clamp meter load
clamp meter load
(OP)
When measuring one leg of a 3 phase (coming out of a phase adder, fyi) with a clamp meter (FLUKE 336) there should be no noticable effect on the current or voltage whatsoever, correct?
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RE: clamp meter load
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RE: clamp meter load
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RE: clamp meter load
RE: clamp meter load
RE: clamp meter load
An example where this effect can be readily observed is when you have short parallel cables - like what you sometimes find from a power transformer secondary to the bus-bars in the distribution system.
The resistance and impedance of each cable is very low and as long as nothing is clamped on, the current will be evenly divided between the cables. If you measure current in one cable and multiply with total number of cables, you will see that the product is a little less than the total current. The reason is that the clamp increases impedance of the cable measured so that current is decreased somewhat. The two other cables (if you have three parallel) then carry a little more. The effect is hardly noticable - but it does exist.
Circuits having milliamp current usually have a high impedance already and they will not be affected by a current clamp. The field around such a conductor is also very low and does not create any significant counter-EMF in a core around it.
Gunnar Englund
www.gke.org
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RE: clamp meter load
The iron in the current clamp will be creating a wireless reactor. These were unwound cores through which the cables of a transformer were passed to raise the effective reactance of a transformer. If only one cable passes through the core or the clamp meter it will raise the impedance of that cable slightly. It's not the burden, it's the iron encircling the magnetic field of the conductor that causes the effect.
respectfully
RE: clamp meter load
JRaef.com
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RE: clamp meter load
But a CT? That would be unusual indeed. The CT is like a current source relatively insensitive to change in impedance of the driven load provided you don't push the CT into saturation. It doesn't sound right to me.
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RE: clamp meter load
A CT (or a clamp-on ammeter) has a secondary winding that feeds a burden. If the burden is low ohms (which it usually is) the core's flux excursions needed to induce necessary voltage is quite low. If the burden is higher, which it sometimes is, the voltage is higher. Strictly proportional to burden.
This voltage is reflected back to primary and produces a counter-EMF in the primary winding, which - if low impedance - will react in such a way as to reduce current. In the three parallel short cables scenario, the effect can be readily observed.
But I do not subscribe to the "not closed yaws" theory. If yaws are not closed, not much flux is developed and the effect is not possible to see.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: clamp meter load
Remember that the meter core encircleing a power conductor also comprises an inductor with one turn. You have encircled a conductor with a magnetic material and eddy currents will result. Admitted, the laminations reduce the eddy currents, but the inductance remains. This will also have the effect of increasing the impedance regardless of burdens and even without secondary turns.
I believe that the inductive affect will in most cases be several times greater than the burden effect.
Respectfully
RE: clamp meter load
And, to take that a bit further, it is the priary winding in a transformer. The secondary winding is usually connected to a low ohm burden, where the measurement is made - be it a relay coil, a meter movement or an electronic amplifier - as is usual in clamp-on meters.
If the secondary were shorted (burden = zero ohms) it would be like shorting the secondary of an ordinary transformer - no counter-EMF and lots of primary current. In other words; no inductor.
That is exactly what happens when you short a CT. No voltage drop across it. It is only when you have a burden where secondary current develops a voltage that you notice the presence of the core. So you can, in effect, have a core circling a conductor without having any inductance in it at all.
It is quite revealing to do a simple experiment: connect a DMM with AC mV range to two points a couple of inches apart on a current carrying conductor. Then put a clamp-on ammeter between them. If you have enough current (abt 10 amps), then you will see voltage increase. Sometimes the voltage is dependent on what current range (different burden) you select.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: clamp meter load
Have you ever tried that experiment using the hall Effect type current clamps which have a bucking winding to drive the core flux to zero? Curious as to the experimental results if you have.
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RE: clamp meter load
I agree there is some very small impedance associated with the primary of a clamp-on ammeter. Since the secondary of the clampon feeds a voltage circuit (very low current), the impedance is simply the same impedance we would get from an iron donut.
A set of parallel cables would have individual cable current unusually sensitive to even small impedances.
So in clamping an ammeter directly onto one of several parallel cables, it seems very plausible that the division of current among those multiple cables might be sensitive to that very small impedacne.
BUT the scenario described on 19 Jan 07 18:02 was a CT whose secondary was feeding a meter. It was claimed that clamping an ammeter onto the secondary circuit affected the reading of the meter and if I follow skogs response the reason would have been that the added impedance of the clampon affected the primary circuit of the CT.
I have two problems with that:
#1 - This impedance of the CT which was very small to begin with is reduced by a further factor of CT turns ratio squared in going from the secondary to the primary of the CT. Very small times something like (1/20)^2 will be ridiculously small.
#2 - The nature of the load on the primary of th CT in this scenario was not described but it would not logically be one of a set of parallel cables because we have a permanent meter on the secondary of the CT (you explain to me why anyone wants to monitor current in individual cables of a parallel cable configuation). Maybe the poster can clarify the primary in thise case, but you can call me skeptical.
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RE: clamp meter load
I do just that. Typicall several times every month. The reason is simply that if you have a set of parallel cables (which you have when a 240 mm2 or 300 mm2 cable is not enough) then the clamp cannot "take" all cables. You then have to measure current in one cable and multiply by number of cables.
It is when you do that that you can see the effect of current "avoiding" the cable you measure.Sorry, I am on my way to Indy now. Staying overnight in Gothenburg. So I do not have acess to measurements I made.
If, to make things very simple, we have a CT with 500:1 ratio and a burden that has 1 V voltage drop. Then you have no less than 0.002 V drop across the CT. And that means a noticable current difference in a 5 m 240 mm2 conductor paralleled to other conductors. Go and try it! It is a fact.
Some of the older clamps were real bad in this respect. The newer, electronic ones seem to have less power consumption in the measuring circuits.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: clamp meter load
"I do just that."
You have a permanently installed meter reading individual cables of a parallel set? I doubt it. I think you're saying you put a clamp-on meter on individual cables. But jraef said this particular CT feeds a permanent meter. My point was that the load on the primary of this CT feeding a meter is not likely one of several parallel cables. That was my point #2 above.
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RE: clamp meter load
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RE: clamp meter load
The discussion then took another turn and was more or less about the possibility that a CT an influence current at all. It can. And that has caused some confusion with my customers in the measurements I mentioned.
So, what I said is that that measurement situation sometimes occurs. And I think it is good to have some knowledge about it.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: clamp meter load
Maybe somewhere along the line there was confusion on my part or another's part about who was talking to who.
I didn't disagree with any of your comments gunnar except in the context it appeared you were defending the scenario posed by jraef. You directed your comments to me, but all of my comments are specifically to question the scenario of jraef.
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RE: clamp meter load
Most clamp on devices seem notably devoid of cooling fans, or heat sinks.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: clamp meter load
Do the maths:
Typical meter burden of 1 VA.
Let load current be 1000A to keep the maths straightforward.
From the above conditions and knowing that primary VA must be equal to secondary VA then the volt-drop in the primary due to the reflected impedance must be 1 mV. The reflected impedance is therefore 1uΩ.
In a circuit comprising several parallel conductors the introduction of an additional 1uΩ into one conductor will cause a redistribution of currents among the parallel group. In the case of a single conductor the introduction of 1uΩ is essentially irrelevant from any practical point of view.
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Sometimes I only open my mouth to swap feet...
RE: clamp meter load
I think the specific condition that you have added ( a circuit with several conductors in parallel and the current distribuition on the conductors) creates confusion to the original posted question. When measuring one leg of a 3 phase (coming out of a phase adder, fyi) with a clamp meter (FLUKE 336) there should be no noticable effect on the current or voltage whatsoever, correct?
Your answer (and those of others) make think that yes, if you insert a clamp on or a CT on a circuit, the original electric parameters as current or voltage are altered.
RE: clamp meter load
One, where there are cables in parallel, the installation of a magnetic circuit encircling only part of the conductors may make a noticible difference.
Two, encircling one phase of a three phase circuit with a magnetic circuit will have the effect of adding a small amount of impedance to the conductor in question. This effect may not be noticable but it may be measurable, as Gunnar proposes, with .
In the field, we are not usually concerned with the affect of a clamp meter on one phase. We do consider the small effect of a clamp meter around one of a group of parallel conductors. This effect is small but it may be noticable.
As Gunnar pointed out, it may be measured with a DMM.
Greg, the increased inductance in one cable of a parallel run of cables diverts current to the other cables. The heat due to the increased current is developed over the length of the other cables into which the extra current is diverted.
respectfully
RE: clamp meter load
You can't treat a transformer as an inductor - the transformer just reflects the secondary impedance, whatever it might be, back to the primary side. So if the burden is resistive it will appear that resistance has been added to the primary, similarly with an inductive burden it will appear that inductance has been added. The reflected impedance is proportional to the square of the runs ratio. If the transformer secondary was open circuit then it would appear as an inductor.
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Sometimes I only open my mouth to swap feet...
RE: clamp meter load
Oh what I would give for an edit facility!
RE: clamp meter load
Thanks for the correction. I guess I should have said impedance. With that correction is my statement acceptable?
respectfully
RE: clamp meter load
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Sometimes I only open my mouth to swap feet...
RE: clamp meter load
RE: clamp meter load
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Sometimes I only open my mouth to swap feet...
RE: clamp meter load
I really didn't want to say that current in a practical circuit is influenced by a clamp around a conductor. No. No way.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: clamp meter load
So with all this detailed information, I'm gathering that the induced current into the other legs is there, but basically insignificant, as well as any change in voltage?
I'm not absolutely positive, but let's assume the cable length is less than 100'. With less than 1 ohm of added impedance on the other lines, I don't imagine the voltage or the current's magnitude would change significantly, or enough to even mention when describing (or having been described) the line's conditions?
(incidentally the fluke # was an example, not exactly what was used)
thank you, I can never get enough here! :)
RE: clamp meter load
I've also seen a very significant measured current imbalance in motor leads that went away when I more carefully controlled the CT-to-cable geometry.
I suspect that the original problem may have more to do with one of these effects than with any burden imposed by the CT.