Connecting to a high voltage AC supply

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Joined: Fri Oct 01, 2021 4:06 am

Connecting to a high voltage AC supply

Post by Tomblueprintauto »

Hello everyone.

I want to look at the voltage and current through motor connected to mains supply. I'll have my "negative" lead connected to earth and and "positive" connected to Line (or Phase for some other countries). Supply voltage is 230V.
Now, because my negative lead will actually be positive about 50 times every second, do I need to put an attenuator on both leads or will one on the positive be enough to protect the scope from high voltage damage?

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Re: Connecting to a high voltage AC supply

Post by victor2k »

Use a TA041 Differential Oscilloscope Probe or a simple transformer 220 to 9 - 12 v for voltage capture with attenuation given by the ratio between primary voltage and secondary voltage(measured with the multimeter).
I think is not safe to use the scope without a good USB isolator on AC supply.
Best regards
Last edited by victor2k on Tue Jul 18, 2023 4:38 pm, edited 1 time in total.

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Re: Connecting to a high voltage AC supply

Post by ben.martins »

Hi Tom,

As already mentioned please don't use anything other than a differential probe if measuring anything on mains electricity. Attenuators only ever attenuate the signal side meaning the ground is unprotected.

It's worth mentioning that depending on where you are measuring you should also be aware of the correct CAT rating. Given the scope, leads and attenuators are not CAT rated these will not offer you protection in the event of any transient voltage which could occur on the mains supply.

Both the TA041 and TA057 are CATIII rated meaning you will be protected but be careful with the probes you use. Ideally they should be equal or higher to the CAT rating of the probe. If lower, then this will derate the differential probe to that of the measurement probes. For example, if the differential probe is rated to CATIII and you connect CATI probes, the whole device is now CATI.

If you're ever in doubt please also refer to the safety guide for the scope you are using or send a message to

Kind regards


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