Figure 1. Example test configuration with appliance draped in cheesecloth.
Recently, Underwriters Laboratories Inc. (UL)
issued an amended version of the safety standard for listing of
electric clothes dryers—UL 2158 Electric Clothes Dryers.1 This new
version of the standard includes aggressive fire tests that all clothes
dryers will have to successfully pass by March 20, 2013 to receive the
UL listing. This article discusses one interpretation of the rationale
behind the new tests, the challenges that these tests pose to the safe
design of clothes dryers, and suggested approaches to meeting the
Description of UL 2158 Fire Containment Tests
section does not attempt to completely describe the criteria for the UL
2158 fire containment tests, but it does intend to provide an overview
of the test conditions as an aid to discussion. The new fire
containment provisions are described in detail in clauses 19.6, “Load
fire containment,” and 19.7, “Base fire containment,” of the standard.
Each of these two clauses calls for a static test with the dryer
energized but not tumbling, and a test with the dryer energized,
heating, and tumbling. Therefore, the test regimen requires a total of
four separate appliances since each test may cause sufficient damage to
prevent reuse of each appliance as indicated below.
1. Static Test – Load Fire
2. Dynamic Test – Load Fire
3. Static Test – Base Fire
4. Dynamic Test – Base Fire
titles for each test clause indicate where the test fire will be
ignited: either on the test load in the drum or in the mechanical
compartment in the base of the cabinet below the drum (i.e., tumbler).
Once the test fire is ignited, the test continues until failure or to a
maximum of 7 hours (without failure). A small negative draft through
the appliance is maintained through all tests using a fan blowing air
at 3.3 L per second (L/s). This is used to simulate natural ventilation
in the residential installation. The failure criteria for the fire
tests are common to similar types of appliance fire standards in which
the appliance is draped/enclosed with a single layer of cheesecloth,
which, if ignited, indicates failure of the test. Cheesecloth is a good
indicator for flame escape from the appliance because it is readily
ignited by flame contact. An example of the test configuration is
depicted in Figure 1.
Tests for both clauses incorporate a standard towel
load with a dry weight of 0.016 kg per liter (kg/L) of drum volume. The
towel specifications are described in the standard, and they may be
commonly referred to as “UL towels” or “UL ballast.” In the load fire
tests, the towels in the drum are directly ignited through the use of a
hand-held propane torch. After ignition, the drum door is closed and
the test is commenced. In the case of the dynamic (tumbling) test, the
dryer is started after the load is ignited. It is fairly typical for
the test load to have a total towel mass between 4.5 and 6.8 kg. When
loosely tumbled, this load will produce flames that project from the
opened door and can extend up the front face of the appliance. However,
once the door is closed, the flames can be observed to die down almost
immediately in many cases. In the enclosed drum, the burning rate of
the towels is controlled by ventilation of fresh air into the drum,
which may be due to the appliance fan, the test fan, or natural
ventilation. Thus, with the door closed and ventilation limited, the
towel load can burn in a smoldering or intermittently flaming fashion
with a much lower heat release rate. In some instances, it has been
observed that the tumbling action of the towel load in the dynamic load
may be sufficient to extinguish the towel load. This is perhaps one
supporting reason for conducting the test in a static mode as well.
two tests where the fire is ignited in the base require an additional
fuel source to the towel load. Commonly, the mechanical compartment in
the base of an appliance may contain plastic air ducting and a plastic
fan housing. These fuels may burn given the right conditions, but the
likelihood of ignition of these components is increased through the
placement of cheesecloth in the base of the appliance. The cheesecloth
can be considered as a rough analog to excessive lint accumulation in
the base of the appliance. The base fire clause requires a layer of
cheesecloth eight sheets thick to be draped over the horizontal
surfaces, components, and wiring located within 15 cm of the base of
the appliance. The cheesecloth must be continuous across these
surfaces, yet it cannot impede or interfere with mechanical functions
or components’ ventilation. The cheesecloth is directly ignited in
these tests, and the fire is allowed to spread within the mechanical
Possible Fire Containment Test Failure Modes
There are many possible failure modes for an appliance in these fire tests. Some possibilities include the following:
- Flame escape through gaps in cabinet panels.
- Flame escape through door due to hinge-mount or door-latch failure.
- Flame escape through seals or gaps due to intermittent flaming (e.g., puffing).
- Flame penetration through polymeric user interface panels due to melting.
- Dripping of molten flaming plastic through the cabinet base.
- Sparks and molten metal globules created by fire attack on energized conductors.
may be other possible failure modes not listed above, but this list
could be considered a set of likely test failures during the appliance
Relation to Other Clauses of UL 2158
objective of the standard is to set a minimum threshold for safe
performance of the appliance. The standard addresses details of the
construction, electrical safety, thermal safety, and human safety in
In general, the appliance
cabinets are designed with ventilation openings and are constructed
with steel walls—both factors assist in containing an internal fire.
Other specialized openings, such as those for ventilation and the drum
door itself, may provide means for fire escape during the containment
tests. UL2158 dictates criteria for several aspects of appliance design
that may constrain the problem of fire containment. Among the most
obvious test clauses are the following:
- 20.6 Entrapment
- 22.5 Bottom openings
- 22.9 Ventilation openings
- 32 Polymeric materials
of three alternative design approaches is allowed; the intent of each
is to mitigate the likelihood of child entrapment and suffocation
within the appliance drum. The first approach is to limit the door
opening force to 67 N (15 lb force), thus simulating a minimum
threshold for pushing the door open from inside the drum. The second
approach suggests that natural ventilation for the drum be
investigated, presumably to provide for a minimum natural ventilation
rate to support respiration. The final alternative requires a two-step
approach, where the dryer door can be fastened shut only through
simultaneously holding the door closed and activation of a secondary
locking control away from the door.
that choose the first or second approach to meeting the entrapment
clause may encounter distinct challenges. In the case of low opening
force, the latch system may fail due to either direct fire attack or
the effects of intermittent flaming or puffing of the burning materials
inside the dryer cabinet. If the door opens during the test, it will
lead to test failure. In the other case, where a minimum natural
ventilation rate is designed, this design will also feed air to the
fire, causing enhanced burning. The enhanced burning rate may lead to
compromising other components within the appliance to allow flames to
escape and fail the test. Given the three alternatives, the sequential
door locking mechanism may prove to be the most robust method to meet
the entrapment clause without defeating the fire containment clauses.
standard dictates that the bottom (base pan) of the dryer cabinet shall
be constructed in a manner “…to reduce the likelihood of molten metal,
burning insulation, or the like…” from falling through the bottom of
the mechanical compartment in the base of the dryer. Openings in the
base pan may be the result of machine tooling during fabrication or may
serve as points for securing internal components in the mechanical
Under the previous versions of
the standard, the most likely materials to pass through such openings
would be failed electrical components or materials ignited by such
failures. With the new fire tests, the likelihood of burning materials
passing through existing cabinet openings is enhanced because a greater
quantity of burning materials is present. The cheesecloth and
thermoplastic components in the mechanical area in the base of the
dryer are new fuels that were not considered under the prior version of
the standard. Thus, openings in the bottom of the appliance that may
not have been a concern under the prior version of the standard may now
directly cause failure of the test due to allowing the passage of
molten plastic, fire-induced arcing of wiring, or passage of other
burning materials. One potential design method to mitigate the effects
of bottom openings would be to remove or shield those openings.
openings are openings on the sides or top of the appliance other than
those considered “bottom openings” that may allow passage of burning
materials. The standard requires that these openings have barriers or
louvers that limit the size of the openings to mitigate the likelihood
of burning materials from passing through. With the fire containment
tests, these louvered openings have the potential to be exposed
directly to the flames from materials burning inside the appliance. If
the ventilation openings allow flames to pass outside the appliance
cabinet, it will lead to failure of the test. This may lead to
relocation or redesign of ventilation openings such that they remain
low enough in the appliance cabinet that they will only experience the
inflow of cool air from the surroundings and not outflow of hot gases
or flames during a test. The optimum location for such openings may be
specific to both the geometry of the given appliance and the type of
fire test being conducted.
standard requires polymeric materials to meet one of three different UL
94 classifications—HB, 5VA, or 5VB—depending upon their use in the
appliance. Current enclosing parts such as electronic component
housings must meet the higher fire resistance of 5VA or 5VB, but other
components such as the drum baffles, air ducting, lint screen housing,
fan housing, and fan must only meet the HB classification.
HB designation refers to the horizontal burning rate test from UL 94 in
which a horizontal specimen will burn at a specified minimum rate.
These types of plastics may be readily ignited by the burning
cheesecloth in the base fire tests or the burning towel load in the
drum fire tests. These plastics are typically thermoplastics that melt
and flow in a fire environment and may allow burning drips to escape
from the cabinet through either small holes in the base pan or gaps in
the cabinet walls. Also, the molten plastic may flow to other areas
inside the dryer cabinet where openings or susceptible components have
not been removed or protected. Some appliance styles incorporate
thermoplastic leveler legs that could melt away and create a hole for
passage of flaming materials. An individual manufacturer may have to
modify the type, quantity, and location of plastic materials inside the
appliance to pass the fire containment tests.
Potential Design Strategies
manufacturer will likely undertake a different design process to either
modify their existing appliance configurations or to develop new
configurations that will pass the fire containment tests. Based upon
the review and discussion above and good engineering practices, there
are many different approaches that can be taken to designing a suitable
appliance. Overall, the objectives of such a design approach should be
some combination of hardening the appliance against internal fire
Examples of possible alternatives that
may individually or in combination achieve the fire containment
objective for a given appliance are listed below:
- Eliminate HB-rated thermoplastic components.
- Increase the fire resistance of plastic components.
- Improve seals and eliminate potential openings in the cabinet and base.
- Improve the fire resistance of door hinge/latch assemblies or door locking.
- Install rigid non-combustible shielding to block openings or remove openings.
- Limit fire-induced natural ventilation.
- Install active internal fire suppression systems (e.g., sprinklers).
list is not exhaustive but suggests several general design
alternatives. An individual manufacturer must undertake the analysis
and testing to decide which, if any, alternatives are suitable and
acceptable for their products. As an example, consider an individual
appliance that contains several internal components such as the air
duct and fan housing that are currently manufactured from HB-rated
polypropylene. The manufacturer may find that those parts melt and form
a flaming pool of plastic inside the appliance that sometimes flows out
of the dryer cabinet in the fire tests.
looking at the list of alternatives above, one may be able to rank them
as potential solutions based upon manufacturability and economics. For
example, replacement of the part in-kind with a V0-rated plastic may
not be reasonable due to the manufacturing requirements for the part. A
more-successful approach may be to install barriers in the cabinet base
that prevent molten plastic from flowing out of the appliance. Another
option may be to redesign the component so that it can be manufactured
from a different material. Successful design for the new fire
containment tests will require some measure of iterative fire testing
coupled with fire dynamics analysis to identify appropriate design
features for each appliance.
summary, the UL 2158 Electric Clothes Dryers standard has recently been
revised to include clauses requiring fire containment tests on listed
clothes dryers. The fire containment requirements may reduce the
ultimate number or severity of residential fires related to clothes
dryers. The new test requirements will take effect in 2013, but the
design challenges that the new testing imposes may take significant
investment of time and resources to overcome.
1. UL 2158 Electric Clothes Dryers. Northbrook, Illinois: Underwriters Laboratories Inc., March 20, 2009.