db/cu and 521.5.1 Ferromagnetic enclosures: electromagnetic effects

Unfortunately, there are many areas where the regulations make silly requirements due to old wives' tales. This is a primary one and as I have explained along with Mike and others the effect of sheet steel boxes is essentially zero. Large cast-iron cases are a different matter altogether, the characteristic being the length of the conductor inside the magnetic material. You can verify this yourself with a length of steel conduit and an arc welder, passing a large current through a single conductor inside the conduit will heat it significantly. Zoomup's picture failed to heat because the length is very small. Essentially this is a badly written regulation, and probably is copied from some IEC document, but getting technical editing of the regulations is very difficult. I did manage to get one completely faulty new regulation dropped, but it took a long technical paper, experimental proof, and a long time. Objections of regulations in the DPC are considered, but it takes a lot of comments to get a change. Keep up the good work!

In order to satisfy the naysayers, I have just measured the added inductance from 4 mm of steel washers threaded wires on a length of wire, it added 2 nH (Two nano Henries) to the inductance of the plain wire! This shows that the magnetic coupling is actually quite small, and at 50Hz the loss is only partly connected to the added inductance, it depends on the resistance of the coupled material too. As a comparison, the inductance of a metre of wire measured as a big loop measured around 1.642 uH, varying somewhat with the exact loop size and position as expected. The measurement frequency for this is much higher than 50Hz, so it may be that the material (steel has poor high-frequency magnetic properties) but measuring such small values at 50Hz is very difficult indeed. Even if the washers add a few micro-Henries at 50 Hz, the effect will be tiny. The poor high-frequency performance of Iron is useful for induction hobs and heaters and is widely used for manufacturing forged parts. Here though the length of the material is large and the coupling is a multi-turn coil, about 6 inches diameter in my induction hob. It heats a vertical wire very little but steel pans with 2 kW. 

Unfortunately, there are many areas where the regulations make silly requirements due to old wives' tales. This is a primary one and as I have explained along with Mike and others the effect of sheet steel boxes is essentially zero. Large cast-iron cases are a different matter altogether, the characteristic being the length of the conductor inside the magnetic material. You can verify this yourself with a length of steel conduit and an arc welder, passing a large current through a single conductor inside the conduit will heat it significantly. Zoomup's picture failed to heat because the length is very small. Essentially this is a badly written regulation, and probably is copied from some IEC document, but getting technical editing of the regulations is very difficult. I did manage to get one completely faulty new regulation dropped, but it took a long technical paper, experimental proof, and a long time. Objections of regulations in the DPC are considered, but it takes a lot of comments to get a change. Keep up the good work!

In order to satisfy the naysayers, I have just measured the added inductance from 4 mm of steel washers threaded wires on a length of wire, it added 2 nH (Two nano Henries) to the inductance of the plain wire! This shows that the magnetic coupling is actually quite small, and at 50Hz the loss is only partly connected to the added inductance, it depends on the resistance of the coupled material too. As a comparison, the inductance of a metre of wire measured as a big loop measured around 1.642 uH, varying somewhat with the exact loop size and position as expected. The measurement frequency for this is much higher than 50Hz, so it may be that the material (steel has poor high-frequency magnetic properties) but measuring such small values at 50Hz is very difficult indeed. Even if the washers add a few micro-Henries at 50 Hz, the effect will be tiny. The poor high-frequency performance of Iron is useful for induction hobs and heaters and is widely used for manufacturing forged parts. Here though the length of the material is large and the coupling is a multi-turn coil, about 6 inches diameter in my induction hob. It heats a vertical wire very little but steel pans with 2 kW.