Help with BX cable grounding issue.

[Takoma Park Volunteer Fire Department Postmaster]

In answering a question on alt.home.repair about why BX cable with no
bonding strip is unsuitable as a grounding electrode conductor I gave
the answer that I was given in apprentice school low these many years
ago. A follow up posting say's that my answer is nonsense. Does anyone
here have the straight info on this question so I can provide the
original poster with good information.
--
TIA
Tom Horne



That's a pile of hogwash. You can't assume the presence of
corrosion. And even if it is present...



Total nonsense. At 60 cycles, the impedance (or more correctly
the Inductive Reactance) is meaningless. Start to transmit RF
over that wire and it's a different story. Sheeeesh.



I'll say. Just look at the wizard "educating" you.



Uhhh, at 60 Hz, so they should. It's not an issue.

 

[SQLit]

In answering a question on alt.home.repair about why BX cable with no
bonding strip is unsuitable as a grounding electrode conductor I gave
the answer that I was given in apprentice school low these many years
ago. A follow up posting say's that my answer is nonsense. Does anyone
here have the straight info on this question so I can provide the
original poster with good information.

Your totally correct. Flexiable conduits USED to be accepted as a ground.
First local authorities out lawed them and then finally the NEC. Flex or
the sheith was considered a ground path 30 odd years ago. Has not been
accepted her in the SW for more than 20-25 years. My journey man always
said. "If you run a ground wire and connected it first and correctly you do
not have to worry about being killed or killing someone else."

I run ground wires in metal conduit. Just my way of knowing it will always
be safe.

 

[Ben Miller]

In answering a question on alt.home.repair about why BX cable with no
bonding strip is unsuitable as a grounding electrode conductor I gave
the answer that I was given in apprentice school low these many years
ago. A follow up posting say's that my answer is nonsense. Does anyone
here have the straight info on this question so I can provide the
original poster with good information.

I have seen research data on this, but I can't put my hands on it at the
moment. It was a grounding study done by Georgia Institute of Technology.
They tested a number of raceway/conductor combinations. There were inductive
effects for some combinations. I know the impedance of BX was higher, but I
don't recall how much was resistance or inductive reactance. I will try to
find it.

Consider this, however. If you assume that in the worst case the coils are
effectively insulated from one another, then the grounding path is along a
helix. I just measured one at 4 turns/ inch. If you assume .375 mean dia,
then you get approximately 4.7" effective path in a 1" run. The
cross-sectional area may also be smaller than other raceways , so you would
have a grounding path resistance possibly over five times that of a rigid
metal raceway for the same length run. Any reactance would makes things even
worse.

Ben Miller

 

[daestrom]

I have seen research data on this, but I can't put my hands on it at the
moment. It was a grounding study done by Georgia Institute of Technology.
They tested a number of raceway/conductor combinations. There were inductive
effects for some combinations. I know the impedance of BX was higher, but I
don't recall how much was resistance or inductive reactance. I will try to
find it.

Consider this, however. If you assume that in the worst case the coils are
effectively insulated from one another, then the grounding path is along a
helix. I just measured one at 4 turns/ inch. If you assume .375 mean dia,
then you get approximately 4.7" effective path in a 1" run. The
cross-sectional area may also be smaller than other raceways , so you would
have a grounding path resistance possibly over five times that of a rigid
metal raceway for the same length run. Any reactance would makes things even
worse.

I agree. But also add to consider that the BX is not copper. The
resistivity is worse for iron/steel or whatever it is.

daestrom

 

[daestrom]

1) as to the short circuit and drain capacity of BX, specifically the
resistivity of BX ground - the numbers -

Hold up a #14 ground wire and look at its end - a #14 wire is 0.003 inches^2 -

Hold up the helix strip carrying #14 and look at its end. Is the helix on the
BX rated for #14 at least ten times as big -- .030 inches^2? (Soft steel
being 10 times the resitivity of copper.)
You could theoretically use it as a conductor, if you insulate it.

As to a ground - indicated by the conductor comment above, a one on one
comparison is not really technically fair . That #14 wire is sized with its
continuous current rating, to not overheat the particular insulation in its
anticipated environment after hours of heavy use.
- as to what it really can carry in a dead short - a hell of a lot more,
since even the best protection will let thru 4000 amps, and the usual is closer
to 50,000 to 150,000 amps -for parts of a second only, however, and that is
important when comparing grounds.
So a ground, i.e., a non-current carrying wire that is used to drain charge
and to carry appreciable amounts of current only a few times in its life, can
normally be smaller than the conductor it serves since it can get hot to near
the point of combustion of adjacent materials as it works while a conductor
may not - as long as the ground does not start a fire itself while the fuse is
opening the circuit. So you need to compare the thermal capacity of steel and
copper, the melting point of each, rise-time resistance and arc limiting, etc.
to fairly compare.

Some valid points. You're right of course about the thermal issues. But I
hope most short circuits in residential wiring don't reach the 50,000 to
150,000 amps you're thinking. A lot of service panel breakers are only
rated for 10K interrupting rating. A 100,000 amp fault would not be cleared
at all. ISTR that the available short circuit current is supposed to be
taken into account when selecting breakers.

daestrom