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Part 2:
Branch
Circuits Are Most Likely to Fail:
Time: Approximately 1/2 Hour
Do you know where branch circuits are
most likely to fail in a house? I’m looking for a physical location. Why do
they fail there?
We have already covered failure modes and
some of physical locations where circuits fail in
Part One but I want to elaborate here both on the reasons for failures and the
locations.
You already know about poor connections.
We have talked about circuits failing in junction boxes in attics and other
locations. The failures we have discussed so far have mainly been related
in some way to poor workmanship. It could be argued that indirectly all
failures are the result of poor workmanship. Or, can they? There are
many mechanism in place to ensure that only safe electrical products make it to
the marketplace. We go to great lengths to ensure that electricians are
properly trained. There are programs to educate consumers. Still
electrical systems fail every cay. The Consumer Product Safety Commission
oversees the new recalls of manufactured goods every single day. Home
owners start fires, cause electrocutions, and electrical explosions every day.
Then we home inspectors come along and take
on the weight of the World. We have to give the house a clean bill of
health or deliver the bad news that something or perhaps, lots of things,
require repair or replacement. We have truly awesome responsibility.
We have to evaluate a complex system that is almost completely hidden from view.
Electrical systems are hidden from view
because people don't like to look at electrical parts (I don't know why?).
There is an incidental benefit to the systems being mostly hidden. They
are protected from physical abuse. As it is, electrical systems take a lot
of abuse. They really do quite well at surviving the abuse.
Aside from problems with workmanship
electrical systems are most likely to fail where they are exposed, where the
components are in plain view. Now this may seem obvious to some of you but I
assure you it isn't obvious to everyone. OK, so this section is about
Branch Circuits so let's take a look at where and how branch circuits fail.
Branch circuits are most likely to fail on
an outside wall near the main panel. There are a couple of different
factors that contribute to outlets in perimeter walls being more likely to fail
than in interior walls. The first reason is that the first outlet box in
a circuit is likely to have been used as a junction. The box may have more
conductors in it and it may be carrying the full load of the circuit. the
part of the circuit that goes back to the panel box is called a "Home Run".
There can be multiple cables going back to
a panel for the same circuit but electricians usually tie as many together as
they can before taking the circuit back to the panel. There is a limited
to the amount of space in a panel and there is a limit to the number of places
to land (terminate) the conductors. The goal, therefore, is to try to
limit the number of conductors that go back to the panel. Tying two or
more cables together in the first box on a circuit limits the number of cables
that have to be terminated in the main panel.
Another factor that contributes to outlets
in perimeter walls being more likely to fail the range of temperatures that the
outlets and their boxes are subjected to. The temperatures in an
outside wall cause expansion and contraction of connections. The cycles of
expansion and contraction can cause connections to loosen and fail.
We old-timers like to talk about the good
old days; especially the days before there were "quick-wire" terminals on
outlets and switches. The quick-wire terminals on the back of a typical
outlets allow the outlet to be placed in series with the circuit conductors.
The first outlet in the circuit connected in this way will carry the entire load
of the circuit at all times, even when nothing is plugged into the outlet.
Quick-wire terminals are not as durable as a conductor wrapped around a screw.
The combination of outlets being placed in series with the circuit conductors,
use of quick-wire termination, and being in an outside wall is inviting
disaster.
Use your IR thermometer if you have one to
check outlets along the walls. Be sure to have a load in the circuit. It is
best to try to put the load at the farthest outlet on the circuit. An inexpensive electric space heater is an
ideal load to use for testing. I don't carry my own load. I usually
find enough loads in a house that it is not necessary to provide one.
Attics and crawl spaces can be dangerous
places for home inspectors and for electrical wiring. Attics and crawl
spaces, as with the outlet boxes on perimeter walls are subjected to temperature
extremes that the wiring on the interior is rarely if ever to exposed to.
Thermal stresses can cause problems but it is not the thermal stresses that are
our main concern in these and similar locations. It is mechanical damage
that we need to be concerned with.
I have probably been in hundreds of attics
where the homeowner threw down some plywood for a makeshift floor in an attic.
The plywood is directly on top of NM cable. Fires are caused by pinched
cables every day. Be on the lookout for things such as plywood or stored
items on top of wiring. Watch for things hanging from electrical wires and
cables. Look for signs of nails and staples driven into cables. I'm
not just talking about the work done by the electricians who installed the
wiring. I am also talking about all the things that are done after the
wiring was installed. Pinched and damaged cables are serious business.
The basic rule here is that electrical
wiring should not be used for any propose other than carrying current for the
electrical system. If you see any evidence of the wiring having been used
for any other purpose, look for signs of damage. Spotting damage to the
wiring is a lot like looking for wood destroying insects. You may have
some trouble finding the damage at first but once you know what you are looking
for, the damaged areas will seem to jump out at you.
AFCI
video demonstrates mechanical damage
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