Script to accompany PowerPoint on Types of Overcurrents & Calculations
Industry trainers are encouraged to use these materials in their sessions.
Slide 1
Suggestion on How to Use
Download both the PowerPoint file (.ppt) and script file (.pdf).
Print the script file (.pdf) and read the script as you view the PowerPoint (.ppt) presentation in
the “Slide Show” view. In this way you see the slides in large format and have animation (if
there is any).
Must have PowerPoint and Adobe reader application software on your system.
Slide 2
This presentation will cover types of overcurrents. You will need the Bussmann Electrical
Protection Handbook - SPD
Slide 3
The National Electrical Code® has a definition for overcurrent in Article 100. (Read the text)
Notice an overcurrent can be an overload, short circuit or ground fault.
Slide 4
Overload is defined in Article 100 as (Read the test). Overload typically can persist for some
period of time without doing substantial damage.
Slide 5
This is a simplified circuit diagram to better understand overloads and faults. There is a source
that provides the electrical power. This typically is the electrical utility, batteries, on site
generators, etc. The load is utilization equipment such as motors, lighting, machinery, HVAC,
etc. Components such as conductors, busway, switchboards, panelboards, transformers and
motor starters serve the function of providing a means to deliver the electric power from the
source to the utilization equipment.
The load and each component has its own resistance (impedance). The load has the
overwhelming largest resistance (impedance) in the circuit. The circuit component resistance is
typically negligible compared to the load resistance.
(click)
The load current is calculated by the formula shown. Since the load is the overwhelming largest
resistance, you can usually just use the load resistance.
Slide 6
What happens when an overload occurs such as when possibly a conveyor belt becomes
overloaded or jammed. In that case, the load current may increase beyond the current rating of
the circuit.
(click)
Overloads are characterized as the current staying within the normal circuit path.
(click)
The resistance of the load decreases when an overload occurs; that is why the current increases.
The load resistance is the major resistance in the circuit.
(click)
Overloads are not dangerous if they do not persist for too long. However, if overloads do not
subside in a prescribed time, the increased heat in the circuit components and the load may cause
damage. The damage can be of a wide spectrum from slight damage to igniting conductor
insulation or adjacent materials. The root cause is thermal escalation as too much current flows
through conductors, components, and utilization equipment that are not capable of withstanding
the overload for a sustained time. Overloads create added thermal conditions that the circuit is
not designed to dissipate for long periods of time. The role of proper overcurrent protection is to
remove the overload before damage is sustained to the utilization equipment and the circuit
components.
Slide 7
Short circuits are another type of overcurrent. Here is an example where a screwdriver shorts out
the two bare conductors.
(click)
Fault or short circuits are characterized by the electricity going outside the normal intended
circuit path.
(click)
The load is bypassed, and can be thought of as not being in the faulted circuit.
(click)
Now using the equation you can see why short circuit currents are so large. Remember in the
normal condition the load resistance is the majority of the circuit resistance. When the load
resistance is no longer in the circuit, the circuit resistance is now very small. The short circuit
current becomes very high. Typically the short circuit current can be hundreds or thousands of
times the normal load current.
(click)
Damage to the circuit components that are still in the circuit can occur rapidly. The damage can
be due to thermal stress or mechanical stress. If components are not adequately protected by
overcurrent protective devices they can be damaged and even violently rupture or catch on fire.
Slide 8
Slide 9
Let’s examine the two types of faults.
A bolted short circuit is characterized by the conductors of different potential being “bolted”
together. Remember, for all practical purposes, the load is no longer in the circuit.
An arcing fault is characterized by the electric current going through the air. In the case of an
arcing fault the air contains contaminates that permits the electrical potential to breakdown the
normally good air insulation. An arcing fault has current flowing through the air. An arc
quickly vaporizes the normal conductors at the points of the arc and this conductive vapor adds
to the conductive nature of the “air path”.
Either type of fault can be devastating since the energy of the source is only abated by the
resistance (impedance) of the circuit components. The bolted fault dissipates the large amount of
energy along the path of the circuit components. The smaller, higher resistance components take
more of the stress. With an arcing fault, the circuit components also take on significant stress
since a large fault current is flowing through them. However, a major difference is that a
tremendous amount of energy is released at the point of the arc.
One last point. In both cases the tremendous release of damaging energy can occur in a few
thousandths of a second.
Slide 10
Let’s first study the bolted faults and let’s use a typical electrical systems of 480Y/277 V, 3 /
4W Solidly Grounded system. This system has three phase conductors A, B, & C, a neutral
conductor and a ground conductor or ground path.
Slide 11
Let’s first look at a three phase bolted fault.
(click)
The A, B, and C conductors all are “bolted” together. This might happen if a ground chain is left
on a bus structure after improperly doing the lockout /tagout procedure when the job is
completed. In essence the three conductors are tied together with no extra resistance.
Typically this case is used as the “worst case” condition for the proper selection of overcurrent
protective device interrupting ratings and assessing component protection.
Slide 12
Next we will look at the line to line bolted fault.
(click)
In this case only two of the three phase conductors are “bolted” together. The current magnitude
can attain a value of 87% of the three phase bolted fault value.
Slide 13
The last bolted fault we will examine is the line to ground.
(click)
In this case a phase conductor is in direct contact with the ground path. Typically the fault
current is considerably lower than the three phase bolted fault current. However, if the fault
occurs near the transformer terminals, it is possible for the bolted line to ground fault current to
be higher than the bolted three phase current.
Slide 14
Let’s move on to the arcing faults. It is much more difficult to try to predict the current
magnitude for these types of faults.
The variables that effect the arcing fault current include the system voltage and the distance of
the air gap the current is conducting over. Also, the amount of available three phase short
circuit current has a great deal to do with this; typically the greater the three phase available
current, the greater will be the arcing fault current. Whether the arcing fault is in open air or
contained in an enclosure makes a major difference. And if inside an enclosure, the volume of
the enclosure is a significant factor. The smaller the enclosure the more condensed the ionized
gas and the higher the fault current. The more the “air path” contains ionized, conductive
molecules, the greater the fault current.
Even though an arcing fault is initiated, it may not be able to sustain itself. All the variables
above affect this situation. For instance, a 120 volt line to ground arcing fault typically will not
sustain itself.
Slide 15
Let’s examine the progression that can occur when an arcing fault occurs.
(click)
What happens when a line to line arcing fault occurs?
(click)
The by-product of the arcing is vaporization of the conductors at the arc tips. The copper vapor
is 67,000 times the volume of solid copper, so the copper vapor will envelope the other phase
conductor and ground and neutral. Also the air becomes superheated and ionizes. An ionized,
conductive cloud engulfs all three phase conductors, ground and neutral.
(click)
This can all happen in a matter of a few thousandth of a second.
Slide 16
We are going to examine a few types of arcing faults. The first is the three phase arcing fault.
(click)
The current that flows can vary widely.
The maximum that can flow is about 89% of the three phase bolted fault.
Slide 17
Next let’s look at the line to line arcing fault.
(click)
Again the actual current that flows is dependent on many variables so a set current is hard to
estimate. However, the maximum value it can attain is about 74% of the three phase bolted fault.
Slide 18
The last one we will look at is the line to ground fault.
(click)
Again the variables make it difficult to estimate. Typically it will be at least 38% of the three
bolted fault.
Slide 19
We already discussed that arcing faults may not always sustain themselves. It is dependent on
the many variables that we have mentioned.
(click)
An industry rule of thumb for 480 volt systems is that the arcing fault will not sustain itself if the
arcing current is less than 38% of the three phase bolted fault current.
Slide 20
Now that we have covered bolted short circuit currents and arcing faults, how do we know what
the fault current is in a real system?
We are going to discuss this a bit further. In the following we will only be discussing three
phase bolted faults. But using the information we already discussed you can better understand
the values for the other type faults.
Slide 21
This is a one line for a typical electrical distribution system. (mention the various components
such as lengths of conductors etc.) The short circuit currents will be different at different points
in your system. That is as you move further into your system, the resistance (impedance)
increases, which reduces the fault current. The short circuit currents are dependent on the circuit
resistance like we already reviewed. The big determining factors for the resistances
(impedances) and level of fault current are the transformers, system voltage, and conductor size,
per phase, and length.
Slide 22
This is an example of what the short-circuit currents might be in a 480 Volt electrical system. At
the secondary of a transformer, which is next to the main switchboard, the available short circuit
current is 60,000 A.
At the motor control center the available short circuit current is 40,000 Amps. The reason for the
lower value versus at the main switchboard is because of the circuit conductors of a specific size
and length. This adds resistance (impedance) to the circuit.
Same holds for the other points in the system.
Slide 23
So let’s do some calculations. Do these problems
Slide 24
Use the Bussmann Electrical Protection Handbook – SPD
Or, use the equation shown.
Slide 25
These are the answers. Notice the differences in the three phase short circuit available for these
examples. It shows that the size of the transformer and the percent impedance of the transformer
are both major determinates of the available short circuit current.
Slide 26
Let’s see how conductors effect the available current. Do these examples.
Slide 27
Use the SPD for the steps to calculate these.
Slide 28
Notice the difference between 4A and 5A. The same size and length conductor. However, 5A
was on a 480 system. The fault current does not drop as quickly on a 480 system.
Notice the difference between 5A and 6A. In this case, 6A is the same length conductor but a
larger size; so the fault current does not drop as much.
Slide 29