Electronics > Projects, Designs, and Technical Stuff

Open HV Probe 40kV

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tggzzz:

--- Quote from: beanflying on April 21, 2019, 09:27:03 am ---And some more 'actual evidence' and a study Study of electrical properties of 3D printed objects

and

https://www.sciencedirect.com/science/article/pii/S1876619616000565

Data on PLA (brand specific) https://ultimaker.com/download/74599/UM180821%20TDS%20PLA%20RB%20V10.pdf

--- End quote ---

A couple of quotes from the first reference that would concern me w.r.t. this application (my emphasis):

  The dielectric strength measurement was difficult to
  complete due to the surface discharges that occurred during
  the increasing of the voltage. Therefore the relatively high
  observational error distorted the measured data. However,
  the highest observed value of dielectric strength for PLA
  was about 33 kV/mm.

  However, several problems could occur, when using
  PLA in insulating systems. This material is derived from
  corn starch and so it is biodegradable, unstable in time. The
  mechanical and other properties could change very
  markedly during aging. The solution could be also the
  development of proper additive to suitable adjustment of
  the PLA properties.

  Generally, an improvement of the electrical properties of
  these materials is required, for example by adding some
  additives to the base material.

The ultimaker data contains nothing relevant to this discussion; I'm not sure why you referenced it.


--- Quote ---The only downside would appear to be surface contamination and that to a degree is true of any materials regardless of process.

--- End quote ---

Of course. The degree is the key point; professionally produced products wiill have characterised the materials and processes, and won't be constrained to use whatever fits in their machines.

My first boss told me a story... He was using a screwdriver around the neck of a TV CRT, and after a while he got a hell of a belt from it. After looking carefully for the cause, he found there was an almost invisible oil track/film on part of the screwdriver's stem and handle.

beanflying:
The Ultimaker data does contain electrical properties and goes toward what DaJMasta mentioned about actual datasheets on Filaments.

Far from being an 'poorly understood properties' but perhaps 'needing more study' is appropriate which is the broad NON Conclusions about aging and extended use they made.

If you take there figure of 33kV/mm as a best case the tip of my design is @ 210mm away from the hand gaurd. So 170X my proposed maximum would be needed in theory be needed to get it to breakdown. The PLA would need to be compromised dramatically for 40kV to be an issue.

tggzzz:

--- Quote from: beanflying on April 21, 2019, 10:33:06 am ---The Ultimaker data does contain electrical properties and goes toward what DaJMasta mentioned about actual datasheets on Filaments.

--- End quote ---

It only contains dissipation factor and dielectric constant. Those are useful for assessing capacitance and HF losses, but have zero relevance to operation at 40kV.

Hence my statement that "The ultimaker data contains nothing relevant to this discussion".



--- Quote ---Far from being an 'poorly understood properties' but perhaps 'needing more study' is appropriate which is the broad NON Conclusions about aging and extended use they made.

If you take there figure of 33kV/mm as a best case the tip of my design is @ 210mm away from the hand gaurd. So 170X my proposed maximum would be needed in theory be needed to get it to breakdown. The PLA would need to be compromised dramatically for 40kV to be an issue.

--- End quote ---

Engineers base their designs on worst case specs, not best case specs.

I gave an example of how smooth and solid metal and plastic could be dangerously compromised. Hombrew 3D printing is worse in that it has non-smooth surfaces with problems stated in your reference. Add to that any porosity and aging, and who knows what the worst case properties might be.

But hey, it is your heart, and it is up to you to do with as you wish.

PartialDischarge:

--- Quote from: beanflying on April 21, 2019, 10:33:06 am ---If you take there figure of 33kV/mm as a best case the tip of my design is @ 210mm away from the hand gaurd. So 170X my proposed maximum would be needed in theory be needed to get it to breakdown. The PLA would need to be compromised dramatically for 40kV to be an issue.

--- End quote ---

Well not really. I'm not going to position myself for or against the use of PLA but let me clarify some terms.
Breakdown is measured across a material and not over the surface. The latter is surface tracking which is highly dependable on humidity and roughness and on the material in question. Some materials are tested for CTI (comparative tracking index) for that reason in some materials used in power equipment.

Now, breakdown can measured at DC or at 50Hz, and it makes a huge difference. That same paper he measured in the video with a dc breakdown of >1kVdc would break down at 200Vac easily.

Most importantly and less known is that for anything high voltage (>10kV) conductivities, permittivities and the distribution of the field lines are of utmost importance yet ignored by mostly everyone.
At pure DC, high voltage only cares about conductivities along its path and will create resistive dividers as a result. At AC material permittivities do matter and capacitive dividers are formed. If a high er material is inserted as an insulator at a certain point near a high voltage conductor for example, field lines will be expelled to the outside, and if that outside is air, the concentration of field lines may create breakdown in this space, something with goes against the idea that inserting a good insulator in a high voltage path is always positive.

Chapter 2 in this interesting book talks about this
https://www.academia.edu/30766171/High_Voltage_Engineering_Practice_and_Theory

PartialDischarge:

--- Quote from: tggzzz on April 21, 2019, 11:27:10 am ---It only contains dissipation factor and dielectric constant. Those are useful for assessing capacitance and HF losses, but have zero relevance to operation at 40kV.

--- End quote ---

Actually the dielectric constant is the top 1 or 2 most important specification or any material used in high voltage, since it dictates how the field and equipotential lines will be distributed, the capacitances that you mentioned. Since most HV applications are for power line frequency this has been extensively studied, the most typical being an air void inside a high er dielectric (due to a problem in epoxy injection in a bushing for example). This will create arcing inside the void (acting as a very small capacitor) which will create UV light, which will destroy surrounding walls, which will create cracks, conductive paths, etc ,etc

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