Not long ago I wrote an article on
Substation grounding for Electrical Business that
raised the issue of whether one should
interconnect the building reinforcing steel with the station
ground electrode. A reader responded with the question of
whether the best approach might be to ignore the rebar
bonding. The reader is well justified in wondering whether the
best approach might be to ignore the rebar bonding. The reader
is well justified in wondering whether there is not an easy
answer to his question as there may not be a precise answer
for every possible situation.
The Canadian Electrical Code, Rule
36-302(6) addresses the reader’s query as follows:
(6) The reinforcing steel members to be
found in building foundations and concrete platforms shall
be permitted to be included as part of the station ground
electrode design provided that:
(a) No insulating film separates the
concrete from the surrounding soil; and
(b) The maximum expected fault current
magnitude and duration will not result in thermal damage to
the steel members or the concrete structure; and
(c) The steel members are connected to
the rest of the station ground electrode with not less than
2 copper conductors of not less than No. 2/0 AWG in such a
way that should one grounding conductor be damaged, no
single metal structure or equipment frame may become
isolated; and
(d) The ground electrode design is made
assuming that the concrete resistivity is greater than or
equal to that of the surrounding soil.
In addition to this long list of
conditions, the Appendix B reference to this rule also
contains this note: "ANSI/IEEE Standard No. 80 should be
consulted for conductor sizing to prevent thermal damage to
the rebar during fault conditions.
You may also have noticed that this
sub-rule is permissive, meaning that rebar bonding is not a
code requirement and the designer can choose to connect or not
make use of this option. However, as indicated, the code does
allow reinforcing bars in concrete pads or building
foundations to be included in the station ground electrode
design. Measurements have shown that interconnecting
reinforcing steel with the station ground electrode usually
reduces the station’s grounding resistance. Consequently the
station’s ground potential rise is reduced as well. This may
be a practical and cost-effective way to improve the GPR,
especially when insufficient property is available to expand
the station ground electrode any further.
IEEE Standard 141 tells us that steel
reinforcing bars in foundation piers in buildings usually
consist of groups of four or more vertical members. These
vertical members are wired to the horizontal members in the
footings at the base of each pier. Measurements have shown
that such piers may have an electrode resistance of about half
the resistance of a ground rod driven to the same depth in
earth. Large buildings would have many such piers, offering
good opportunities to use them for this purpose.
This reference indicates that connection to
such piers has good potential to improve substation grounding.
In regions where the ground resistivity is high, a better
grounding resistance can often be accomplished by including
the reinforcing steel in the final design.
A question you should ask yourself: By
reference to the IEEE No. 80 Standard, does the Canadian
Electrical Code intend that the designer achieve the required
5000 volt maximum GPR before interconnection with rebar steel?
There may be a wide range of opinions on this subject and we
would welcome our readers’ views. In conclusion:
a) Ignoring rebar bonding does not
violate the code. This may be a convenient way to improve
the substation GPR.
b) Reinforcing steel in electrical
equipment pads and building foundations may be used as part
of the ground electrode design.
c) When interconnecting rebars with the
station ground electrode, pay special attention to the
requirements of this rule. This should include discussing
with the electrical inspection authorities to obtain their
interpretation of Rule 36-302(6)(a) to (d).
d) Finally, must the ground electrode
design in every case satisfy the code on its own before
interconnection?
As in all cases, for an exact
interpretation of any of the above, you should consult locally with
the electrical inspection authorities in each province
or territory 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. |
