I have an injection molding machine that has some odd behavior* going on in it which I need to diagnose. I've probed around with a multimeter, but I think I need to understand deeper what is happening. So, I want to connect up an oscilloscope. However, I'd also like to avoid frying my o-scope or electrocuting myself.
The house main circuit breaker runs to a 3 phase converter, which then serves out 240v, 3-phase power to the machine. The machine's protective earth (PE) is tied to the phase converter, which is tied to the house grounds.
I want to probe the different individual legs being used to power heating elements and probe between them to compare what is going on. Here are my questions:
Can I use a passive probe for this with either 1x or 10x attenuation? The datasheet for the o-scope has max input voltage of 400 Vpk. If I tie the ground of the probe to machine PE/ground, this should be ok, right?
Would a differential probe be more appropriate? I ordered 2x of these, thinking they would be the way to go, but I'm concerned that if I have an inappropriate voltage (ie: short) on two of the legs at the same time, the common mode will reject this. It would be more appropriate to use the probes on separate legs, measuring leg to PE, and then use the o-scope to subtract the two waveforms, right?
* More detailed description of what is going on: when the heating bands of the machine are turned on, power is applied from a 240v leg through a solid state relay, and then goes back to another leg. All of the solid state relays are brand new (I replaced the old ones). You can see the schematics in reference . Pages 6 and 7 are the heating elements. My machine has the K85, K84, and K81 zones (no K82). I have verified that points 85, 89, and 95 are not tied together (no neutral wire for these elements). I'm having an issue where if I turn on K84, K81 also has energy flowing through it. I've tried to use a multimeter to find a short, but I haven't had any luck so far. What's worse is when I pull out the plugs for E81, E84, and E841 that would go to the heating bands and measure point 94 to PE with the SSRs energized, I see 115v RMS (expected) and when I measure point 95 to PE with power on, I see ~80v RMS (not good).