Guarding Multimeter at several kV from ground? - Page 1
Forum section: electronics / general

author: babysitter
author_rank: Frequent Contributor
timestamp: November 06, 2023, 10:03:02 am
content: Hi all,knownI'm building a test fixture for testing dielectric breakdown of a component that has no applicable product standard, and as it is a combination of solid and liquid non-conductors and as such I was not ale to find any standard fitting it fully, but consider MIL-STD-202G as guideline.The fixture will have a integrated access for measurements (1:1000 divider) , where the multimeter potentially floats single-digit kV above ground, depending on the user.Now i'm not certain if I should add a guard to the hand multimeter, that is, support it by either a isolator or preferrably a metal plate that is attached by resistor to the measured potential as a "local ground" to prevent electrostatic field effects instead of having it in unknown proximity to a different potential. (And also have it at a given location, heh.)I consider it beneficial, but I have quite little HV experience.Any hints?Best regardsHendrik

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 06, 2023, 10:08:58 am
content: If the DMM is going to be floating, regardless of the differential voltage it is measuring, then it has to be physically isolated, caged, not accesible by anyone

author: babysitter
author_rank: Frequent Contributor
timestamp: November 06, 2023, 10:21:53 am
content: Quote from: PartialDischarge on November 06, 2023, 10:08:58 amWell, true.But my question is about a "electrostatic guard" - to prevent influences due to the multimeter <airquote>realizing</airquote> that it floats.Not if and how and where and when the material under test, the hv, the operator or the bystanders which might or might not deserve it realize it.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 06, 2023, 10:51:30 am
content: The DMM only measures differential, it does not care or is affected by floating voltages. I've even floated battery scopes to 5kV, not that I recommend anyone do it, and they work fine.

author: babysitter
author_rank: Frequent Contributor
timestamp: November 06, 2023, 11:04:15 am
content: Quote from: PartialDischarge on November 06, 2023, 10:51:30 amI'm aware of it measuring differential, I was just considering that I could break the influence of the very big round capacitor plate (tm) and the resulting capacitive voltage divider that may or may not inject voltages, possibly upsetting things, even if its just extending the equalisation time until the mulimeter shows the right value.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 06, 2023, 11:58:03 am
content: I think you should draw a schematic of all the relevant resistances/capacitances in your system and see if they could affect each other.

author: TizianoHV
author_rank: Regular Contributor
timestamp: November 06, 2023, 05:18:19 pm
content: What kind of DMM are you using?I used my DMM multiple times floating (in the photo it was sitting at 100kV in a spherical shield, measuring leakage current).When floating the DMM at a few kV’s, I always shield it with some aluminum foil (connected to common, leaving out just the display). Without shield, in case of a breakdown, it would reset due to EMI.The shield should be connected to the source side (not to the measured side).*If the floating shield is accessible you should add a second shield connected to ground to avoid contacts...

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 06, 2023, 05:57:57 pm
content: Nice one, where did you get that transformer and accesories? I have a 100kV HaefelyAre you measuring the leakage of the isolation leg? What did it measure?

author: babysitter
author_rank: Frequent Contributor
timestamp: November 07, 2023, 07:48:52 am
content: I went the experimental route, have folded a guard from thin metal, and it seems the guarding question is valid.I attached the guard metal to one output of a 1:1 mains transformer (240 V 50 Hz country) the other output to ground, and dependent on the position and what i attached to the DMM inputs, i got between 5 mV (shorted input) and 900 mV (higher U with rising Z) of "extra voltage" in AC mode.This is a strong indicator that the guarding might have benefits.BRHendrik

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 08, 2023, 05:12:22 pm
content: Quote from: babysitter on November 06, 2023, 10:03:02 amis the dependency related to the impedance of the user?