Electrical grounds

[e hilton]

Need help, in a dumbed down way. On another forum there is a discussion about using conduit as a grounding conductor and the “experts” claim it’s allowed. “We always do it that way”. Seems to me there is a distinction between a grounding conductor and an equipment ground, or something like that which confuses me. Can somebody please explain it in a 5th grade way.

 

[Scipio]

Yeah, that discussion is a bit confusing on that site.
I believe that the OP was talking about using conduit as an equipment grounding conductor, which is allowed, though not in all applications.
Chapter three of the NEC describes the uses for different kinds of conduit and their uses and where they can not be used.
It can not be used as a grounding electrode conductor, and I think that is where the confusion lies.

Andy.

 

[wwhitney]

Seems to me there is a distinction between a grounding conductor and an equipment ground.

No, those are the same.

Perhaps this will help: some conductors are meant to carry electrical power, and those are called circuit conductors. Often one of the circuit conductors is connected to earth, and it is called the groundED conductor. Other metal parts in the installation should not be carrying electrical power. Those metal parts are all connected (bonded) together by a groundING conductor. And this system of bonded metal parts is always supposed to be connected to earth.

The list of what may be used for the groundING conductor is in NEC 250.118, and it includes most types of metal conduit (FMC and LFMC are significantly limited due to their flexibility).

Note also that at the service disconnect, or at the source of a separately derived system (e.g. isolation transformer), there is one connection between the groundING conductor and the groundED conductor. That is so that any downstream fault between an ungrounded circuit conductor (any circuit conductor other than the groundED conductor) and any of the bonded metal parts will complete a circuit that should allow enough current to flow to trip an OCPD on that ungrounded circuit conductor. Which is better than the alternative of a fault leaving an unbonded metal part energized, as someone could possibly contact that part.

Since the groundING conductor is not supposed to carry circuit current, there should be no other connections between the groundED conductor and the groundING conductor, as a second connection would create a circuit or loop, causing current meant to be on the groundED conductor to be split between the two.

Cheers, Wayne

 

[Gregg Harris]

250.118 Types of Equipment Grounding Conductors. The
equipment grounding conductor run with or enclosing the
circuit conductors shall be one or more or a combination of
the following:
(1) A copper, aluminum, or copper-clad aluminum conductor.
This conductor shall be solid or stranded; insulated,
covered, or bare; and in the form of a wire or a busbar of
any shape.
(2) Rigid metal conduit.
(3) Intermediate metal conduit.
(4) Electrical metallic tubing.
(5) Listed flexible metal conduit meeting all the following
conditions:
a. The conduit is terminated in listed fittings.
b. The circuit conductors contained in the conduit are
protected by overcurrent devices rated at 20 amperes
or less.

d. The combined length of flexible metal conduit and
flexible metallic tubing and liquidtight flexible metal
conduit in the same ground-fault current path does
not exceed 1.8 m (6 ft).
e. If used to connect equipment where flexibility is
necessary to minimize the transmission of vibration
from equipment or to provide flexibility for equipment
that requires movement after installation, an
equipment grounding conductor shall be installed.
(6) Listed liquidtight flexible metal conduit meeting all the
following conditions:
a. The conduit is terminated in listed fittings.
b. For metric designators 12 through 16 (trade sizes
3∕8 through 1∕2), the circuit conductors contained in
the conduit are protected by overcurrent devices
rated at 20 amperes or less.
c. For metric designators 21 through 35 (trade sizes
3∕4 through 11∕4), the circuit conductors contained in
the conduit are protected by overcurrent devices
rated not more than 60 amperes and there is no flexible
metal conduit, flexible metallic tubing, or liquidtight
flexible metal conduit in trade sizes metric
designators 12 through 16 (trade sizes 3∕8 through 1∕2)
in the ground-fault current path.
d. The combined length of flexible metal conduit and
flexible metallic tubing and liquidtight flexible metal
conduit in the same ground-fault current path does
not exceed 1.8 m (6 ft)

 

[e hilton]

I remember back about 1990 my electrician said we could no longer use BX cable (i think) which was 2 conductors wrapped in a flexible sheath, because the sheath wasn’t a proper ground conductor. This was commercial construction in Dallas.

 

[ICE]

Ground Fault.
An unintentional, electrically conductive connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.

Effective Ground-Fault
Current Path. An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.

"intentionally constructed, low-impedance" EMT (the most common) is tested and Listed by an NRTL as an equipment grounding conductor. Loose set screws and fittings create a high-impedance electrically conductive path. I don't care how scrupulous the electrician might be, it only takes one loose screw or fitting and you have lost the ballgame. As an inspector, do you check it all?

 

[Gregg Harris]

Somebody has their nuts backwards!

The cogs on the nut are to penetrate the painted surface and create a clean ground.