High Voltage Caps Insulation

My friend is building a high voltage vacuum tube amplifier. Part of the design is some 6000uF (500V rating) Nippon caps. Four of these caps are in series. The network is charged to 1500V. My question is regarding the mount for these capacitors. Are the outside of the caps well isolated from the terminals, or are they connected to negative terminal in some way? I measured the resistance from the terminals to the case and it was basically "open", but there is some concern about whether the cap that's at the highest voltage will start arcing between the mount (at ground potential) and the case of the cap. As it is there is a 0.050" gap around the caps to the metal mount. The caps are held in position with silicon o-rings, but I think this is all unnecessary.

Please advise,

Matt

You'll find a specification of 2000 VAC/60 sec isolation withstanding for screw mount caps. This means, that 1500 V is slightly above rated continuous voltage, but most likely still O.K.. Personally, I would prefer an isolating base plate. As another point, you would normally use parallel resistors to guarantee equal voltage share for the series connected capacitors.

What does 2000v/60sec mean? That means a 2000v potential for 60 seconds before the insulation breaks down?

If there is an air gap and the caps sit there floating, will this be okay? What would I do if I had 40 caps in series and 15000 volts?

Thanks,
Matt

What does 2000v/60sec mean?

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It's a test voltage specification and means, that the isolation resists for at least 60 sec. Most likely more, but it doesn't tell about long term effects. If you review common safety rules, e.g. IEC 1010, you'll notice that usual working voltages are only a fraction of the test voltage.

What would I do if I had 40 caps in series and 15000 volts?

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You surely would use an isolating base plate to mount the capacitors, as already suggested.

What is an isolating base plate? I'm assuming it isolates electrically but what are the requirements for this?

We have silicone o-rings/air gap, is this sufficient?

We have silicone o-rings/air gap, is this sufficient?

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It's sufficient for 1500 V, I suppose. For 15 kV, you want to have several cm creeping distance which requires to mount the capacitors on an isolating board, e.g. PVC, acrylic glas, pertinax, whatsoever.