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Oscilloscope input safety ratings

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Salas:
Hi, oscilloscopes have input marks like 300VRMS CAT I. Probes have some max voltage spec too. Are they safe enough to those claims after possible high voltage practical experiences and tear down observations? Are some brands better than others to work with vacuum state circuits for instance, or to watch mains noise?

Lightages:
I am no expert on oscilloscope front ends, but you should never connect an oscilloscope directly to mains. It should be done with a differential probe or other isolating device.

alm:
CAT I 300 V would be enough for low energy circuits up to 300 V. This includes measuring on the secondary side of any reasonably sized power transformer. Any mains circuit will be at least CAT II.

Many scopes are also rated for something like CAT II 100-150 V. A 10x probe rated for CAT II 300 V will attenuate signals up to 300 V by 10x, so the scope only gets to see 30 V. Pay attention to the probes limits, if you exceed them the attenuation resistor/cap might short and the scope suddenly gets to see the full voltage. If the probes are of the switchable type, make sure you don't accidentally switch them to 1x. CAT II 300 V should be fine for connection to mains circuits as long as there are no high power motors or substations nearby. See Fluke's appnote 'The ABCs of multimeter safety' for a description of the overvoltage categories. Note that these ratings are for DC or low frequency AC; they are derated with frequency. The probe won't be able to handle 300 V of RF.

Since 99% of the scopes out there are grounded, you can't directly clip a probe across a mains circuit like you would do with a DMM. Never defeat the scope's ground connection, it's critical for safety. You can get a decent approximation of the mains signal by just measuring between ground and live. In most circuits there is only a few volts between ground and neutral. Don't clip the ground lead to the neutral, though, large currents can flow. If you want to directly measure between live and neutral, you can either use an isolation transformer or a differential probe. A poor man's differential probe (common-mode rejection ratio will suck) can be made by using two channels, connecting the ground clip together, and put them in subtract mode. You can then connect the tip of the two probes to live and neutral.

Of course common precautions for working with dangerous circuits should be followed, like avoiding working live and following the one hand rule.

kg4arn:
The scope input (at the BNC has a CAT rating) e.g. CAT 1 300 Vrms.
Adding the probe may increase the CAT rating of the scope plus probe system.

For example:  My Agilent DSO-X-3024A has a CAT 1 300Vrms input rating at the BNC  on the front of the scope.
The supplied N2863B probe is rated 300Vrms CAT I, II.
So the scope plus the probe may be used CAT II.  This is my understanding.

Also beware the probe's voltage derating curve with frequency.
As frequency increases the probe may be damaged by fairly low voltages (certainly lower than the 300Vrms figure). 

I attached a typical curve.

The curves are not exactly the same for all the different passive probes/manufacturers, but they seem to have the same general shape, in my experience.
Also, some of the less expensive probes seem to be fairly well made but the manufacturer wont provide any derating curves.

kg4arn:

--- Quote from: Salas on October 28, 2012, 02:48:23 am ---... to watch mains noise?

--- End quote ---

Take alm's advice.  Get a step down transformer, say mains voltage to 12 or 6 Vrms and monitor the mains noise from the isolated low voltage side.  Fuse the primary on the HOT wire not the neutral.

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