Bonding and Grounding, What they mean and which is more important.
Chelan Home Inspections
Bonding or grounding what do you think is more important? Heck many do not even know the difference?
So let’s start off with terminology .
From the 2014 NEC Article 100-
Bonded (Bonding).
Connected to establish electrical continuity and conductivity.
Bonding Conductor or Jumper.
A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.
This material was extracted from Mike Holt Training Materials copyright 2007 by permission. Visit mikeholt.com or call 1.888.NEC.CODE (632-2633) for more information.
Ground.
The earth.
Grounded (Grounding).
Connected (connecting) to ground or to a conductive body that extends the ground connection.
I am sure that helped.
Here is a little more terminology from article 100
Effective Ground-Fault Current Path.
An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.
I am sure that helped also ;)
First off all electricity wants to go from the source back to the source, it can go through the ground but is not prefered. This is a general concept and there are more to it than this but for all intents and purpose this is true in residential. So the ground is just another path to the source and there can be several paths. This is why we want to create better paths to the electricity to take.
Grounding and its primary purpose-
The main purpose of grounding electrodes at a home is for lightening, line surges or other unintentional dangerously high voltages that can develop in the electrical distribution system. The grounding electrode connects to the earth to provide a safe, alternate path around the electrical system of your house and hopefully minimizing damage from these high voltages.
Due to this connection we can also induce current back to the home from the earth.
Bonding and equipment grounding conductors primary purpose-
The primary purpose of these is to create a low resistance path back to the overcurrent devices so they can open during a fault condition.
The equipment-grounding conductor (EGC) is used to ground those things that we do not want carry current, such as metal or conductive casing of equipment or appliances. By providing a proper sized and installed EGC we hope to be as close as possible to ground potential and provide a safe low resistance path for ground-fault current to flow.
This is where the terminology really has it wrong. The equipment-grounding conductor (EGC) is really a bonding conductor in purpose and function, and this causes confusion between grounding and bonding.
By bonding any metal objects and systems, such as metal water pipes, we again are trying to create that low resistance path back to the overcurrent device.
Before we do the math that shows how this all plays out first lets look at this statement from Schneider (Square D) on Breaker tripping characteristics. This is important to understand.
“The tripping characteristics of molded case circuit breakers can be represented by a characteristic tripping curve that plots tripping time versus current level. The curve shows the amount of time required for a circuit breaker to trip at a given overcurrent level.” (Schneider Electric)
So lets dispel a few myths. How many people think a 40 amp breaker will trip at 42 amps? How about that they will trip immediately or when an electrical event happened? Or that you will not get shocked. These are a few myth examples and these examples are far from the truth.
Using a 40 amp circuit breaker as an example it takes about 1200 amps (multiple of 30 to the breaker rating) to instantly trip the breaker. Lower amperages depending on the trip curve will take longer to open the circuit breaker.
Circuit breakers are basically designed to protect wiring from both a short circuit and overload damage. Typical breakers in residential are a Thermal Magnetic Circuit Breaker (Inverse-time) type.
The breakers thermal part works by the use of a bimetallic strip which causes a spring-loaded latch to release and trip the breaker when a certain temperature is achieved. The faster the rise of heat, the faster the breaker reaches temperature threshold of the bimetal strip and trips. Heat needed to activate the trip mechanism is directly proportional to the power (watts) and current (P=I2 x R). This is for an overload condition.
So let’s use an overload of 250% on the 40 amp breaker, that is 100 amps, it takes approximately 60 sec before the bimetal will bend far enough to trip the breaker. At an 135% load, 54 amps, it takes about 1800 seconds(30 minutes) to initiate the breaker to trip. So you can see that is not even close to instantaneous.
The Magnetic trip function is used for a short circuit conditions using an electromagnet. As load current passes through the electromagnet coils if that current spikes from a short circuit it will create enough electromagnetic field strength to attract a nearby armature to also cause the breaker to trip.
From Schneider
“Thermal-magnetic circuit breakers include both a magnetic tripping function, for short-circuit protection, and a thermal tripping function, for overload protection. As the alternate name “inverse-time” implies, the higher the overload, the shorter the time in which the circuit breaker will open.”
Now back to bonding and time for a little ohms law math, Amps- E/R=I (Volts/Ohms=Amps) and Watts is P= E x I (Watts= Volts x Amps).
Many think that volts kill but this is not exactly true. Lets say you have been a bad boy or girl you may be hit by a policeman’s taser, it may be 5 seconds of 50,000 volts at 26 watts, this equates to 0.00052 amps. The result is no fun and an instant loss of neuromuscular control and any ability to perform coordinated functions but you are normally not dead (well if you had a heart issue this may kill you).
With normal household voltage of 120 volts any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock and currents of 100 to 200 milliamps can be deadly. This is why GFCI devices are set to trip at 5 to 6 milliamps (0.005 or 0.006 amps).
So now comes the math again,
If we have 120v and 25 ohms of resistance (this is the required maximum resistance of a ground rod) you would reach only 4.8 amps. Well below any trip threshold of a circuit breaker.
Here is the math, E/R=I, 120 volts/25 ohms=4.8 amps
Typically the equipment and equipment grounding conductor design is that all grounding circuits within equipment should ensure a resistance of one tenth ohm (0.1) or less at any point. (Here again is a problem with this terminology that leads us to some confusion).
Now back to the design principle from above, when we apply ohm's law to this design we reach the 1200 amps required for instant trip on a 40 amp breaker.
The math, E/R=I , 120 volts /0.1 ohms =1200 amps
Now armed with this information we can now see that having a proper equipment grounding(which really is a bond) and proper bonding of metallic/conductive systems is actually really about the safety of the wiring system to prevent shock and fire.
We as home inspectors need to be well aware that a properly bonded system is essential to the safety of the electrical system.
Here are two very good articles on grounding and bonding.
“An investment in knowledge pays the best interest.”
Benjamin Franklin
NCW Home Inspections, LLC is a Licensed Washington State Home Inspection service located in Wenatchee Washington serving Chelan County, Douglas County, Kittitas County, Okanogan County and Grant County Washington and the cities of Wenatchee, Leavenworth, Cashmere, Oroville, Cle Elum, East Wenatchee, Quincy and many more…
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Thank you though.
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