Equipment Ground Through Ferrous Conduit

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ike5547

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Why does a similar requirement to bond a GEC at both ends of a ferrous pipe (as in 250.64(E)) not apply to equipment grounds that may be in a steel conduit, but not grouped with any other circuit conductors? Example would be to ground an existing outlet while using a steel protective sleeve.

Why a GEC but not an EGC?

Looking for a technical explanation.
 
I don't know the real answer, but my guess is that
Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation. This is generally the job of a GEC, which involves high voltages at crazy frequencies.
An EGC is for an Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point
on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path. This is high current, not necessarily high voltage or weird frequencies.

Just my guess
 
This is high current, not necessarily high voltage or weird frequencies.
Click to expand...

An engineer once told me many years ago that, like lightning, a dead short (or maybe just an arc) has "a component of all frequencies." I don't understand it and maybe he didn't either.
 
ike5547 said:
Why does a similar requirement to bond a GEC at both ends of a ferrous pipe (as in 250.64(E)) not apply to equipment grounds that may be in a steel conduit, but not grouped with any other circuit conductors? Example would be to ground an existing outlet while using a steel protective sleeve.

Why a GEC but not an EGC?

Looking for a technical explanation.
Click to expand...
What do you consider to be a ferrous pipe? What exactly is it?
 
Besoeker3 said:
What do you consider to be a ferrous pipe? What exactly is it?
Click to expand...

It has magnetic properties related to iron.

250.64(E)(1)
Ferrous metal raceways and enclosures for
grounding electrode conductors shall be electrically continu‐
ous from the point of attachment to cabinets or equipment to
the grounding electrode and shall be securely fastened to the
ground clamp or fitting. Ferrous metal raceways and enclosures
shall be bonded at each end of the raceway or enclosure to the
grounding electrode or grounding electrode conductor to
create an electrically parallel path. Nonferrous metal raceways
and enclosures shall not be required to be electrically continu‐
ous.
 
ike5547 said:
It has magnetic properties related to iron.

250.64(E)(1)
Ferrous metal raceways and enclosures for
grounding electrode conductors shall be electrically continu‐
ous from the point of attachment to cabinets or equipment to
the grounding electrode and shall be securely fastened to the
ground clamp or fitting. Ferrous metal raceways and enclosures
shall be bonded at each end of the raceway or enclosure to the
grounding electrode or grounding electrode conductor to
create an electrically parallel path. Nonferrous metal raceways
and enclosures shall not be required to be electrically continu‐
ous.
Click to expand...
Yes, but ferrous metals include cast iron, wrought iron, and of course steel.
 
The Code refers to ferrous (containing iron) metals. But it properly should refer instead to ferromagnetic metals. Some iron alloys, including most types of stainless steel, are not ferromagnetic. As a result a magnet will not adhere to them (a useful test) and they will not produce a choke effect any different from an open copper wire.
But that distinction is apparently beyond the capacity of the NEC.

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ike5547 said:
You didn't understand my question and not necessarily. Or, rather, you didn't understand all of it. It was a bit wordy.
Click to expand...
That is actually the correct answer.

The issue is the inductive reactance that increases the impedance of the conductor where a single conductor of an AC circuit is installed in a ferrous raceway. That is not the case with the EGCs as they are installed in the same raceway with the circuit conductors that supply the current into the fault. Where all of the conductors of the circuit are in the same raceway, the magnetic fields cancel, and do not act to create an inductive choke like they do where there is only a single AC conductor in a ferrous raceway.
 
Besoeker3 said:
What do you consider to be a ferrous pipe? What exactly is it?
Click to expand...

Ferrum is the Latin name for Iron, and the reason why Iron's elemental symbol is Fe. Ferrous refers to ferromagnetism, a property of a select few metals, where it enhances the strength of a magnetic field, and allows the metal to become a permanent magnet. Iron is the namesake of ferromagnetism, because it is the most common material used for applications of magnetism, and the first metal discovered with the property. Other metals that exhibit ferromagnetism in their elemental forms are Cobalt, Nickel, Neodynium, Gadolinium, Terbium, and Dysprosium. Iron is the most common material used for magnets because it is the most cost-effective ferrous metal to obtain. Neodynium is what you would use, in applications that require extreme strength.

Any raceway constructed of ordinary ferrous steel is ferrous. Whether it is RMC, IMC, EMT, LFMC, FMC, or a ferrous steel cladding of a cable.

A plastic raceway is obviously not ferrous. An aluminum raceway is an example of a raceway that is metallic, but not ferrous. Less intuitive is the stainless steel, where you might initially think it is ferrous due to iron being its top ingredient, but due to the mix of metals in the alloy (the way iron and nickel interact together), the magnetic properties of stainless steel are nullified, and stainless steel (specifically austensic stainless steel) is not magnetic. There are types of stainless that are mildly magnetic.
 
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ike5547 said:
An engineer once told me many years ago that, like lightning, a dead short (or maybe just an arc) has "a component of all frequencies." I don't understand it and maybe he didn't either.
Click to expand...

If a waveform has only one frequency, it takes the shape of the function of A*sin(B*t), where A is the amplitude, and B is a term that is directly related to the frequency (B=2*pi*f to be specific). This is called a sine wave, and is the waveform of an ideal AC power source.

For waveforms that aren't sine waves, they can be calculated to be a summation of multiple sine waves, in a mathematical process called a Fourier transform. That summation of sine waves is called a Fourier Series. This is how we generate square waves, triangle waves, and sawtooth waves from sine waves. There is a pattern specific to the waveform, where it is an infinite series of sine waves, with ever increasing frequencies and decreasing amplitudes (called harmonics), which can form that particular shape of waveform. The lowest frequency in the series is called the fundamental, and is usually the greatest in amplitude.

Lightning isn't going to be a sine wave like an ideal AC power source, but you can still take the Fourier transform of a lightning strike, if you were to measure its waveform on an oscilloscope. You will not get an organized pattern of harmonic frequencies. Instead, you will get a chaotic mix of all frequencies.
 
Speedskater said:
And much of the power in a lightning strike is in the neighborhood of 1 Megahertz !
Click to expand...

And that means, inductance is a property of the circuit that you need to reduce as much as possible, when trying to control the total impedance of the EGC's path. Because the impedance due to inductance increases in proportion to frequency. This means reducing the length and the turns in the path, so that it connects to the closest possible grounding electrode. It also means bonding both ends of the ferrous raceway, to mitigate the effects of currents that develop in the steel from the high frequency currents in the GEC inside.
 
Speedskater said:
Because with the EGC both the Hot & Neutral are going thru that same metal conduit. So the opposite fields cancel out.
In the case of the GEC there is only one conductor, so the metal conduit acts as a choke.
Click to expand...

This is the correct answer. More specifically, when the circuit conductors are ran together the physical area made by the loop formed by the short-circuit path back to the source is kept narrow and hence the circuit inductance is lower because the flux is mostly canceled than if the conductors were ran separately to form a wider loop. The more flux that is allowed to be linked and permeate space, the more inductance the circuit has and the lower the fault current will be such that protective devices may never trip.


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Carultch said:
Ferrum is the Latin name for Iron, and the reason why Iron's elemental symbol is Fe. Ferrous refers to ferromagnetism, a property of a select few metals, where it enhances the strength of a magnetic field, and allows the metal to become a permanent magnet. Iron is the namesake of ferromagnetism, because it is the most common material used for applications of magnetism, and the first metal discovered with the property. Other metals that exhibit ferromagnetism in their elemental forms are Cobalt, Nickel, Neodynium, Gadolinium, Terbium, and Dysprosium. Iron is the most common material used for magnets because it is the most cost-effective ferrous metal to obtain. Neodynium is what you would use, in applications that require extreme strength.

Any raceway constructed of ordinary ferrous steel is ferrous. Whether it is RMC, IMC, EMT, LFMC, FMC, or a ferrous steel cladding of a cable.
Click to expand...
It was this I was commenting on:
"Why does a similar requirement to bond a GEC at both ends of a ferrous pipe (as in 250.64(E)) not apply to equipment grounds that may be in a steel conduit, but not grouped with any other circuit conductors? Example would be to ground an existing outlet while using a steel protective sleeve."
The reference to steel rather than ferrous.
 
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