# Scope probe across AC outlet

I was told something and don't completely understand it.
I was told: If I place a scope probe across an AC outlet, I'll blow up the
probe because I'm placing the ground of the scope probe onto the AC outlet and shorting it to ground.
That makes sense because the scope probe's ground is Earth ground and I'm shorting 120V to ground.
But if I place the scope probe's ground on neutral and the scope probe tip on hot, then I should be OK, correct?
Also, I can float the scope but risk being electrocuted because it will float to a 120 potential.
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Peter wrote:

If you get the probe ground on the outlet's hot side, yes.

The neutral might have a few volts on it W.R.T. ground. That might not damage anything, but it will give you screwy readings as your probe and scope provide a second path back to the panel for appliance return currents.

What are you trying to measure? It might be sufficient to use a 1:1 transformer between the outlet and the probe (with a suitable voltage rating, of course).
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Well, yeah. You don't just randoming connect the "ground" lead of the scope probe.
If you are "probing" about a power circuit you simply connect the ground lead to GROUND. Of course, you should ensure that you don't exceed the voltage rating of the probe or the scope input.
If you need to know the voltage between two points you either can "kludge" an isolation transformer OR use two scope probes and have the scope substract the two voltages (differential input.) You would well serve yourself to educate yourself on "common mode rejection ratio" before the place too much confidence on what you see on the scope.

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On 6/13/07 5:25 PM, in article UoCdncmbmIBvF-3bnZ2dnUVZ snipped-for-privacy@comcast.com,

For safety's sake, the cabinet signal ground are connected to the green ground wire. This screws up many measurements. I have seen the grounding pins on the scope plugs snipped off. This is a very unwise thing to do.
If you want to measure live ac lines, use two probes and a dual input amplifier set up in the difference modes. With that, you can connect to any pair of ac conductors with relative impunity. You still want to avoid exceeding voltage specs on the probe.
The problem with using a single probe is that sooner or later, you will connect the probe ground clip to a hot conductor and that conductor will be shorted to ground.
Bill -- Support the troops. Impeach Bush. Oh, I forgot about Cheney.
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wrote:

So theoretically I can measure directly across an AC outlet, however, netural may be slightly above 'Earth ground' and the probe's ground needs to be on netural - and assuming my probe and scope can handle that much voltage.
Otherwise, I'd have to use a transformer, float the scope, or do a differential measurement.
I interpreted everything as: you can't put a scope probe across an AC outlet. Obviously you can providing the probe's ground wire is connected to neutral and not hot.
Make sense.
Thanks
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Peter wrote:

AND, provided that the scope probes are rated for the applied voltage.
Working with *any* live power circuit is best avoided unless it really is necessary. They tend to be very unforgiving of mistakes and making mistakes is an essential part of the learning process.
--
Sue

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On 6/15/07 3:44 PM, in article ZPudnddEGtjRi-7bnZ2dnUVZ snipped-for-privacy@comcast.com,

NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO! NO!
You missed my point. Use the two probes connected to the scope's differential amplifier simultaneously. If you want to look at the waveform between two points, connect one probe to each of those points. You do not need to connect anything to the case ground of the scope. You could connect the scope case to the protective ground obtained from a purely protective green conductor if you wish. Do not connect the scope ground to neutral.
Bill
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Iraq: About three Virginia Techs a month