Use of "Pot METAL" Electrical Outlet Boxes in Outdoor Masonry Walls by Joe Mehaffey, MSEE and Electrical Contractor Revision 2, 21 September 2008
I
once believed that the use of cast metal boxes in outdoor masonry walls
would insure permanence. UNfortunately, this has turned out to be
false in some cases. In August of 1997, I had 14 Thomas and
Betts (Red Dot) cast metal outlet boxes installed in the retaining
walls of a terrace garden. Seven of the boxes were double duplex
outlet boxes and five were single outlet boxes and two boxes were used
to house outdoor light switches. Unused holes in the boxes were
sealed and a hole was drilled in the lower front of each box cover to
insure that if any water did get into the box, it could drain out.
All went well for about ten years.
In
the latter part of 2007, the Ground Fault (GFCI) circuit breaker on the
circuit began to trip occasionally when rains occurred. By the
summer of 2008, the breaker could not be reset even in dry weather.
An investigation revealed a DC resistance to ground of about 100K
ohms with an ohmmeter on the black (hot) wire, but with 110vac applied
to the black wire, the current reading of about 50ma was more than enough to trip the
GFCI breaker.
The
covers to the boxes were removed and in the bottom of each box a
variable amount of "grey-white dust" and larger metal particles were
found.
Some boxes had a quarter inch of this "grey dust" on the bottom
of the box. The paint coating on the inside (and outside) of the
boxes had
completely disintegrated. Some boxes had part of the bottom, and
top eaten away, cracked, and loose ready to drop out when the cover was
removed. Virtually all boxes had the sides and top "bulging
inward" as if a powerful hand had squeezed the sides of the box
together. The bending was from zero in some boxes to 1/4" in
others. I believe this was due to the corrosion of the box.
Much of the wall metal was so fragile that grabbing the wall with
a pair of pliers and pushing toward the inside would easily break off a
hunk. Obviously, the corrosion damage was so severe that there
was no option but
to replace all the boxes.
As
a test, and starting before removing any receptacles, we read an ohmmeter attached to the black (hot) wire as the
receptacles/switches were removed from the boxes. Most (about
70%) of the time, as a receptacle was cut free, the resistance shown by
the ohmmeter increased. When we finished, the resistance
had gone to over 2 megohms and the current measured on the black wire
with 110vac applied was less than 1ma AC. At about 30pf per foot
and 400ft of wire, the .012uf gives an AC impedance of about 220K
ohms. This would produce an AC current of 110/220000 = 0.5
ma due to the capacitance of the hot wire to ground. This then is in reasonable agreement with our
measurements and indicates that the leakage fault is corrected.
Next,
we examined the electrical outlet devices which were standard "spec
grade" outlets from P&S. Almost all were in fairly good
condition considering they had been in a high dampness location for ten
years. However, we noted that some devices had a layer of
grey/white dust, sometimes pretty thick, covering the portion of the
outlet that was INSIDE the box. We noticed
that between the "hot" screw and grounded mounting strap which
runs vertically along the back of the device, many had signs of a
blackened stripe. On these we measured and found there was
conductivity, but only when we moistened the "dust" and applied 110vac
across the hot terminal and ground on the device. Here I think we
found the current path that was tripping our GFCI breaker.
So..
It turned out to be the combination of a) the disintegrating pot metal
box causing potentially conductive dust to fall on the outlets and
stick and b) moisture to wet the dust, make it conductive and thus generate a fault. We replaced the
outlets with new WR/CR rated receptacle devices.
I
have used and seen used literally thousands of these cast metal boxes
in masonry before without problems. However, I have no
prior personal history of what happens if these "pot metal" boxes
are placed in a masonry wall that stays damp most of the time.
Most brickwork on homes and offices drys out pretty quickly after
a rain. This particular wall system apparently stays damp long
after a rain occurs. In
this case, I observed that IN THE SAME WALLS, identical boxes
used for telephone jacks did NOT exhibit the same degree of
deterioration as shown by the grounded outlet boxes. My
conclusion is that the electrical outlet boxes were tied to the
electrical SAFETY GROUND and this allowed currents generated by the
electrolysis of the dampened "pot metal" (a cheap Zinc Alloy) to
flow
to the electrical ground and this electrolysis effect caused chemical
reactions to destroy the integrity of the cast box. Since the
phone boxes had no such path to electrical ground, they were affected
to a much smaller extent by corrosion.
To
effect a (hopefully) permanent repair for this problem, the old Pot
Metal boxes were cut out of the masonry wall with a hammer and chisel.
While this was not easy, the boxes had become extremely brittle
and most boxes came out of the wall with about 30 minutes of careful
hammering. Some took an hour. Much care had to be
used to avoid damage to the wires which entered the box through a hole
in the rear of the cast box. The cast metal box was replaced by
an outdoor PVC PLASTIC box made by Carlon. I believe this will
rather permanently fix the problems because PVC boxes a) are moisture
resistant, b) are not conductive to ground and have no path to
ground for the safety ground wire and c) do not corrode when
exposed to moisture. In addition, Hubbel Corrosion Resistant (CR)
and Weather Resistant (WR) receptacle devices were used as
replacements.
This combination should be stable in the presence of moisture and
temperature cycling that is sure to occur in this application.
In
addition, all of the seven Red Dot double outlet covers with weather
resistant doors were defective. None of the doors would fully
close. Many appeared bent in the hinge area, possibly through use. Others
had the hinge+spring corroded or contaminated with dirt
and/or insect debris and the door would not close under spring
pressure. I selected mating Carlon double duplex covers but these
do not seem especially robust. Time will tell. However,
they ARE easy to replace if needed.
Summary and Recommendations:
Cast
Zinc Alloy (Pot Metal) cast electrical boxes failed in a high
dampness masonry wall application in 10 years time. The nature of
the failure was determined and replacement PVC Plastic boxes and
Weather and Corrosion Resistant electrical outlet devices were used to
replace the prior defective boxes and devices. This is an
unusual failure but I would recommend that cast "pot metal" outlet
boxes NOT be used in outdoor EMBEDDED INTO MASONRY walls or other
construction which might remain wet for periods of time.
Definitely, I would not use such metal boxes in any application
near salt water.