Electrician Job Question:

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The UL stands for "Underwriters Laboratory". It used to be
an Insurance Industry organization, but now it is independent
and non-profit. It tests electrical components and equipment
for potential hazards. When something is UL-listed, that means
that the UL has tested the device, and it meets their requirements
for safety - ie: fire or shock hazard. It doesn't necessarily
mean that the device actually does what it's supposed to, just
that it probably won't kill you.

The UL does not have power of law in the U.S. -- you are
permitted to buy and install non-UL-listed devices. However,
insurance policies sometimes have clauses in them that will
limit their liability in case of a claim made in response to
the failure of a non-UL-listed device. Furthermore, in
many situations the NEC will require that a wiring component
used for a specific purpose is UL-listed for that purpose.
Indirectly, this means that certain parts of your wiring
must be UL-listed before an inspector will approve it and/or
occupancy permits issued.