Mike Dolt Enterprises
Mike Dolt Enterprises
User: sparkyguy545, Location: Durham, NC
Asked on: Wednesday, June 1, 2011 at 7:07 pm EDT
Answered on: Tuesday, May 14, 2024 at 6:06 am EDT
Question: OK, It's been WAY too long since I had a class in magnetics.I understand the basic reason for bonding both ends of a ferrous conduit when used for GEC is to eliminate the choke effect that is widely described in every NEC handbook However, 250.64E clearly states that the requirements apply to ferrous conduit only. Therefore, NO bonding would be required if the GEC was installed in rigid aluminum conduit.So, what is unique about this effect and why does it apply with ferrous conduit only? Electrically, and magnetically, wouldn't rigid aluminum conduit functionally create the equivalent of a single-turn, shorted, air-core transformer secondary and therefore ultimately have the same "choke" issue? Obviously, magnetic fields can induce current in aluminum conductors and windings. Many (if not most) xfmrs now are manufactured with aluminum windings. Clearly, it must be due to the fact that ferrous conduit reacts to magnetic field and aluminum does not. However, since aluminum windings can still have a current induced into them from magnetic fields, I don't clearly see why the phenominia doesn't also occur with aluminum conduit.
Magnetic Properties of Ferrous and Non-Ferrous Conductors
Ferrous materials, such as iron and steel, are ferromagnetic, meaning they exhibit strong magnetic properties and can become strongly magnetized. Aluminum, on the other hand, is non-ferrous and has negligible magnetic properties.
GEC and Conduit Choke Effect
GEC (Grounding Electrode Conductor) carries fault current to the grounding electrode. When fault current flows through a ferrous conduit, it creates a magnetic field. This magnetic field induces an opposing electromotive force (EMF) in the conduit, which opposes the flow of current. This is known as the choke effect.
Why Ferrous Conduit Requires Bonding
The choke effect in ferrous conduit can significantly increase its impedance and reduce its effectiveness as a ground path. To eliminate this effect, both ends of the conduit must be bonded together to provide a low-impedance path for fault current to flow.
Aluminum Conduit and Choke Effect
While aluminum conduit can also carry fault current, it does not exhibit the choke effect because it is non-ferrous and has negligible magnetic properties. The induced EMF in aluminum conduit is negligible, and it does not significantly affect the flow of fault current.
Transformer Comparison
The analogy of a single-turn transformer secondary is not entirely accurate because the conduit is not a transformer winding. However, it is true that aluminum conduit can have currents induced in it by magnetic fields. However, these currents are typically very small due to the low magnetic susceptibility of aluminum compared to ferrous materials.
Conclusion
The choke effect is a specific phenomenon that occurs in ferrous conduits due to their ferromagnetic properties. It does not occur in non-ferrous conduits, such as aluminum, because they have negligible magnetic properties. Therefore, bonding is only required for ferrous conduits to eliminate the choke effect and ensure proper GEC functionality. Flag for review