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g/tools • Posted by u/fluke_fanatic •

Six months with the Fluke 378 FC — the clamp that measures power factor without breaking the circuit Review

Six months of daily use, mostly commercial service work and motor troubleshooting. Short version: it changed how I approach a service call, and I did not expect that.

What it does that other clamps do not: FieldSense measures voltage through the insulation, so you get true power, power factor and harmonics from a single clamp with no leads and no exposed conductors.

Where it has earned it:
Motor troubleshooting. Being able to see power factor on a running motor without opening anything means I can survey a whole MCC in fifteen minutes instead of scheduling a shutdown. Found a failing capacitor bank in a customer's plant that way and had the answer before I had my gloves on.

Where it has annoyed me:
FieldSense needs a good ground reference through you. On a fiberglass ladder in dry conditions it gets flaky and the readings wander. Fluke documents this. It still catches me about once a month.

It also does not like conductors in tight bundles. Anything close together and the field measurement is unreliable — you need a single conductor with some space around it.

Would I buy it again: Yes, and it has largely replaced my 87V for troubleshooting, which I did not see coming. The 87V still lives in the bag for anything where I need the accuracy or a real four-wire measurement.

Would I recommend it as a first meter: No. Learn on a meter with leads. FieldSense makes it too easy to skip understanding what you are measuring.

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u/milwaukee_mark •

The "not as a first meter" call is right and I wish more reviews made it. A tool that removes a step is only safe for somebody who knows what the step was.

Question — how does it handle VFD output? I have found most clamps lie badly on drive output current and I am curious whether FieldSense does better or worse.

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u/vfd_victor •

Not the OP, but I use one on drive output daily. It is better than an average clamp and it is still not a substitute for the drive's own metering.

The problem is not the meter, it is that PWM output current is not a sine wave and any true-RMS clamp is integrating something with substantial high frequency content. The 378 is honest about it and gives you harmonics, which at least tells you when to distrust the number.

For drive output I read the drive. For everything upstream of the drive I use the clamp.

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u/fluke_fanatic OP •

That matches my experience exactly and is better stated than I would have put it. Upstream, trust the clamp. Downstream of a drive, the drive knows more than you do.

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u/sparky420 •

The ground reference thing is real and worth expanding on for anyone considering it. It is not just fiberglass ladders — it is anywhere you are well insulated. Bucket truck, rubber mats, dry boots on a dry epoxy floor.

The symptom is that voltage reads low or wanders rather than reading zero, which is worse, because a wandering low number looks like a real measurement. Know the failure mode before you rely on it.

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u/gen_set_gina •

Bought one on the strength of a thread like this six months back and the MCC survey use case is exactly why it stays on my truck. Walking a customer's motor room and handing them a list of which machines are drawing what, with no shutdown and no permit, is a service I can now sell that I could not sell before.

The tool did not just make the job faster. It made a job exist.

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u/snap_on_stan •

Price check for anyone shopping — street is around $600 and it has not moved much. The 376 FC is roughly $250 less if you do not need FieldSense, and if you are honest about whether you will actually use power factor, a lot of people should buy the 376.

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