Eddy current clarification

[davesparks]

Not borne out by John Ward's video, or Julie's experience. I'm just perplexed now!

But both of those examples are cables passing through steel plates whereas I was talking about cables passing through a few feet of steel conduit.

 

[R-fur]

Eddy currents are a real thing, I have seen several instances of damage by them.
The worst case was on the LV box of a 2MW transformer, four conductors per phase all neatly grouped by the phase and taken through a steel plate. The paint on the plate had burnt off around the cables and discoloured the steel. We changed the gland plate to brass to solve the problem. Interestingly at the other end the singles were again taken through steel into the switch board but there was no sign of heating, I think this was due to the singles being taken in randomly rather than bunched in phase groups.
As said above there will always be induced currents from singles but it is much worse if they pass through ferrous metal. In another case we had four singles passing through a steel plate, again there was signs of heating. This was solved by cutting a slot to link the four holes (which were in a line) then brazing it shut again to retain strength. The brass in the brazed joint was enough to prevent further eddy current damage.

 

[Avo Mk8]

But both of those examples are cables passing through steel plates whereas I was talking about cables passing through a few feet of steel conduit.

Ah! OK - Sorry I didn't get that at all. Re-reading your post I see what you mean. Thanks.

 

[IAmSparkytus!]

Eddy currents are a real thing, I have seen several instances of damage by them.
The worst case was on the LV box of a 2MW transformer, four conductors per phase all neatly grouped by the phase and taken through a steel plate. The paint on the plate had burnt off around the cables and discoloured the steel. We changed the gland plate to brass to solve the problem. Interestingly at the other end the singles were again taken through steel into the switch board but there was no sign of heating, I think this was due to the singles being taken in randomly rather than bunched in phase groups.
As said above there will always be induced currents from singles but it is much worse if they pass through ferrous metal. In another case we had four singles passing through a steel plate, again there was signs of heating. This was solved by cutting a slot to link the four holes (which were in a line) then brazing it shut again to retain strength. The brass in the brazed joint was enough to prevent further eddy current damage.

I always thought they had the same effect on brass as I was once told brass bushes wouldn’t prevent the issue or is that because the brass bushes are drilled and fitted to the steel casing of a distribution board?

 

[newfutile]

would not a brazed up slot still count as a shorted turn of a transformer os sorts?

 

[R-fur]

would not a brazed up slot still count as a shorted turn of a transformer os sorts?

It is conductive but not magnetic. The braze stops the magnetic circuit and forces the magnetic flux from all the cores to go around all four cores. The sum of all the cores should be zero so there is no magnetic flux left to induce currents.

 

[R-fur]

always thought they had the same effect on brass as I was once told brass bushes wouldn’t prevent the issue or is that because the brass bushes are drilled and fitted to the steel casing of a distribution board?

You are correct. Brass bushes in a steel plate will not make any difference. You have to look at the magnetic circuit, all the cores have to pass through the same hole in any ferrous material.

 

[newfutile]

I thought any conductive material in a moving magnetic field will induce a voltage.
Magnetic flux is a thing but it's the moving flux or conductor moving though a magnetic field that induces a voltage.
I was also taught that below 100 amp current is unlikely to be a concern.
Having said that I bring all my conductors in thought he same hole in a metal enclosure.

 

[R-fur]

I thought any conductive material in a moving magnetic field will induce a voltage.
Magnetic flux is a thing but it's the moving flux or conductor moving though a magnetic field that induces a voltage.

That is correct, A changing magnetic flux will induce current in a conductor, but by putting all the cores through the on hole in a ferrous material the magnetic fluxes cancel out, hence no induced current.
If the cores are brought through separate holes in a ferrous material each will induce a current. These currents will summate/negate and produce losses which can result in hotspots. I agree with a comment above that below 100A there will be no heating, But it is still an inefficiency.

 

[westward10]

I remember working at the large Debenhams in Manchester city centre and they had installed a new sub panel in 600.0 singles on a tray. They entered the existing panel through separate entries of the ferrous panel and it was buzzing like crazy but there was no heat build up.

 

[IAmSparkytus!]

I remember working at the large Debenhams in Manchester city centre and they had installed a new sub panel in 600.0 singles on a tray. They entered the existing panel through separate entries of the ferrous panel and it was buzzing like crazy but there was no heat build up.

Surely my 1.5mm and 2.5mm earths going through their own hole will be OK then?

Complete opposite end of the spectrum!

Wonder if my NIC inspector would have hard feelings about it

 

[westward10]

Surely my 1.5mm and 2.5mm earths going through their own hole will be OK then?

Complete opposite end of the spectrum!

Wonder if my NIC inspector would be have hard feelings about it

Your NIC inspector probably doesn't understand the science.

 

[davesparks]

Your NIC inspector doesn't understand

Corrected that for you

 

[bjstar]

The first picture I assume is steel in which case it is wrong. The aluminium panel in the second picture is fine but the lock nuts assuming they are steel are not.

Hi i dont know if this is the right place for this question here goes, can eddy currents be used to to 8dentify diffrent metals? Tia.

 

[davesparks]

Just annoying when you’re a bit of a neat freak with a distribution board and know that taking the earths through their own hole at the top of the board makes it look so much better

Whilst neatness is important you should not be putting form over function.

Putting all the earth's through a separate hole might look neat but it can make the next person to work on that boards job a lot harder.

 

[IAmSparkytus!]

How so?

 

[nicebutdim]

Hi i dont know if this is the right place for this question here goes, can eddy currents be used to to 8dentify diffrent metals? Tia.

I imagine they could, but it doesn't seem an ideal method of identifying materials. The process would require the possession of quite a bit of information and strictly controlled conditions.

 

[Mike Johnson]

How so?

I would imagine by not being able to identify what earth lead applies to what circuit, unless each earth lead is identified/labelled.