Conductive or Not Cable Clamp

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beanland

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Vancouver, WA
Medium-voltage cable, single-conductors, can I clamp each phase with its own metal cable clamp without having to worry about current induced in the clamp?
 

don_resqcapt19

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Medium-voltage cable, single-conductors, can I clamp each phase with its own metal cable clamp without having to worry about current induced in the clamp?
If the clamp and what it is attached to is ferrous and will completely encircle a single conductor you will have heating in the clamp. If the current is high enough, the heat could damage the conductor insulation. If there is a gap or if the clamp is non-ferrous you won't have a problem.
 

GoldDigger

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I don't know either but I was thinking that the shielding may have some effect since it already encircles each conductor.
Normal conductive shielding will reduce the electrostatic field (making non-contact voltage sensors ineffective) but will not noticeably reduce the magnetic field outside the shield.
There would be a reduction in magnetic field only an equal return current were being carried by the shield/neutral, as would be the case in SE.

Magnetic heating will affect primarily ferrous metals, while induced current heating requires the conductive clamp to cover some distance parallel to the current flow, or for the ferrous clamp to also encircle another conductor subject to induced current.
 

beanland

Senior Member
Location
Vancouver, WA
Split or not

Split or not

What you are indicating is what I have been otherwise informed. (did I say that?)

I know that if I have a ferrous (magnetic) clamp, I need to put an "air gap" in it to make sure there is no current flow.

What if I have a ferrous clamp, with a gap, but that gap is bridged by a stainless steel (non-magnetic but conductive) bolt.

What I am having a hard time accepting is that if I place a non-magnetic conductive loop about a single phase conductor there will be no current induced in the loop.
 

GoldDigger

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Location
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Retired PV System Designer
What you are indicating is what I have been otherwise informed. (did I say that?)

I know that if I have a ferrous (magnetic) clamp, I need to put an "air gap" in it to make sure there is no current flow.

What if I have a ferrous clamp, with a gap, but that gap is bridged by a stainless steel (non-magnetic but conductive) bolt.

What I am having a hard time accepting is that if I place a non-magnetic conductive loop about a single phase conductor there will be no current induced in the loop.

The reason that current is induced from one wire to another is that they enclose the same area of varying magnetic field, not that they are in any way intertwined with each other.
The magnetic field around a single straight conductor goes in circles around that wire. There will be little or no magnetic field inside the cross section of the wire with field lines parallel to the wire. But that is the orientation of magnetic field that is needed to induce current in a loop surrounding the wire.

An elongated loop of wire roughly parallel to the straight wire, whether it encircles the wire or not, can have current induced in it. That is the reason that we worry about induced current in the conduit when a single phase runs by itself in that conduit.
 
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eHunter

Senior Member
What you are indicating is what I have been otherwise informed. (did I say that?)

I know that if I have a ferrous (magnetic) clamp, I need to put an "air gap" in it to make sure there is no current flow.

What if I have a ferrous clamp, with a gap, but that gap is bridged by a stainless steel (non-magnetic but conductive) bolt.

What I am having a hard time accepting is that if I place a non-magnetic conductive loop about a single phase conductor there will be no current induced in the loop.

What is the cable type, size, operating voltage and expected current(continuous & max)?
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I know that if I have a ferrous (magnetic) clamp, I need to put an "air gap" in it to make sure there is no current flow.

You are not putting in an "air gap" to prevent current flow, you are putting in a "no-iron" gap to prevent large magnetic fields. Because of hysteresis effect in ferromagnetic materials, just the alternating magnetic field, without any large scale current flow, is what is heating the material.
Hysteresis heating is one of the heat sources for a transformer core, particularly if it is made of the wrong material.
 
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