clear to me if fixing this requires only mechanical protection or an insulation for at least 1kV. It is only clear if the insulation of K101 is damaged already.
Gyro:
Any solid dielectric has a finite life, this depends on aging, voltage stress etc. and it's only a matter of time. By keeping voltage stress at a low percentage of its breakdown voltage the MTBF can be extended to the point that it doesn't cause failure at anything like the expected life of the component or product that it is installed in. Class Y caps are a good example of this.

The ADuM4160 USB isolator datasheet has a nice concise summary of this in the Insulation Lifetime section (page 12 and Table 8 )...  https://www.analog.com/media/en/technical-documentation/data-sheets/adum4160.pdf

On that PCB revision of the 34401A, you have a an uncharacteristic HP major cock-up. The GDT is in contact with the metal case of K101, separated only by a poorly controlled layer of paint, which might even have pinholes. If it is never exposed to more than, say, 20V the MTBF of that paint insulation layer could well be so long that it is never noticed (just think how many 34401As there are out there that will never see a problem). Start exposing it to hundreds of volts and that MTBF of the paint layer will plummet.

Yes, things like vibration, pressure of the GDT against to paint will affect things (although the flow soldering process would have limited this) may affect things, but ultimately it is just a matter of voltage dependent time. The important thing is to make sure that the breakdown voltage of any added insulation fix is a decent multiple of the measurement voltage it will see.
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