Transcript Bonding and Grounding

Bonding, Grounding
and the NEC
Presented by The National Association of Certified Home Inspectors
www.NACHI.org
The New Code …
• The 1999 and 2002 editions of the
NEC* have now clearly defined the
separate and vitally important
purposes of grounding and
bonding in making safe electrical
installations
*The National Electrical Code (NEC) is a registered trademark of the National Fire Protection Association,
www.nfpa.org.
The New Code …
• Section 250-4 establishes new
performance requirements which
clarify what grounding and
bonding are required to
accomplish
The New Code …
Section 250-2 introduces and defines
the new terms:
• Ground Fault
• Ground Fault Current Path
• Effective Ground Fault Current
Path
The New Code …
• These new definitions are in
addition to the two important
definitions in Article 100 which
apply to Section 250-2
The New Code …
These definitions are:
• Grounded - Connected to earth
• Bonded - The permanent joining of
metallic parts to form an electrically
conductive path that ensures electrical
continuity and the capacity to conduct
safely any current likely to be imposed
The New Code …
• The importance of grounding
•
electrical equipment seems to be
well understood
The purpose and intent of bonding
to create a low impedance ground
fault return path seems to be less
understood
The New Code …
• The importance of bonding is best
described in the following graphics
which review how bonding
performs in a typical circuit
A Typical Circuit …
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• 100’ of Overhead Distribution Line
• 25’ of Service Drop
• 25’ of Service Entrance Conductor
• 100’ of Branch Circuit Conductors
A Typical Circuit …
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Current flows…...
Path of Current Flow Normal Operation
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From the transformer to our
service …
Path of Current Flow Normal Operation
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Through the overcurrent device to
our load …
Path of Current Flow Normal Operation
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Through the load returning to
the service …
Path of Current Flow Normal Operation
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And back to the transformer.
Path of Current Flow Normal Operation
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What determines the amount of current
that will flow in this circuit?
Path of Current Flow Normal Operation
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The total resistance or impedance in the
circuit will determine the amount of current
that will flow in the circuit.
Things You Can Count On …
• Ohm’s Law Works
– We can change the code, or
– Hire a different contractor, or
– Use romex instead of EMT, but
• E = I x R still works!
Overload and Short
Circuit Conditions
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How is our circuit protected against
overload and short circuit?
Overload and Short
Circuit Conditions
15A Circuit Breaker
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The overcurrent device protects this
circuit from both overload and short
circuit.
Ground Fault Condition
• So let’s talk about a ground fault
condition …
• Which certainly sounds like the one
condition where grounding would be
important and decide for ourselves
whether …
• Grounding provides protection for
equipment or personnel under a ground
fault condition
Ground Fault Condition
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What happens if the hot conductor
comes into contact with our metal box?
Ground Fault Condition
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And our friend comes along and
touches it?
Is he in jeopardy?
Ground Fault Condition
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No … not at all … and why not?
Ground Fault Condition
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Because the transformer we’re looking
at is not grounded so there is no
path through the earth for current to
return to the transformer.
Ground Fault Condition
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Yes, that was a “trick” question …
sorry about that … but the intent
was to make a point.
Things You Can Count On …
• No circuit – no current
• Current does not flow unless there is a
continuous path from one side of the
source of supply to the other
• Current cannot travel through the earth
to return to a transformer unless the
transformer is grounded
Ground Fault Condition
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So our friend in this situation is
perfectly safe … however …
Ground Fault Condition
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What do we know about utility
company transformers?
Ground Fault Condition
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They’re grounded … and, with this
transformer grounded, our friend is in
serious jeopardy.
Ground Fault Condition
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Because utility transformers are grounded, we
need to do something to our equipment to keep
our friend from being electrocuted.
Ground Fault Condition
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Can we protect our friend by grounding
our metal equipment? Let’s take a look.
Ground Fault Condition
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Grounding our equipment provides a
second path for fault current.
Ground Fault Condition
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The first path is through our friend to
earth and back to the transformer.
Ground Fault Condition
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The new second path is through our metal
equipment to earth and back to the transformer.
Fault Current Path
• We need to open a 15A Circuit Breaker
as quickly as possible. This will require
a fault current of 60A to 75A (four to five
times the rating of the breaker)
• We can use Ohm’s Law to find out how
much current will flow on our new path
Ground Fault Condition
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The voltage is 120V. We need to know the
resistance in this circuit to calculate current.
Fault Current Path
• Assuming a minimum of 5 ohms
resistance through each grounding
electrode, we know there is at least
10 ohms resistance in the fault path
that we created by grounding our
equipment
Fault Current Path
• Therefore, using Ohm’s Law:
–E = I x R
– Transposed to: I = E / R
– Where: I (current) = E (voltage) /
R (resistance)
– And so, I = 120 / 10 = 12A
Fault Current Path
Only 12 Amps …
• Will 12 Amps trip our 15A circuit
breaker?
Absolutely not!
With Equipment Grounded
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So the overcurrent device does not open
and we have fried our friend!
Conclusion …
• Grounding does not
protect equipment or
personnel from a
ground fault!
The Bonding Connection
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The vital connection left out of our
discussion until now is the bonding of
metal equipment to the service neutral.
The Bonding Connection
• Every piece of conductive metal
•
which is a part of our system or
likely to become energized …
Must be connected together by an
electrically continuous metal-tometal contact or by an equipment
grounding conductor
The Bonding Connection
• These connections create an electrically
continuous, low resistance path from
every part of our system back to the
service equipment
• At the service, these connections
terminate on the neutral bus creating
an:
• “Effective Ground Fault Current
Path”
The Bonding Connection
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These bonding connections let us use
the neutral as a return path for fault current.
The Bonding Connection
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Bonding provides a third path for fault
current to return to the source of supply.
Fault Current Path
• We need to open a 15A circuit breaker
as quickly as possible. This will require
a fault current of 60A to 75A (four to five
times the rating of the breaker)
• We can use Ohm’s Law to find out how
much current will flow on our new path
Fault Current Path
The resistance in this path includes
• 100’ - #2 AL OH Distribution
.032
• 25’ - #4 AL Service Drop
.013
• 25’ - #2 CU Service Entrance
.005
• 100’ - #14 CU Branch Circuit
.307
Resistance to the point of fault
.357 ohms
The Bonding Connection
.357 ohms
.3 ohms
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The resistance from the point of fault through our
metal equipment back to the neutral is assumed to
be the same as the branch circuit wiring and 100’
of #14 cu has a resistance of .3 ohms.
The Bonding Connection
.357 ohms
.57 ohms
.3 ohms
The total resistance in this path
created by bonding is .714 ohms.
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Fault Current Path
• Therefore, using Ohm’s Law:
–E = I x R
– Transposed to: I = E / R
– Where: I (current) = E (voltage) /
R (resistance)
– And so, I = 120 / .714 = 168A
The Bonding Connection
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The effective ground fault current path
allows 168A of fault current to flow and
forces the overcurrent device to open.
The Bonding Connection
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This path does not rely on grounding and
works even if our system is not grounded.
Conclusion …
• The overcurrent device protects against
ground fault conditions provided
that …
• Our circuits have been installed so that
all conductive metals are bonded
together and to the service neutral
In Review …
• Grounding is a connection to
earth intended to protect our
electrical system from lightning
and high voltage
In Review …
• The overcurrent device
protects our electrical system
from overload and short circuit
In Review …
• The overcurrent device
protects our electrical system
from a ground fault condition
if …..
In Review …
• Proper bonding …
• Has created a permanent, electrically
continuous and low impedance path
• Which allows fault current to return to
the neutral at the service
Wiring Methods Are Critical
• Clearly, an effective ground-fault current
path must be created throughout our
wiring system
• This is accomplished through the proper
installation of a listed wiring method
Wiring Methods Are Critical
• The safety of our electrical system
relies on
– The wiring method selected, and
– Its proper installation
Wiring Methods Are Critical
• A wiring method that has been
specifically designed as an equipment
grounding conductor, and
• Engineered to assure a low impedance
fault current path, is
• Clearly the best choice for a safe
electrical installation