The Canadian Electrical Code in some
instances limits maximum applied voltages to protect
the general public and inexperienced people from
electrical shock hazards. Unqualified persons are at
greater risk due to their inability to identify
electrical hazards and understand electrical shock
risks. This article reviews some of the circumstances
where the code prescribes maximum voltages to minimize
exposure to serious electrical shock.
The first and most obvious voltage
restriction is in our home. Rule 2-106 prescribes 150
volts-to-ground as the maximum voltage in dwelling
units. A dwelling unit is where we live, such as a
single family home, townhouse or apartment. It follows
that this rule limits the supply voltages to dwelling
units to either 120/240 volts single-phase or 120/208
volts three-phase. As much as possible, the intent is to
protect occupants and other persons considered
unqualified against undue electrical shock risks.
However, multi-family residential
buildings often employ qualified and experienced
maintenance staff. When so, Rule 2-106 does permit some
exceptions for multi-family buildings where the
electrical load exceeds 250 kVA and if such maintenance
staff is on hand. Here up to 347/600 volts may be
supplied to permanently installed:
· Space
heating controlled by low voltage wall thermostats
· Water
heating
· Air
conditioning
Rule 2-106 is relaxed for such
applications only when assurance is given that
management will employ qualified people, capable of
recognizing and dealing with the personal risks when
maintaining this equipment.
Similarly, Rule 30-102 further
supports Rule 2-106 by requiring that lighting circuit
voltages in dwellings be limited to either 120/240 volts
or 120/208 volts. But for other than dwellings, Rule
30-102 allows up to 347/600 volts. Other than dwellings
includes industrial or commercial lighting, but also
lighting in common areas of apartment buildings or other
multi-family buildings such as hallways, basements and
parking lot lighting. Here again, it is expected that
this equipment would be maintained only by qualified
people.
A similar shock issue applies to
ballast type lighting, which depending on its design,
may produce high open circuit voltages. Rule 30-706
addresses this safety issue. Some ballast type lighting,
in particular instant start fluorescent lighting
produces extremely high open circuit voltages. High
starting voltage allows the lamps to light quickly
without cathode heating or other means to assist
starting.
And to further limit shock risks,
Rule 30-706 prohibits fluorescent fixtures having open
circuit voltages above 300 volts from use inside
dwellings unless the live parts are enclosed to prevent
inadvertent contact during lamping or relamping, and
Rule 30-802 prohibits outright the use of any lighting
fixtures having open circuit voltages more that 1000
volts within dwellings.
Neon tubes operate at even higher
voltages and they can be very dangerous if incorrectly
installed, maintained and when unqualified people may
come in contact with them. To reduce such risks, Rule
34-300 restricts maximum open circuit voltages for neon
signs and outline lighting to a maximum of 15,000 volts
and 7500 volts phase to ground if neon tube circuits are
provided with secondary ground-fault protection. If not,
open circuit voltages must be limited to:
· 6000 volts
for transformers that have isolated secondary windings
(not autotransformer types)
· Equipment
that has factory-installed (no interconnecting field
wiring) between the transformers and the neon tubing.
The reason for the above voltage
restrictions is to limit exposure to electrical shock to
a general public that is usually unaware of the risks.
Dry niche type underwater lighting
fixtures are installed in the walls of swimming pools
behind a glass lens, sealed to exclude water. It is well
recognized that people in or around swimming pools are
vulnerable to electrical shocks due to the presence of
moisture and low body resistance to current flow. It’s
particularly important to prevent stray currents in the
water, normally achieved with correct grounding and
bonding methods. But to further limit electrical shock
risks, Rule 68-066(3) also limits both the lighting
circuit supply voltage and lighting ballast open circuit
voltage to maximum 300 volts during both starting and
operation.
The code also limits electrical shock
hazards in outdoor electrical substations supplied at
more than 7500 volts in the event of a system ground
fault. Currents may flow through the earth and away from
the fault location, thereby creating ground potential
rise on station grounding systems and on all metallic
objects such as electrical equipment and structures in
the station.
A person who happens to be inside a
high voltage substation during a ground fault is at risk
from electrical shock. Rule 36-304(2) and Table 52 helps
reduce this risk by limiting step and touch potentials
to "tolerable" levels. For example, during a
one second ground fault in an outdoor station with a
crushed stone surface, Table 52 specifies maximum step
and touch voltages of 2216 volts and 626 volts. You will
recall that step voltage is voltage between your feet
and touch voltage is voltage between an energized object
and your feet during a ground fault. The 150 mm crushed
stone surface of the station further reduces the shock
hazards.
As in the case of previous articles,
you should consult the electrical inspection authority
in each province or territory for a more precise
interpretation of any of the above.
Leslie Stoch, P.E. is principal of L. Stoch & Associates,
providing electrical engineering and ISO 9000 quality systems
consulting. Prior to that, he spent over 20 years with Ontario Hydro as
an electrical inspection manager and engineer. Les holds a B.S. in
electrical engineering from Concordia University in Montreal.
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