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Clamp meters: Peak vs. In-rush?

Discussion in 'Electrical Engineering' started by notme, Nov 11, 2009.

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  1. notme

    notme Guest

    Fluke clamp current meters have 2 features that seem similar: peak and
    in-rush. The older models have Max (some: Peak). The recent advent in Fluke
    clamps is "In-rush".

    How do these differ? Isn't in-rush current the short, max current at
    motor-turn on? Shouldn't meters with a Max feature capture this accurately?

    Compare, for example, my old Fluke 36 (Max):


    and the 334 (In-Rush):


    (click "Explore" then "Selection Guide").

    How do Max & In-rush differ? Only in the marketing department?
    Or is there a real-world difference?

  2. notme

    notme Guest

    I found this description on the Fluke 33x virtual demo page:

    "Note: In-rush current measurements done with a 330 Series Clamp Meter will
    differ from Min/MAX, Peak, or Peak-Hold measurements which are not triggered

    Sounds like other than auto-triggering, the results are the same.

  3. PeterD

    PeterD Guest

    Like this?

    Apply power, inrush current is 10A for .2 seconds.
    Current drops to 1.2A for 2 seconds.
    Current rises to 12.5A for 1 second.
    Current drops to 1.2A until termination.

    So the Inrush reading would be 10A, and the Max reading would be
    12.5A. I can see some usefulness for this, since a max only meter
    would falsly read 12.5A which the user might attribute to the inrush
    current, instead of the event at 2 seconds.
  4. Shaun

    Shaun Guest

    In the usual Max measurement, the input (current) is sampled and the maximum
    sample is displayed, but the true maximum could have occurred between the
    samples and in that case you miss the true maximum or inrush current that
    your looking for. In inrush current measurements first off it's a triggered
    measurement and measures for a very short period of time and it doesn't
    depend on samples, I think it's an analog approach.

  5. notme

    notme Guest

    In the usual Max measurement, the input (current) is sampled and the maximum
    After a short phone conversation with a tech support person at Fluke, I think
    I understand the difference: it's the acquisition speed. (The new clamps also
    have triggered event feature, but that's icing on the cake.)

    In the clamp meters in Fluke's present product lineup that have the "In-rush"
    feature, the acquisition speed is listed as 100 mS. In the older clamp meters
    (eg. my model 36) that have the "Max" feature, the acquisition speed is
    listed as 250 mS.

    In other words, old (model 36) meters sample 4 times a second. New (model
    33x) meters sample 10 times a second (overhead aside).

    Help me understand the implications of the faster acq. speed. Obviously for a
    quick event to be measured, the speed needs to be quick or the event will
    pass unnoticed. Having said that, as long as the event overlaps *any* period
    of time with the acquisition window, the peak value will be measured. Yes?
    It's kind of a random chance of getting the acquisition (for events <
    acquisition speed) isn't it? But not impossible.

  6. Access Test - Please Ignore
  7. Phil Allison

    Phil Allison Guest

    ** The term actually used is "integration time " - very important .
    ** Not at all what Fluke claim.

    See page 2 of this pdf.

    The 33x meters are actually sampling the current surge wave a " large number
    " of times in the crucial first few cycles of applied AC power, so that the
    peak value can be found.

    This is quite unlike your typical DMM that *ANALOGUE * samples a DC input
    voltage a few times a second - with these, an AC to DC converter ( true rms
    or average rectified value ) is needed to measure any AC wave.

    .... Phil
  8. notme

    notme Guest

  9. Phil Allison

    Phil Allison Guest

    ** The link was very hard to find, Fluke's site alluded to its existence but
    was not clear on where it was.

    Google helped out ....

    ** DMMs baffle the masses, it seems.

    Mainly cos the name is so misleading.

    ..... Phil
  10. daestrom

    daestrom Guest

    Now you've introduced another nuance to the discussion.

    Reading the app note it seems that Fluke designed there 33 meters to
    read the symetrical currents for motor starting.

    But as you mentioned, large inductive loads often have a DC offset
    component to their starting current. This comes as an artifact of
    closing the starter when the sine wave is not at zero-crossing
    (inevitable in a three-phase motor).

    You can see this in oscillograph traces, or hi frequency samping such as
    your set up. But it's hard to get repeatability unless you have a
    zero-crossing motor starter. Each time the motor/transformer is
    energized, it's likely to be at a different point on the sine wave.

    (some large motor/transformer protection schemes avoid false-tripping on
    this in-rush by using various techniques such has harmonic-restraint, or
    simple time delays)

    I wonder if Fluke deliberately filter out the DC offset just so they
    don't have to explain why the reading changes each time you start the
    motor :)

  11. Phil Allison

    Phil Allison Guest

    ** Fraid you have got that all wrong.

    The way to *guarantee* very large inrush surges ( with transformers and
    transformer based PSUs) is to switch on at the zero crossing of the AC

    Cos doing this generates the maximum degree of magnetic saturation in the

    ** Fluke make no specific claims about the accuracy of their "inrush surge"
    detection circuitry.

    But I would not doubt is does the job required, as far as motors and
    circuit breakers are concerned.

    ..... Phil
  12. Shaun

    Shaun Guest

    *******I'm afraid that YOU have it all wrong

    Switch on power at or close to the voltage maximum on the AC sinewave
    creates the biggest transient ( surge current) in equipment with a magnetic
    core. Think about it.

  13. Phil Allison

    Phil Allison Guest

    ** Fraid that is a very silly and persistent myth.

    You have obviously NEVER checked it out.

    ** That is very stupid and very rude.

    Read this then **** off.

    ..... Phil
  14. Phil Allison

    Phil Allison Guest


    ** I see no sign of thought here whatever.

    ..... Phil
  15. Phil Allison

    Phil Allison Guest

    "Andy is a Fucking Tenth Wit "
    ** Fraid he said the direct opposite - pal.

    " Switch on power at or close to the voltage maximum on the AC sinewave
    creates the biggest transient .. "

    Stinking FUCKWITS like YOU have no reason to live.

    Top yourself, ASAP

    Rat bait would be ideal.

    And appropriate.

    .... Phil
  16. Shaun

    Shaun Guest

    I looked up inrush current for transformers and I stand corrected, it's
    maximum will occur at the voltage zero crossing point. I thought it was the
    same as an L R circuit in which case if switch closes as the peak of the ac
    waveform it causes the maximum transient.


    BTW: Phil you are very RUDE!!

  17. The meter READS the inrush signature, you idiot. Not create it.
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