Current Clamp Question

[Deleted member 105166]

I have a Megger MFT1741 with current clamp, also a Fluke 117.
As the MFT can't record min/max, I'm using the current clamp with the Fluke multimeter, following Megger's instructions and understanding the 1000:1 step down ratio.
Just taking some basic readings on known values, the results are well outside what I'd expect to see.
For example, a line load of 8.29A on the MFT translates as 14.4V on the DMM
An 0.8mA earth leakage on the MFT shows as 2.3mV on the DMM
As the DMM is true RMS, I understand peak voltage being 1.41 x the RMS, however the measurements I'm achieving are significantly outside any adjustment to take this into account.
All equipment is recently calibrated and works very predictably on its own.
The current clamp also hums when used with the Fluke DMM if clamping a load over a few A.
It does not hum when used with the MFT, irrespective of load size.
Any thoughts/ideas welcome (other than "why did you buy that over priced huge I-clamp instead of the much more useful and compact Megger DCM305 clamp?"...which I've asked myself many times...

 

[M-Ty]

Sounds like your 1-ohm terminating (burden) resistor is faulty.

 

[Lucien Nunes]

Agree. The clamp produces 1mA/A into 1Ω, which develops the 1mV/A that the meter displays. If the resistor is open-circuit then the clamp will output an arbitrarily high voltage into the meter's input resistance. The clamp contains a limiter that restricts the output to 28V peak, to avoid damage to the secondary coil if the connection to the burden resistance is lost when measuring a heavy current. However even at the low currents you are testing, the core flux is much too high hence the buzzing. What kind of resistor are you using?

 

[Deleted member 105166]

Sounds like your 1-ohm terminating (burden) resistor is faulty.

But if that were the case, would the clamp not give inconsistent results when used with the MFT? Or am I missing something?

 

[M-Ty]

The MFT1741 is designed to be used with that clamp without an additional resistor.
But you need the resistor if using a millivoltmeter because the clamp is a current transformer.

 

[Lucien Nunes]

Exactly. Because the MFT is designed specifically to use the clamp, it incorporates the correct resistor.

 

[Deleted member 105166]

Thanks both, so the resistor in the Fluke 117 is likely to be faulty. If I borrow another DMM I should be able to confirm it is the 117 causing the problem.

 

[Lucien Nunes]

The DMM does not and cannot contain the resistor; you have to provide it externally. A normal external CT works with the meter's current ranges, not voltage, and your clamp might produce acceptable results that way. However it is calibrated into a 1R load and if the DMM integral current shunt differs from this value, the calibration will be affected. So it is best to load with 1R and use the voltage range.

 

[M-Ty]

Indeed. Imagine trying to measure 230V with a DMM with an input impedance of 1R (instead of the usual 10 megohms)!
Your original post said that you were following Megger's instructions, not just shoving the leads into the Fluke...
Your reading of 14.4V across what should have been 1R was a clue that the resistor was faulty, but it seems that you haven't actually provided it yet.
The clamp measures currents up to 1000 A with a 1000:1 step-down ratio,
which gives an output of 1 mV per amp into 1 ohm.
The resistor will dissipate 1 W measuring 1000A.
(A common 0.6-W 1% tolerance resistor available for about a penny would only be good for measuring 774A.)