The Canadian Electrical Code has a long and precise
definition for grounding as: "a permanent and continuous
conducting path to the earth with sufficient ampacity to carry
any fault current liable to be imposed upon it, and of
sufficiently low impedance to limit the voltage rise above
ground and to facilitate the operation of the protective
devices in the circuit." When we talk about grounding, we
are usually thinking about electrical systems.
The code has an equally precise, but not as lengthy
definition for bonding as: "a low impedance path obtained
by permanently joining all non-current-carrying metal parts to
assure electrical continuity and having the capacity to
conduct safely any current likely to be imposed upon it. When
we talk about bonding, we are usually thinking about metallic
items such as equipment enclosures.
The rules for grounding and bonding are designed to protect
people and property against the dangers of electrical shocks
and electrically caused fires by:
-
Limiting electrical circuit
voltages
-
Minimizing differences in
potential between metal objects such as equipment
enclosures and other grounded metal.
-
Making sure that fuses and
breakers trip promptly when they should
-
Reducing voltage surges caused
by lightning or other causes.
The code specifies that alternating current systems must be
connected to ground for voltages up to 150 phase to ground or
if the system carries a neutral.
The code also provides a number of exceptions where it
would be more hazardous or impractical to operate with a
grounded electrical system.
One such exception is electrical arc furnaces. It would be
easy to see why a grounded system would be impracticable and
maybe even hazardous, since the return currents might find
many paths back to its source.
Another exception would be isolated circuits such as wiring
to the underwater speakers in a swimming pool. If these
circuits were to be grounded, under fault conditions, there
could be a higher possibility of current flow in the water,
which could be very hazardous to swimmers.
A third exception to the rule is an electric crane running
across a hazardous area containing combustible fibres. It
would be easy to see why it is advisable to eliminate some of
the arcing and sparking risks so as to minimize the
possibility of fire.
The code has some very specific requirements for grounding
an electrical system. The system must be connected to ground
once at the owner’s main substation or by the electrical
utility and once again in the owner’s main service. The
code prohibits any connections to ground once past the owner’s
main electrical service.
This requirement has two main purposes:
-
To maintain control of return
currents by ensuring that they flow only on the insulated
neutral conductor and not select random paths back to
their source; and
-
To ensure that the ground
fault protection in switchboards will operate accurately
and when it should.
The code is also very specific about bonding. It specifies
that except for double insulated devices, metallic electrical
equipment enclosures and raceways must be bonded to each other
and to ground by a number of acceptable means such as bonding
wires, busbars, metallic cable sheaths or metallic raceways.
The code prohibits using the identified circuit conductor
(neutral) as the means of bonding electrical enclosures. Once
again, there are several valid reasons:
-
To maintain control of return
currents as above; and
-
To avoid grounding the
electrical system past the main service, which the code
prohibits; and
-
To avoid the possibility that
equipment enclosures could become live should the
identified conductor (neutral) become accidentally
disconnected.
The code goes to great lengths to ensure that all related
metal objects remain at ground potential and do not develop
differences in potential to each other. For these reasons, it
specifies that in addition to the above, we must also bond to
ground all of the following with a building:
In dairy barns and other agricultural buildings where small
differences in potential can have harmful effects upon animals
(tingle voltages), all metal is bonded together, including all
water piping, stanchions, water bowls, vacuum lines an even
the floors where cattle are milked.
Another area where very small potential differences can be
dangerous to people is that area in or around swimming pools.
Section 68 of the code includes some very precise requirements
for bonding all of the metal objects in the vicinity of a
pool, in particular when electrical equipment is used within
three metres of the pool.
As in previous articles, you should seek the advice of your
local inspection authorities for an exact interpretation of
any of the above as they apply in each province or territory.
Some Special Requirements
Several months ago in two previous articles, we covered
some of the standard grounding and bond fundamentals contained
in the Canadian Electrical Code. Now let’s look at some less
usual or more interesting code requirements covering some
special circumstances. It’s unlikely that all of our readers
will have had an opportunity to employ all of the following.
One excellent example is Rule 18-132(2), which deals with
bonding electrical equipment in Class I, Division 1 hazardous
locations. Such locations contain dangerous concentrations of
flammable gases or vapours which may ignite or explode on
occurrence of a spark due to any cause including a loose
bonding connection.
You may not be aware that Rule 18-132(2) has some
exceptional bonding rules for electrical circuits, not only
within the hazardous location, but also within the
non-hazardous location from which the hazardous location
circuit is supplied. (See Sketch A). This rule refers us to
Rule 10-606(1)(a), (c) and (d), and Rule 10-60(2) (Means of
Assuring Continuity at Service Equipment).
In simpler terms this combination of CEC rules tells us
that circuit bonding in Class I, Division 1 hazardous
locations and the rest of each circuit originating outside the
hazardous location, must satisfy the special CEC conditions
normally reserved for bonding on the supply side of customer
main services.
As a direct result, electrical circuits in Class I,
Division 1 hazardous locations and all the way back to their
points of supply are restricted to the following methods of
bonding and bonding connections:
Standard locknuts and bushings are insufficient and you
will notice that EMT connections are not anywhere included in
this list.
I have always interpreted the above rules as applying to
only a complete circuit from its source (such as a
distribution panelboard or motor control centre) in a
non-hazardous location to electrical equipment in a hazardous
location. But do these rules also affect the feeders upstream
from these sources and perhaps back to the main substation? I
hope someone can enlighten me. The code is not very specific
on that point and I’m afraid you’ll have to discuss that
issue with your local inspector for the best interpretation.
Livestock buildings and facilities also necessitate special
grounding and bonding practices. CEC Rule 10-402(4) specifies
that livestock waterers must be bonded with minimum #6 AWG
copper conductors. Also, Rule 10-406(5) requires that all
metal such as water pipes, vacuum lines, water bowls and
stanchions must be bonded with #6AWG copper conductors.
For what reasons do we need these heavy wire sizes and
special precautions? Tingle voltages (between electrical
objects and the floor or between metal objects) in livestock
buildings or other areas may originate from electrical ground
faults in farm buildings or neutral potentials from primary
electrical distribution systems. Cattle are extremely
sensitive to very small voltages and will refuse to drink if
tingle voltages are present in water bowls. They may also
withhold milk during milking. Both of these actions along with
their general unease in the presence of tingle voltages may
lead to deterioration in health and a loss of production.
For these reasons and for safety reasons the CEC has
created some special grounding and bonding requirements in
livestock areas where these conditions may exist. To
supplement the effectiveness of these rules, a device
sometimes called tingle voltage filter is also available.
I am sure you already know that Rule 10-204 specifies that
AC services, when required to be grounded, must only be
grounded once at the customer’s main electrical service and
once again back at the transformer supplying the service. Rule
10-204(1)(d) also prohibits any grounding or bonding
connections to the system neutral downstream from the main
service.
This sub-rule has two important purposes. It is designed to
prevent grounding and bonding conductors, metal piping, ducts,
cable sheaths or building structures from becoming parallel
paths for unbalanced neutral currents. It also ensures that
electrical system ground fault protection will operate
effectively to eliminate dangerous and destructive arcing
ground faults when called upon.
If that be the case, how do we treat an emergency standby
generator which must have its neutral interconnected with the
building system neutral? If the generator neutral is
interconnected with its case inside the machine, there is no
choice but to provide multi-pole switching to disconnect the
generator neutral simultaneously with the phase conductors. If
the neutral is isolated from the case and if not separately
grounded, the neutral does not require disconnection from the
electrical system neutral, but a separate grounding conductor
must be installed to the generator case.
As with past articles, please consult your local electrical
inspector in each province or territory for an exact
interpretation of the above as applicable.
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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