O-scope to measure 110VAC mains

Other Hardware General Electronics
1

I have a dimmer circuit and need to look at the 110VAC side of the circuit. I have a Rigol DS1102E with the below probes.
I connected this to a socket, taking care to ensure the house grounded wire (neutral) is connected to the shield, and the house live/hot wire was connected to the probe tip. The probe is set on 10X.
It trips my house breaker. I have not tried swapping, as I am certain I got the wires correct measuring with my DVM.

Any thoughts on what might be causing this? I read some other forums that recommend floating the o-scope or lifting the ground wire, or using a resistor network to bring down the voltage to the input. I have measured much higher voltages on o-scopes before and have a hard time believing any of that is necessary. Maybe I got the wrong hardware?
Any pointers are appreciated.

Scope
The front of the unit states 300Vrms all inputs.
It states the following at the end of this document.
Maximum Input Voltage
400V (DC+AC Peak, 1MΩ input impedance)
40V (DC+AC Peak) [2]
and another section states:
Maximum input voltage on analog channel
CAT I 300Vrms, 1000Vpk; instantaneous overvoltage 1000Vpk
CAT II 100Vrms, 1000Vpk
RP2200 10:1:CAT II 300Vrms
RP3200 10:1:CAT II 300Vrms
RP3300 10:1:CAT II 300Vrms

Probes
Specifications

  • Attenuation Ratio: 10:1 and 1: 1 switchable (slide switch on the handle)
  • Input resistance of 10 megohms (10:1) or 1 Mohm (1:1)
  • Input capacitance: 10×(14-18P)、1×（120P）
  • Compensation range : 8 pF - 35 pF
  • Bandwidth: 100 MHz (10:1)
  • Rise Time: 3.5ns (10:1) and 56ns (1:1)
  • Maximum operating voltage : 600V (10:1) or 300V (1:1)
2

It is not good to connect the probe ground to the neutral wire because there may be several volts on the neutral wire, depending upon what loads are energized. The probe ground should connect to ground, or you can follow the dangerous practice of floating the oscilloscope. Even better (and safer) would be an isolation transformer.

If you have a normal circuit breaker and not a GFCI or AFCI circuit breaker, then it should just be looking at the current on the hot wire. Nothing you have posted indicates that a lot of current would flow there. Perhaps there is something else wrong.

3

vaj4088:
Nuetral to ground was the problem. My breakers are Eaton AFCI except in the kitchen, bath, etc. (per NEC). Eaton documentation shows GFCI function is included, and warns of what you stated. I tested in the kitchen and tripped the local GFCI. Then connected to the probe ground to the wall ground, not neutral, and did not trip.

With 9 Mohms in the circuit (which is what my DVM reads when probe is on 10x), the probe cannot be passing 4 or 5 mA from the wall. The GFCI must be picking up a nuetral imbalance due to current sourced from the o-scope, and not the probed wall circuit.

Thanks!!

4

Nuetral to ground was the problem.

Yes it is. You are lucky the circuit breaker tripped, otherwise things would have gone bang.

See:- Using oscilloscope safely with AC mains - Electrical Engineering Stack Exchange

Or how about this from our antipodean frend https://www.youtube.com/watch?v=xaELqAo4kkQ