Clamp Meters

[Diarmuid]

I've had a question hanging around in the back of my head for quite a while...

My understanding of a clamp meter, is that you just take a cable, put the cable in between the jaws of the clamp and then it tells you the current going through the cable,and dependent upon how good the clamp meter is, it tells you the voltage etc.

Is that right?

The reason for all this confusion, is that we have a clamp meter at school, and when we clamp it around a cable, it doesn't work... and the rather ambiguous instructions don't say whether they refer to a complete cable, or just one conductor of that, as in a live wire...

It might just be that our clamp meter is dodgy, but I thought I'd make sure, before I get one in the post christmas sales..

Thanks for your help!

[derekleffew]

First, the clamp meter must be used only around a single wire conductor, not around cable. Second, the clamp meter will only tell you amperage, not voltage, without physical contact using the included probes. Disclaimer: Always use every available precaution when working around live electrical circuits! Here's the one I use, Fluke 30--it's not the best, but adequate for my purposes. It has been replaced by the Fluke 333A, MSRP $149.95 US.

Sounds like yours is not a very good manual. Out of curiosity, what make and model is your meter? Here is a link to the manual for the Fluke 333. It will probably help clear up your confusion.

[SteveB]

Originally Posted by derekleffew

First, the clamp meter must be used only around a single wire conductor, not around cable. Second, the clamp meter will only tell you amperage, not voltage, without physical contact using the included probes. .

I believe an Amp meter, as a clamp around a single HOT conductor meter is called, will only do a reliable test on loads whose sine wave is not distorted in some fashion - I.E. they don't work to get you the load of a Sensor 96 dimmer rack (or any dimmer rack, for that matter).

Or am I thinking of something else ?.

Steve B.

[derekleffew]

I certainly hope you are mistaken, SteveB, as I've used my Fluke 30 to measure amperage loads on many touring dimmer racks and ML Distros, and never been challenged on the results. I think you may be confused on the measurement of output voltage of a given dimmer, for which you need a "true RMS" (Root Mean Square) meter.

[Diarmuid]

I'm not sure the make and model of ours, because it is at school, but I'll try and find out...

And if it can only be clamped around the live conductor, would I be right to suggest that in most applications it can't be much use? I guess if you had like a 3 phase system, with all the different phases on seperate inputs, then it would be ok... but as it can't be used for actual cables, for installed systems and such, it can't be used very effectivley?

Thanks for the help.

[David Ashton]

Clamp meters do have practical limitations in getting to the single conductor which generally involves getting close to live wires, a clamp on meter loses accuracy on dimmed settings as the waveforms become too complex to analyze accurately and they do not read the dc content of the waveform, but they are near enough for all practical purposes.There are cheap rms meters on the market which are perfectly useful for the type of work we do.

[STEVETERRY]

Originally Posted by Diarmuid

I'm not sure the make and model of ours, because it is at school, but I'll try and find out...
And if it can only be clamped around the live conductor, would I be right to suggest that in most applications it can't be much use? I guess if you had like a 3 phase system, with all the different phases on seperate inputs, then it would be ok... but as it can't be used for actual cables, for installed systems and such, it can't be used very effectivley?
Thanks for the help.

If you want to measure the current in a single-phase, two wire plus ground branch circuit or feeder, just make yourself a male-female adaptor with the outer jacket of the cable removed so that you can get the clamp around the insulated hot or neutral conductor.

BTW, the clamp meter does not care if it's measuring the neutral or hot conductor--the current is the same in both for a single-phase circuit.

For higher current feeder applications where you may be clamping around live conductors on equipment with the covers off, you are now under the rules of NFPA 70E "Standard for Electrical Safety in the Workplace" in North America. This defines the protective gear you must wear, and the various boundaries and voltages where different rules apply. DO NOT ATTEMPT THIS without training and expert help!

Of course, this does not apply if you are clamping around insulated, single conductor feeder cables or branch circuit conductors outside equipment or switchgear.

ST

[Diarmuid]

Originally Posted by STEVETERRY

If you want to measure the current in a single-phase, two wire plus ground branch circuit or feeder, just make yourself a male-female adaptor with the outer jacket of the cable removed so that you can get the clamp around the insulated hot or neutral conductor...

Would I be right to assume that removing the insulation from a cable, to just leave the insulation on the wires, wouldn't be possible to do in a way that would pass electrical safety testing? Basically would it be safe to do it, or could it be put through a junction box or something like that to make it safer?

Thanks

[SteveB]

Originally Posted by Diarmuid

Would I be right to assume that removing the insulation from a cable, to just leave the insulation on the wires, wouldn't be possible to do in a way that would pass electrical safety testing? Basically would it be safe to do it, or could it be put through a junction box or something like that to make it safer?
Thanks

There's no need or purpose in removing any insulation. A clamp around Amp-meter will measure the current with insulation intact. It's measuring the EMF.

As further clarification, the typical clamp around amperage measuring meter - here's a photo of an Amp-Probe model, has a set of jaws that open to allow you to wrap the jaws around an insulated conductor, the jaws then close to create a loop around the conductor. There's usually a few inches of clearance between the conductors and the insulated jaws of the meter. Care and attention needs to be in place to do this measuring safely and Steve T. has listed a link as to the methods now required.

As to usage. One typical example would be attempting to get a rough measurement of the normal load on an electrical panel that has space(s) for additional breakers, but whose existing loading is undetermined. Removing the panel cover and testing the hot legs, presumably while the loading is as close to maximum current usage, would then yield a reading.

SB

[SteveB]

To Steve Terry

By the way - a Merry Christmas to all you folks at ETC !.

A question arose as to whether a clamping amp-probe will accurately measure a dimmed load ?. Does the distorted sine-wave affect the readings ?. That's what I was told years ago and is why I stopped using my personal Amp-Probe to measure portable dimming systems.

Steve B.