Safest means to measure high voltage transients on mains ac

[FingerSling]

So, first off newbies and AC shouldn't mix.  What are the safe options for measuring transients on the mains ac and what are the trade-off's?

There is the isolating transformer, but wouldn't that actually remove some of what I want to measure?

There are various high voltage probe options, but again, what would this do the signal under test?

There is the direct connect with a fast blow fuse in-line option, but is this akin to wearing a thick jumper just in case the parachute fails? 


Finally, there are magic boxes, where it's not really clear what's in them?

[Grandchuck]

Many use something like the Micsig DP10013 High Voltage Differential Probe

[Terry Bites]

The flux density in the core is frequency dependent. E=4.44fNABm [Volts RMS]. So a regular mains transfomer will not be a good choice for monitoring fast transients.

100:1 Voltage dividers on both neutral and hot fed into an isolation amp would be my choice.
eg   AMC1100DUBR is low cost and has a rise time of 6uS. You'll also need a small iso DC -DC converter. eg NMV1S0505SC

Make sure the 100k resistors are rated at 350V or higher. Or chain 250V types to get 100k. Fusing both lines with fast fuses is a good idea.
Mains transients may be as high as 6kV Put TVS across the 1k's so the iso amp doesn't get fried. eg ESDB5V0WT

You might also think about a DIY an isolated current sense amp:https://www.ti.com/lit/pdf/slau521 Use an of the shelf DC-DC like a NMV1S0505

[CaptDon]

First, remember there can be common mode transients and differential mode transients. The best way hands down is with properly rated differential probes which are sadly very expensive. I have one of the LeCroy 70/700 volt (15MHz?) probes and they are identical to the Fluke model. If you have a suitable supply of these (or perhaps use one probe and change how it is connected for a day at a time) you could monitor 'differential' by connecting across the mains. Monitoring 'common mode' will be a bit more difficult. If you are absolutely sure your scope is floating by using an isolation transformer you could clip the normal scope probe ground lead to the best earth ground you can find (metal gas pipe or water pipe) and with a suitably rated normal probe (perhaps 10:1 with a 300 v.a.c. max input) you could probe for abnormal transients on the neutral conductor. Sometimes weird things show up on the neutral, particularly in three phase factory environments. During close lightning strikes you could see both common mode and differential mode transients that are very different from each other and yet either alone is big enough to damage equipment and combined it's a no win situation.

[pdenisowski]

High voltage differential probe.  Safest and easiest way to do this.

[tszaboo]

High voltage differential probe.  Safest and easiest way to do this.

And an adapter like this one. Current measured with a clamp.

[jonpaul]

filament or LV PS transformer eg 240V or 120V ><12V

Place light load reistor on secondary, VOILA simple cheap floating line monitor, with known ration of voltage.

j

[FingerSling]

Thanks for all the thoughts and feedback!

Thoughts on Micsig DP10013 or DP750-100 if the latter is available for less?

This video would indicate the DP750-100, but just wondering if real-world experience indicates otherwise, based on the DP10013 recommendation here.

[David Hess]

I cannot access the Micsig website so cannot evaluate those specific differential probes, however a high voltage differential probe would be my first choice.

[S. Petrukhin]

You can isolate the circuit under study from the mains voltage and use a conventional voltage divider.
Not forgetting the difference between PE and Gnd.

But it is more convenient and reliable to use a differential probe.
I have a Missing DP10007 - it is inexpensive and good enough.