ELECTRICAL ISOLATION/INSULATION

[ROBOJOE77]

In my place of employment we have a vacuum vessel that is heated by resistive elements located inside the vessel. The elements are connected to copper electrodes which are connected to a rectifier through cables. The rectifier outputs 25VDC @ 8000A max power. It is common in the process to check for electrical isolation with a DMM by checking resistance from the electrodes to the vessel. Historically, a "good" value has been in a range of low 600's Ω to 700's Ω. I've only worked here for about 3 yrs now, so this seems surprisingly low to me. I would typically expect to see a value in the thousands of ohms. Typically a good 3 phase 480 motor will megger 550MΩ. Does anyone agree that this seems too low, or can explain why this would be totally acceptable? Is it because of the low voltage that this would be just fine? What do you think?

 

[JoeStillman]

It does seem very low. Is this some kind of impedance heating system, where the vessel itself is energized?

 

[ROBOJOE77]

Nope, resistive elements made of graphite. Uses alumina sleeves and k-wool around the electrode extensions for insulation

 

[winnie]

I'm used to small motors where gigohm resistance is the norm, so a 600 ohm acceptable value is shocking but plausible.

All insulation has some leakage. The bigger the system, the greater the leakage.

Consider that your system can deliver 200kW to the heating elements but the allowed leakage resistance would dissipate 25^2 / 600 = 1 W. Doesn't seem a problem to me.

Jonathan