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| Photo 1 |
| Forming shell in pool wall. |
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| Photo 2 |
| Flexible cord coiled around wet-niche luminaires as it is installed in the forming shell. |
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| Photo 3 |
| Luminaire installed as intended in forming shell. |
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| Photo
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| Pool illuminated by a wet-niche luminaire. |
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IAEI
News>Issue Listing>July/August 2003 >Mismatching Wet-niche Swimming Pool Luminaires with Forming Shells Can Be a Shocking Combination
| Mismatching Wet-niche Swimming Pool Luminaires with Forming Shells Can Be a Shocking Combination |
Installing an incompatible wet-niche swimming pool luminaire (lighting fixture) into the forming shell in the wall of a swimming pool or spa can increase the risk of electric shock or injury to the user. |
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| by Steven Holmes, staff engineer, Underwriters Laboratories, Inc. |
Installing an
incompatible wet-niche swimming pool luminaire (lighting
fixture) into the forming shell in the wall of a
swimming pool or spa can increase the risk of electric
shock or injury to the users. The luminaire may
physically fit into the forming shell, but the
combination may not meet the applicable safety
requirements.
Manufacturers of listed forming
shells that mate with one or more listed wet-niche
swimming pool luminaires intend the forming shells and
luminaires to be used only in specific combinations. How
does one know which wet-niche luminaire is intended to
be used in a particular forming shell? The listed
forming shell and listed wet-niche swimming pool
luminaire are both marked to identify the proper
combinations.
The listed luminaire bears a marking
that identifies the specific forming shells with which
the luminaire may be used. Similarly, the listed forming
shell bears a marking that identifies the luminaires
with which the forming shell may be used. These markings
are required to be located where they can be viewed
during luminaire installation: on the interior surface
of the forming shell and on the exterior of the
luminaire. These markings are part of the requirements
in the Underwriters Laboratories Standard for Safety for
Underwater Luminaires, UL 676.
This marking information is included
in the UL Guide Information for Swimming Pool and Spa
Equipment under the product category Fixtures, Lighting
and Fixture Housings (WBDT) located on page 108 in the
2002 UL General Information Directory for Electrical
Construction Equipment Directory (the White Book). This
information can also be accessed on the internet on UL’s
Online Certification Directory at www/ul.com/database
and clicking on the Category Code/Guide Information
search function and entering the category code WBDT.
What are the safety risks of using
the wrong combination of luminaire and forming shell?
Compromised luminaire mounting. A
luminaire not intended for a particular forming shell
may not mount as securely as required. Although the
mounting features of many luminaires and forming shells
may appear to mate properly, the critical dimensions of
certain features may not be compatible. Exact dimensions
and the relative position between mounting brackets and
fasteners, and the thickness, size, or shape of the
involved parts often critically affect how securely a
luminaire is mounted in the forming shell. UL evaluates
the compatibility of specific luminaire/forming shell
combinations.
Photos 1, 2, and 3.
If the luminaire does not securely
mount at all intended securement points on the luminaire
and forming shell, normal mechanical stresses
encountered during pool usage (water motion and contact
by pool users) may lead to loosening or failure of one
or all of the mounting features. The luminaire may fall
out of the forming shell or may be readily pulled from
the forming shell by a pool user. If this occurs,
multiple safety risks become more likely.
• Risk of drowning partially
attributable to user entanglement. A pool user may
get one or both legs entangled in the now loose,
flexible cord, which may cause him or her to panic, thus
increasing the risk of drowning. Similarly, the user may
have his hand, foot, hair, etc., trapped in the forming
shell if the luminaire does not mate properly.
• Risk of electric shock due to
unconfined flexible cord. UL 676 requires the
flexible cords that are part of a wet-niche luminaire to
be water resistant and to have a relatively thick
jacket. When a flexible cord is properly confined behind
a wet-niche luminaire in the forming shell, any electric
current that may escape from a damaged flexible cord
into the water will tend to be confined and collected in
the volume of water behind the wet-niche luminaire and
in the forming shell. The relatively small amount of
electric current that may conduct into the user area of
the pool water will be inherently low enough to not
exhibit a risk of electric shock to a pool user. If a
damaged or deteriorated flexible cord becomes accessible
in the user area of the swimming pool, the magnitude of
electric current escaping the flexible cord may present
a significant risk of electric shock to the pool user.
• Risk of electric shock due to compromised electrical bond. Proper electrical
bonding is required by Article 680 of the National
Electrical Code, NFPA 70, to prevent voltage
gradients in the pool, and the luminaire shall be bonded
to the forming shell. The brackets and fasteners
securing the luminaire to the forming shell are, in
conjunction with the luminaire equipment grounding
conductor, the bonding path to the pool bonding grid and
branch-circuit equipment grounding conductor. Low
bonding path impedance between the luminaire and forming
shell is needed for the bonding grid to function as
intended during an electrical fault condition involving
the luminaire. If the bonding path impedance between the
wet-niche luminaire and the forming shell is not
sufficiently low, a voltage gradient may be created in
the pool water that might increase the risk of electric
shock for a pool user. Leakage current into the pool
water from a luminaire with a fault may exceed the
escape current density limits assigned to listed
swimming pool luminaires that are associated with a
reduced risk of electric shock to the pool user [600
microamperes per square centimeter in fresh water pools
and 2.4 mA per square centimeter in sea (salt) water
pools]. See sidebar for additional discussion of how the
bonding path impedance between the luminaire and forming
shell is critical in limiting this voltage gradient.
A ground-fault condition in the
luminaire should be sensed and interrupted by the
ground-fault circuit interrupter required in
680.23(A)(3). Even so, 680.23(A)(1) requires that, even
without the GFCI installed, there is to be no shock
hazard with any likely combination of fault conditions
during normal use.
• Risk of injury due to contact
with empty forming shell. A pool user can readily
get his or her foot or hand into a forming shell not
adequately containing a luminaire. The brackets,
terminals, and other parts inside and at the opening
present a potential risk of a cut-type injury.
Overheated forming shell in low water
condition. Many
forming shells are formed mostly of plastic. During a
low-water condition (where the water level is low enough
to not immerse part or all of the luminaire), the
non-immersed inner walls of the plastic forming shell may encounter excessive temperatures
from the heat radiated off of the hot luminaire
enclosure.
While listed luminaires and forming
shells are tested to address this condition during a
listing evaluation, only those specific luminaire and
forming shell combinations identified by the
manufacturers are confirmed not to create excessive
temperatures on the plastic forming shell. A luminaire
other than that which is specified for use with the
forming shell may operate with hotter enclosure
temperatures when not immersed, even with the same or
less lamp wattage. Selecting the wrong luminaire may
overheat the forming shell plastic. If the forming shell
plastic begins to distort or deteriorate, the
metal-to-metal joints in parts of the forming shell, or
between the forming shell and the luminaire, may become
compromised. This may affect the bonding path impedance
and increase the risk of electric shock to the pool user
as discussed above.
In the case of new installations,
care should always be taken to confirm that the
wet-niche luminaire and the forming shell are both
marked for use with each other. The same markings must
also be checked when replacing a wet-niche luminaire in
an existing forming shell. Over time, a luminaire
manufacturer may develop new luminaires or change model
numbers. The correlation marking on the forming shell
already installed in the pool wall may not identify
newer generation luminaires. In this case of wet-niche
luminaire replacement in an existing forming shell, care
must still be taken to confirm that the correlation
marking on the luminaire identifies the model of forming
shell that it is to be installed in. If not, safety
risks can develop. In the case of uncertainty, UL or the
manufacturer may be contacted to verify suitability of
the combination in question. Photo
4
For more information on wet-niche
swimming pool luminaires as well as other types of
swimming pool luminaires and equipment, access the UL
Swimming Pool Equipment, Spas, Fountains and
Hydromassage Bathtubs Marking Guide as well as other UL
Marking Guides on the UL Regulators page at: http://www.ul.com/regulators/guides.htm .
Why a low impedance bond from a
wet-niche luminaire to its forming shell and pool
bonding grid reduces the risk of electric shock.
The equipment grounding conductor in the flexible cord
of the wet-niche luminaire may be relatively small (as
little as 16 AWG in accordance with the NEC), and
may have a length of one hundred feet or more. The
relatively small AWG and long length will result in
increased impedance through the flexible cord equipment
grounding conductor.
If the electrical bond between the luminaire and the
forming shell is poor or absent, in an electrical fault
condition within the luminaire this increased flexible
cord impedance causes a relatively greater voltage
potential between the dead metal of the luminaire and
other dead metal connected to the premises’ equipment
grounding conductor.
The supplemental equipment grounding conductor in the
form of corrosion resistant metallic conduit connected
to the forming shell or the 8 AWG copper conductor
routed with the flexible cord in nonmetallic conduit
provides a second, more conductive, path for fault
current originating in the luminaire. If the luminaire
securement does not provide an effective electrical bond
to the forming shell, there is an equally ineffective
electrical bond to the supplemental equipment grounding
conductor.
Section 680.26 of the NEC requires essentially
all of the dead metal within 1.5 m (5 ft) horizontally
from the pool wall and the dead metal of equipment
associated with the pool water circulating system to be
connected to a low-impedance common bonding grid, using
a solid copper conductor not smaller than 8 AWG.
Likewise, the electrical impedance of the electrical
bond from the dead metal parts of the wet-niche
luminaire to the pool bonding grid must be low. UL 676
requires the impedance between dead metal parts of a
luminaire and the 8 AWG pool bonding grid conductor
connected to the applicable terminal on the forming
shell not to be more than 0.020 ohm. For perspective,
this is approximately the DC resistance of 10 m of solid
8 AWG uncoated copper conductor at 25oC, a
relatively low value.
Wet-niche luminaires and forming shells can be
fabricated to provide this low impedance. However,
compromising the luminaire securement parts (due to use
of the wrong luminaire/forming shell combination) can
very readily cause a significant increase in this
bonding path impedance. In some circumstances, if
metal-to-metal connection is not achieved because of
mutually inconsistent design features, it might even
prevent the bonding of the dead metal of the luminaire
to the forming shell (and, in turn, the bonding grid).
Increasing use of plastic luminaire and forming shell
face rings (bezels) raises this risk of a possible
compromise or absence of a bond. During an electrical
fault in the luminaire, increased bonding path impedance
through the forming shell to the pool bonding grid can
result in a voltage potential difference between the
dead metal of the luminaire and other objects connected
to the bonding grid. Such a voltage potential difference
may cause electric current to conduct from the luminaire
into the pool water, instead of through the forming
shell to the pool bonding grid. |
Steven Holmes is a staff engineer at
UL’s Santa Clara, California, U.S.A. office. For
fifteen of his twenty years with UL, Steven has
evaluated swimming pool luminaires and has also
evaluated electric water heaters and boilers and various
types of household appliances. He has a B.S. in
engineering physics and is a member of the National
Society for Professional Engineers and is a licensed
electrical engineer in the state of California.
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