Magnetic Heating

Magnetic heating is caused by a resistive connection.

Any oxidization on the bolts, bolt hole, Nut will cause some resistance to the current. The current will try to find different paths around it. This can cause arcing around different parts of the connection or simply heating (as in any load). Oxidation can build up in as little as 10 minutes after cleaning. Best practice is to clean the outer surfaces (especially the parts coming in contact with the bus- bar) of the nuts and bolts immediately before installation. Also the Bus Bar holes should have the holes cleaned. If they are tinned do not remove the tinning!

If the bolts are made of a material that do not have the same conductivity as the Bus bar, It can appear as a load. Each type of metal has a different resistance. Stainless steel may help as it has a lower amount of oxidation.

Before changing the bolts I would try cleaning or polishing the old ones and the connection points with 0000 steel wool.

http://en.wikipedia.org/wiki/Induction_heating

http://www.ameritherm.com/aboutinduction.php

No - do not put stainless steel bolts in a copper bus bar.

The most likely explanation is that the bolt in question is either loose, dirty/corroded or too small for the hole in bus bar.

If all the bolts on the bus bar are the same type and material then localised heating will be the result of one of the above, obviously if the individual bolts in question are different in material or size from other bolts then there is the answer to your question.

Bolts or set screws should either be plain brass on bare copper or cadmium plated steel on tinned copper. or plain brass on a brass bus bar

I have seen a number of heated joints on bus bars, but never heard the phrase magnetic heating applied to these circumstances. Magnetic heating is where the magnetic field around a conductor affects adjacent steelwork as a result of Eddy currents.

If you are using copper bus duct, use silicon-bronze bolts and washers. This material has an almost ideal combination of high tensile strength, high electrical conductivity, and is non-magnetic.

Bert

I once had a problem with bus bars where the threaded part was welded onto one side and the thread was damaged by the welding process. The screw went correctly solid, but was not quite pressing the two bus bars together!!

That was a shit problem to find!! It worked fine for 99% of the time.....eventually current caused corrosion between the two bus bars.......

With removeable electrical connectors such as photographic flash cords, I have been successfully using a non-flammable spray contact cleaner: Radio Shack Control/Contact Cleaner & Lubricant (#64-4315). In addition to a propellent, it contains white mineral oil (CAS#64742-21-8) as well as decamethylcyclopentasiloxane (CAS# 541-02-6). Are similar lubricants applied to bolted connections? What is the function provided by these ingredients? I am thinking of carrying a small container of white mineral oil with a cloth or Q-tip applicator on photographic locations when using electronic flash equipment: open circuits can be a nuisance due to vibration, strain, oxidation, moisture, crud, etc. Would mineral oil alone suffice for this purpose?

Mineral oil is a fairly clean lubricant and has will little to no cleaning ability (it will dissolve other oils). In fact it will cause more dirt to collect later. The other chemical is a rinse aid used in drycleaning.

The best cleaner for electrical contacts is isopropyl alcohol which does not leave a residue. I use Chem Wipes (isopropyl alcohol soaked cloth) that you can get in the drug store. They come in a box of little sealed packages.

For heavier crud and old oxidation, the use of a white eraser or 0000 steel wool will work.

I apologize if I was not quoted correctly. It is not the bolt on the busbar that was overheating but it is on the bus duct wherein the busbar is inside the duct. There are two (2) busbars each on Phase A, B and C. There is no insulation between them except on the joints on the busbar that has the bolt. My observation is on the busduct and not on the busbar.

Heat on the bus duct will either be coming from the Bus bar (and joints) or from an external source.

Hi Specscebu,

Q1: What are the busduct and the busduct joint bolts made of respectively? Are they of magnetic materials?

Q2: Do the both two busbars on each phase share the same busduct?

Thanks!

Magnetic heating or induction heating is due to expanding and collapsing magnetic fields. The effects of changing fields at 60 Hz could cause this problem. Most bus bars come with bolts and this should not be a problem unless the were changed. Suggest you look for corrosion between the bars first and if the bolt is torque down properly

Dear Guest your problem is really interesting one to me. I would like to get few points confirmed from you to understand the problem .

1) Is all the nuts and bolts are of the same metal.I mean weather the one

getting heated is of same make as others or different make.

2) Have you removed and re tightened those bolts and found whether there is still

heating happening.

3) Do you feel any vibration or humming sound on the bus duct.

4) How long is the Bus duct and what is the frequency of the power supply of Bus .

If we get these points clear probably we can reach at some reasonable cause.

Thanks and good day

Thank to all for your comments and am sorry that it took over two years for me to reply. To reiterate, the heating observed by thermal scanning are on the bolts of the bus duct joint and not on the bus-bar. Because we could not open the bus duct to scan the bus-bar, we scan the surface of the bus duct and we found out some hotspot on the bolt are observe.

On this bolts with horspot, they put a wire on it and connect to another bolt on the next section of the bus duct. Temperature went down but when they measured the current on the wire, whew, current ranges from 50 to 105 amp. We suggested to them to replace the nuts and bolts with stainless steel bolts and the quantity of the hotspot has decreased together with a decreased in temperature.

To answer the question on last comment.

1. There is no vibration and or humming sound on the bus duct.

2. The bus duct is 10 years old and the frequesncy is 60 HZ. The bus duct is for a 77MW generator.