Message Board Thread - "Magnetic Heating?"

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Magnetic Heating? neatster77 3/15/2007
I’m having trouble explaining (and understanding) the physics behind magnetic heating. This is an image of metallic cable tray containing 4 ea. 1000 MCM cables connected to the low voltage winding of a power transformer. The structural support for the cable tray forms a closed loop around the 1000 MCM cables. We’re seeing hot spots where the cable tray and structural support intersect. As an electrical guy, I want to say there’s a voltage-less current flowing from the structural support to ground via the cable tray. But I can’t come up with the physics to explain this. I may just give in and say its vibration.
 
Re:Magnetic Heating? neatster77 3/15/2007
This figure is my attempt to explain the cause of the hot spots.
 
Re:Magnetic Heating? IRJay 3/15/2007
as an electrican you understand the generator principal of moving a field across a metal will generate voltage. Well in this case you have a fluctuating field around the conductors and this piece of metal (which is not grounded correctly) is in that field and is having a voltage generated in it. Proper bonding of this section will eliminate this potentially dangerous situation.
Note this is not voltageless. You have a voltage imposed on this piece and the result is a circulating current within the piece.
 
Re:Magnetic Heating? jvoitl 3/15/2007
I don't believe bonding will solve this problem. Each cable tray should have one conductor of each phase in it. That way the flux from the current cancels and there is no inductive coupling to the tray itself. Just like using a clamp-on ammeter. If you put it around all three phases you will read zero. This is covered in the National Electric Code.
 
Re:Magnetic Heating? Bob Berry 3/16/2007
Bonding will not cure this.

As suggested you can split the phases between the trays. An alternitive is to use a non-ferrous supports, i.e. plastic or stainless steel.
 
Re:Magnetic Heating? Laland 3/18/2007
I agree that each cable tray should contain all three phase of conductor not that each cable tray will have each phase.
 
Re:Magnetic Heating? ies_surfninja 3/30/2007
If that cable tray has all the phases separated like the diagram shows, the engineer or electrical contractor should fix the code violation and not worry about your IR problem.
Good find it it that is the problem though!!!!
 
Re:Magnetic Heating? BG 4/1/2007
I believe that this is from induction due to each phase ran in a different cable tray. As previously stated, each phase cancels out and gives a zero reading on an amp clamp if wrapped around all phases (including neutral if applicable).
I have seen a similar application. The secondary cables from a utility transformer were ran in separate cable trays ie. all of A phase in one cable tray etc. The cable tray showed heat on it and was greater at the metal supports, which created a metal loop around individual phases.
Recommendation is to divide phase conductors evenly per cable tray.
 
Re:Magnetic Heating? Shanshal 4/2/2007
Dear IR user, my advice is to first make sure of your theory of the magnetic heating , simply by placing a magnetic compass near by the cable tray , that will ensure or eliminate the theory of the magnetic heating, please note that IR imaging can only be considered as additional support (visual) to other essential tests.

Yours
Engineer
Abdulrazak Khalid
PO Box 99203, Dubai, Deira , UAE
AbdulRazak.Khalid@dewa.gov.ae
Dubai Electricity & Water Authority
 
Re:Magnetic Heating? Kaj L 4/4/2007
The problem is well known by experienced electricians. You have here got a current transformer with one single winding.
Never put single leaded conductors through separated ways in conductive material. The simple’s way here to put things in order is to change the centre support to something of non conductive material.
Kaj L.
 
Re:Magnetic Heating? Kaj L 4/4/2007
Addition to my former statement
It is also useful to break the current circuit in the centre support by some isolator so the current not can flow trough the material.
The voltage is low but the currency can be very high in the frame, if there is a high current flow in the main cables, and it will be really high temperature if there for instant occur a short circuit.
Best regards Kaj L.
 
Re:Magnetic Heating? sw 4/12/2007
Are you sure there is not some other anomaly going on? If there is some circulating current, why are the virtical supports not being heated?
 
Re:Magnetic Heating? Kaj L 5/21/2007
sw wrote:
sure there is not some other anomaly going on? If there is some circulating current, why are the virtical supports not being heated?
Yes, in this mater I am fairly sure. There is a circulating currency even in the other parts but in lesser degree. The balance between phase B and C is 2 to 3, and gives only 1/3 of the total magnetic influence and with that depending currency in the girder. I will point out the danger: It can gives rely hot spots in case of a short cut combined with transformers with high short cut power factors.
 
Re:Magnetic Heating? neatster77 5/21/2007
Kaj L wrote:
Yes, in this mater I am fairly sure. There is a circulating currency even in the other parts but in lesser degree. The balance between phase B and C is 2 to 3, and gives only 1/3 of the total magnetic influence and with that depending currency in the girder. I will point out the danger: It can gives rely hot spots in case of a short cut combined with transformers with high short cut power factors.
Thanks for all the input.

The vertical members are heating up on the structural support closest to transformer. I'll have to upload another image to show this. There's also hardware higher up on the vertical support that is glowing hot. And the final kicker is the grounding conductors connected to the cable trays are glowing hot also. I measured approx 400 amps in bare #2 copper! Looks like I have a lot more images to upload to completely communicate the situation. This site needs more pretty pictures anyways.

Thanks again for all the insight!
 
Re:Magnetic Heating? Bob Berry 5/22/2007
400amps on a ground cable for me makes alarm bells ring straight away. I would suggest you could have a major problem. I have only come across this twice, once it was a Neutral connection broken in a transformer and the other time there was a ground problem being imported through the earthing system.

You need to get this right, and sort it out fairly quickly.
 
Re:Magnetic Heating? neatster77 5/22/2007
This is the ground conductor connected between A and B phase cable trays.
 
Re:Magnetic Heating? neatster77 5/22/2007
This is the hot spot in the upper left is the top vertical member.
 
Re:Magnetic Heating? jvoitl 5/24/2007
neatster77 wrote:
the hot spot in the upper left is the top vertical member.
Your image shows that the cable trays are warm from the circulating currents. The hotter spots are where sections of the trays are joined with higher resistance connections. These connections are warmer just as any bad or high resistance connection gets hot in a standard electrical circuit.
 


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