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clamp meter load
- Thread starter ILCML
- Start date
Greg,
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 1u[Ω].
In a circuit comprising several parallel conductors the introduction of an additional 1u[Ω] into one conductor will cause a redistribution of currents among the parallel group. In the case of a single conductor the introduction of 1u[Ω] is essentially irrelevant from any practical point of view.
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Sometimes I only open my mouth to swap feet...
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 1u[Ω].
In a circuit comprising several parallel conductors the introduction of an additional 1u[Ω] into one conductor will cause a redistribution of currents among the parallel group. In the case of a single conductor the introduction of 1u[Ω] is essentially irrelevant from any practical point of view.
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Skogsgurra.
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.
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.
- Moderator
- #23
I also agree that we seem to be considering two possibilities.
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
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
Hi Bill,
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...
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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- Moderator
- #26
Yes, I think so: adding a magnetic core does just what you said: adds inductance. If that core has another winding which has a burden (load) on it then it behaves as a transformer and the impedance of the conductor passing through the core is modified depending on the characteristics of the burden.
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Sometimes I only open my mouth to swap feet...
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That would introduce an equal impedance into each of the cables in the group so it would prevent the error caused by current redistribution when one clampmeter is placed over one cable of the group. A colleague describes this as 'current running away from the meter'! The overall circuit impedance would increase by a small amount - 1u[Ω] per cable in the example above, or a total of 250n[Ω] for a four cable group. If that is significant to overall circuit behaviour then it is either a very low impedance circuit or you're working to remarakble tolerances.
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Sometimes I only open my mouth to swap feet...
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Skogsgurra
Electrical
I am sorry if I brought confusion to this thread. What I wanted to say - but only spoke halfly (not sure if that is correct wording) - is that the salesman in the OP probably had heard of the "current escape effect" and used it also in a situation where this effect is nil.
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
- Thread starter
- #31
wow...ok...didn't mean to cause a stir 
....although this thread has been quite educational!
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!
....although this thread has been quite educational! 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!
The ideal CT is impervious to nearby magnetic fields. Real ones aren't. When clamped on near a transformer or motor you will get some influence. I've had "fluctuations" in measured current that weren't really there twice in the recent past. Once due to the influence of a contactor coil and once due to stray flux from a bus CT core.
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.
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.
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