Is it safe to measure low current (<2 A) at high voltage (~2000-3000V)

Thread Starter

aaronshenhao

Joined Aug 11, 2017
2
I don't think it is, but I would just like to double check. My multimeter can measure up to 10 A, but the maximum voltage is 500 V. I will just be measuring only the current of an AC Transformer, which would be quite minute (~0.5 A). However, the voltage would be (~2000 V).

Would this not be possible due to the voltage rating on the multimeter (I'm only measuring current)? I would just like to be sure before purchasing a new multimeter.
 

profbuxton

Joined Feb 21, 2014
419
Not recommended! What are your meter leads rated at?. May be possible if one side of AC grounded(connected to earth) and meter in in series with lead connected to ground. But need to ensure that connection on meter cannot go o/c.
If you are gonna do this keep all leads clear and yourself away from any contact.
 

Thread Starter

aaronshenhao

Joined Aug 11, 2017
2
The leads themselves are rated higher than the meter (1000 V @ 20 A). The meter is rated (500 V or 10 A). The current I will be measuring will only be around 0.5 A, but I'm guessing it won't be possible since the voltage is too high for the meter (2000 V)?
 

Bernard

Joined Aug 7, 2008
5,743
Does meter have an AC Amps range ? If all connections can be made with power off with meter on
a suitable surface & hands off, re check connections and settings, apply power- record- shut off power.
High V rating of meter has nothing to do with current measurements as long as meter case,& leads are isolated from ground by appropriate V.
 

WBahn

Joined Mar 31, 2012
26,398
I don't think it is, but I would just like to double check. My multimeter can measure up to 10 A, but the maximum voltage is 500 V. I will just be measuring only the current of an AC Transformer, which would be quite minute (~0.5 A). However, the voltage would be (~2000 V).

Would this not be possible due to the voltage rating on the multimeter (I'm only measuring current)? I would just like to be sure before purchasing a new multimeter.
The nature of the questions you are asking leads me to suspect that you are not ready to attempt these kinds of measurements.

Keep in mind that currents as low as 0.01 A can cause painful and severe shock and the threshold for fatal currents can be less than 50 mA depending on the waveform used. So your "quite minute" currents are an order of magnitude greater than that need to end you.

Also, when measuring current, your meter better not have 2000 V applied to it. Do that and both you and your meter are likely to end.
 
I don't think it is, but I would just like to double check. My multimeter can measure up to 10 A, but the maximum voltage is 500 V. I will just be measuring only the current of an AC Transformer, which would be quite minute (~0.5 A). However, the voltage would be (~2000 V).
There are a LOT of unknowns in what you said and not enough to reccomend.

You mention AC current, so waveform and frequency and crest factor matter as well as the need for RMS current. Voltage burden maybe.
Isolation? Accuracy?

More important is can you measure it on the low or ground side?

I set up EBIC and beam current measurements on an SEM. Voltage was 15 kV and I could break the ground side. The currents were very low. Usually nA or pA. That was no issue. I had to pay attention to how much ground could be floated above ground of the instrument used.

I did work on a 15 kV 1.5 A power supply. It did have an analog current meter, but 'm not sure how they did it.

Connections need to be made with power off. Discharge procedure have to be followed. Sometimes keepers have to be used. The one hand in pocket rule.
 

DickCappels

Joined Aug 21, 2008
6,968
I have floated electromechanical current meters at 20 KV to 40 KV, both times mounted in plexiglas boxes to protect us from getting too close to them. If such precautions are taken with a DVM and the voltage and frequency are low enough so that corona and capacitive coupling to the environment are not issues (this relates to accuracy and reliability when using a DVM) then it should be fine to measure one or two KV with a DVM rated to measure lower voltages.

Just my opinion based upon experience.

I also remember floating a DVM set to measure voltage and floating it at 17 kV (more or less DC) to measure leakage currents, using the 10 Meg input resistance as a shunt. That time there were no plexiglas boxes in use, but consider that I was working in a lab in which everybody was working on cathode ray tube circuitry and understood high voltage safety issues.
 

ian field

Joined Oct 27, 2012
6,539
Don't try it. It will kill you.
There's Darwin candidates and people who learn - lets hope the TS is one of the latter............

If there's a grounded side; sometimes the current sensing element can be inserted there.

For reasonably clean sinewaves, a current transformer is probably the best way to get enough isolation.

Dc probably needs a hall effect type current sensor - great if you can insert it where the voltage is low, or you end up with big isolation on floating power supplies. Don't use any old opto for the signal - most top out around 1500V or so!

DMM leads probably have adequate insulation (just about) but you can't just chuck the meter casually on the bench like you were measuring mains.
 

Reloadron

Joined Jan 15, 2015
5,781
I don't think it is, but I would just like to double check. My multimeter can measure up to 10 A, but the maximum voltage is 500 V. I will just be measuring only the current of an AC Transformer, which would be quite minute (~0.5 A). However, the voltage would be (~2000 V).
You may want to measure the current on the primary side of the transformer you mention assuming 120 or 240 VAC at 50 or 60 Hz. Then do the math. While this will not render a perfect number with some loss it will get you close enough in most cases. Unless one is experienced with measuring current in a high voltage circuit, like what you mention, they really should not try as really bad things may result.

Ron
 

strantor

Joined Oct 3, 2010
5,468
You may want to measure the current on the primary side of the transformer you mention assuming 120 or 240 VAC at 50 or 60 Hz. Then do the math. While this will not render a perfect number with some loss it will get you close enough in most cases. Unless one is experienced with measuring current in a high voltage circuit, like what you mention, they really should not try as really bad things may result.

Ron
If OP is using a microwave oven transformer, that kind of measurement might not be close enough. I have done extensive testing with rewound MOTs (never with HV windings) and they always draw more current on the primary than they should. Not very efficient transformers.
 

ian field

Joined Oct 27, 2012
6,539
If OP is using a microwave oven transformer, that kind of measurement might not be close enough. I have done extensive testing with rewound MOTs (never with HV windings) and they always draw more current on the primary than they should. Not very efficient transformers.
And they have magnetic shunts for some strange reason - someone explained it to me once, i'm not entirely sure why I have no recollection of what they explained...........

Judging by the way they make the steel case buzz - a fair bit of energy into the primary isn't making it to the secondary.

AFAICR: the secondary is 2kV and good for at least 1/2A - Darwin's curious onlookers can get quickly upgraded nominee's..............

If you really must measure the current; i'd lift the secondary ground wire from the solder tag on the frame. That totally needs putting back in a way that won't fall off when you're done.

The main hazard would be anything getting open circuit during testing - solder all connections instead of using prods or clips.

Passing the bottom lead from the secondary through a current transformer is also an option - but an open circuit secondary on one of those can also be dangerous.
 

strantor

Joined Oct 3, 2010
5,468
And they have magnetic shunts for some strange reason - someone explained it to me once, i'm not entirely sure why I have no recollection of what they explained...........

Judging by the way they make the steel case buzz - a fair bit of energy into the primary isn't making it to the secondary.
I observed the premature saturation with the magnetic shunts removed. I found the 120V MOTs much more efficient when powered from 80-90V
 

Reloadron

Joined Jan 15, 2015
5,781
If OP is using a microwave oven transformer, that kind of measurement might not be close enough. I have done extensive testing with rewound MOTs (never with HV windings) and they always draw more current on the primary than they should. Not very efficient transformers.
Agreed but based on his original post I was not about to suggest otherwise and I did mention loss. :) Another option is use a meter designed for the intended purpose. A good meter will give the voltage limits when used in a current measuring mode, for example: 11A 1000V Fast Fuse** for a Fluke 87 in the AC current mode. Anyway, I do agree that primary current, depending on the transformer, is not always a good indicator of secondary current. The end user should always understand their meter and its limitations.

Ron
 
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