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Pearson current monitor model 1330 teardown (100kA)

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Mads Barnkob

Thu Feb 25 2016, 08:18PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

January 2015.
So, there I was, happy about my latest purchase, a huge and extremely noise immune 100kA current monitor.

Testing it on a small cap bank did however reveal that something was wrong with it.

I tried to write Pearson tech support for help with the above image attached.

I have a second hand Pearson current monitor model 1330 that is very ill behaving compared to my model 101.

I made some different measurements with a little description on the bitmaps from my oscilloscope. The measured currents are around 12-16kA.

The 50 Ohm attenuator mentioned in the image is a 50 Ohm -10dB attenuator.

I also tried looping the coax cable through large ferrites for common mode noise suppression without any luck. Neither did it help with a 50 Ohm termination at the oscilloscope input.

I put the different measurements together in a imag

The reply from Pearson

Thank you for the Imgur data. First off, I would not recommend using our current monitors with external terminations other than 50 Î© and 1 MÎ©. Our current monitors have a 50 Î© output impedance and the sensitivity is designed to work into an oscilloscope at 1 MÎ© input impedance. Using the 50 Î© setting on the oscilloscope with cut the sensitivity in half. Also, the 1330 has an internal series resistor to make the output impedance 50 Î© so putting different loads in parallel with the unit may not have the intended result.

What is the input setting on your oscilloscope? Are you using an external 50 Î© termination with the 50 Î© attenuator? From #1, at ~14 kA I would expect the 1330 channel to be ~10-20 V with a -10 dB attenuator, depending on termination. Since the sensitivity appears off it could be a damaged series resistor or termination resistance. From #2, it looks like the internal termination of the 1330 could have been damaged. Has this unit been used for measuring damped ringing waveforms in the past?

Have you measured the terminal resistance of just the 1330 with the DMM? The 1330 should measure ~46 Î©. If it is much higher this usually indicates a damaged series resistor which can affect the sensitivity when using a 50 Î© load.

I requested help on repairing the unit and all communication stopped after that.

As I had nothing to loose, except money already lost, I decided to take it apart to find a faulty component and maybe repair it on my own.

The green paint is thick, hard and sticks very good to the copper surface, it was a messy and long job just to remove enough of it to find out how it was assembled.

Prying the first copper shield off was a whole evening job. Notice that the two shields are not connected in the through-hole, there is only a tiny narrow slot for the magnetic field to enter the CT.

Carefully chipping the epoxy off was a job that took some evenings week upon week, it was over a long period.

After I had removed the inner copper shielding I lost all hope and interest when there was just another layer of epoxy and this time chipping it off was not an option, now there is components and a ?brittle? core involved. I left it on a shelf for a long time.

Last week I found a large pan on the scrap yard and thought, I will try to cook it and see what that does to the epoxy. Just add salt and pepper for a tasty transformer soup.

The last remaining copper part with connector in it came loose in first boiling run and a cable revealed where the series resistor was located. Carefully removing the softened epoxy, the series resistor came apart in tiny pieces and from the look of it, it was once warm, maybe too warm. This was however also the only visible damage I found.

Connector got out in the free

On the inside of the through-hole, which by the way was only accessible for electromagnetic waves through a thin 2-3 mm wide slot between the shieldings from each side of it, there was another weird coil of 5-6 hair thin wires, wave shaped, in parallel on a wide piece of tape, tapped for each 30mm with a gold wire into the distributed load resistors on the regular large coil. See schematic.

The thin wave-shaped wires should be visible from these two different angles

Schematic made from my best guess on how its connected, number of distributed load resistors it not accurate.

As you can see, I tried replacing the 39R series resistor, identified by the rings it left in the epoxy, and not with a great result, DC resistance is still far from specification.

This way the money was not all a waste. End of story, so far. February 2016.

Dr. H.

Fri Feb 26 2016, 06:23AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Big BIG thumbs up for the Perseverance Mads. Very interesting info indeed.

Thanks for shearing.

Cheers

Chris_Knight

Fri Feb 26 2016, 07:27AM

Registered Member #58280 Joined: Sat Jan 09 2016, 06:48AM
Location:
Posts: 43

Have to commend you for sticking through so long. My curiosity would have met its limits long ago!

Finn Hammer

Fri Feb 26 2016, 11:38AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Mads Barnkob wrote ...

snip

Prying the first copper shield off was a whole evening job. Notice that the two shields are not connected in the through-hole, there is only a tiny narrow slot for the magnetic field to enter the CT.

Great autopsy report. I do , however, find it safe to assume, that the magnetic field passes trough the complete structure (-the core) whereas the copper shielding serves to shield against electrical fields.
But I may be wrong.

Cheers, Finn Hammer

Dr. H.

Fri Feb 26 2016, 11:48AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

My guess is that this little gap is there to prevent the shielding from acting as a shorted turn and that the magnetic field, as Finn sad, will pass through the copper shield.

Finn Hammer

Fri Feb 26 2016, 12:00PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Sorry to double post, but I couldnÂ´t add pictures in the edit mode, and they are on another pc.

I took a 110 apart some 10 years ago, and it had a different distribution of balancing resistors:

I thought it would be so difficult to explain how the wires went, so I rendered it in 3DS to try and make it clear.

Cheers, Finn Hammer

Mads Barnkob

Fri Feb 26 2016, 09:26PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Finn Hammer wrote ...

You are of course right, what I meant was shielding from other than what is passing through the hole. There is also a difference, outer shield is not grounded to the CT output, inner shield is.

It is interesting to see that the smaller models only uses the resistive hair thin wave shaped wire and not distributed resistors as in the large. Or interesting that the large uses both, yours only had 1 wire, mine got 5-6 in parallel, maybe it was just not enough.

Did it have any series resistor near the connector?

Finn Hammer

Sun Feb 28 2016, 10:36AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

No resistors in the 110, and frankly, I felt that it looked a bit cheap with the ordinary resistors, in the 1330 of yours, but perhaps not.

I often have been thinking about making a homemade current monitor with distributed resistors on a pcb, where each turn has 1/4 of the winding as a track on the pcb, then the other 3/4 of each winding as U-shaped loops soldered in. Then the resistors could be surface mount types soldered on the pcb too. Not sure it would make any difference for our applications, though.

Cheers, Finn Hammer

Mads Barnkob

Sun Feb 28 2016, 12:20PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I did not crush any of the 1K, but the 39R was carbon composite.

We are missing something about the different terminations, the thin wave-shaped wires was only for each 30 mm, the 1K resistors was for each 5-10 mm,

Did you makes any notice of the core being made of anything special or was it just a large ferrite ring core? The 1330 is square, as you can see I knocked one of the inner corner filings out. It has a metal band all around it on the outer side to keep it all together.

When we talk home made current transformers, it always boils down to calibration issues and that is one reason to use commercial CTs. But still interesting idea about the core being soldered to the PCB with all the clamp shaped wires and sure makes it easier to solder in distributed load resistors.

Finn Hammer

Sun Feb 28 2016, 05:14PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

The core was a strip wound, very thin lamination strip, wound from something looking rather funky, I guess high nickel or cobalt content, from the shinyness of it.
Calibration of a Pearson is a joke, as calibration is same price as a new one, and guesss why that is so.
My guess is, they toss the old one, and ship a new unit.

Cheers, Finn Hammer

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