Power
Distribution for Branch Circuit Protection
by
ED
Cordial, Senior Project Manager, AMETEK Solidstate
Controls
and
Jim
Murrill, Pan Tech Engineering
Overall protection of Branch
Circuits connected to critical loads can be a very complex issue. These
branch circuits are typically connected to UPS Systems so this type of source
power will be discussed. There could be a very wide variety of UPS System types
and critical loads to be considered. Likewise, there could be a wide variety of
issues involving the protection of the overall system through the branch circuit
protection devices; usually these protection devices would be either circuit
breakers or fuses. Within each of these two categories of protective
devices, there exists another wide variety of choices. When dealing with branch
circuit protection, the primary factors to consider are the time-vs-current
characteristics of the protective device, the time-vs-current characteristics of
the power source, and the time that the critical connected equipment can operate
at zero volts (“ride-through”).
When evaluating the UPS system
involved, one must first determine what type of UPS system is being used: True
On-Line, Standby, or Off-Line. Generally, Industrial customers would tend to use
a True On-Line system, but not always. A true on-line system would be one
in which the Inverter section of the UPS system is intended to be connected to
the loads during all normal operating conditions. This is normally accomplished
through the use of an Automatic Static Transfer Switch. With this configuration,
the output of the Inverter is connected to the primary position of the Static
Switch, and the secondary position, or Bypass position, of the Static Switch is
connected to an Alternate, or Bypass Source. The Bypass Source should normally
be a separate electrical feed to the system rather than being "wrapped around"
from the primary feed to the UPS system normal input. Additionally, in these
on-line systems, the separate Bypass source is typically delivered to the UPS
through a Transformer (usually a step-down/ isolating type) that is compatible
with the UPS system’s Inverter output rating. This is important when trying to
determine the overall "fault clearing" capability of the
system.
Determination of the overall
fault clearing capability of the system is important since the primary intent of
the system is to maintain constant regulated power to the critical loads. This
can only happen if the selections of UPS system and fault protection devices is
done properly.
Selection of the overcurrent
devices depends on the type of excessive current the interrupting device will be
required to operate under. An inrush overload can be as much as twelve
(12) times the normal operating current level. These currents are temporary in
nature and are normal during the start up of the applied load. A good example of
this would be a motor or power supply starting with a surge current. The short
duration of this inrush current does not cause any problems and should not open
the overcurrent protection device. A sustained overload that would occur because
of a defective component in the load but doesn’t go to a complete short must
operate the protective device after several minutes to prevent overheating of
the protective device, wiring and portions of the affected load. A dual element
fuse or circuit breaker is the best circuit protector when this overload
condition exists.
A short circuit or fault
current, on the other hand, could be many hundred times the normal
operating current. If this high current is not cut off in few thousandths of a
second, damage and destruction of insulation, melting of conductors,
vaporization of metal, ionization of gases, arcing, and fires will result. To
protect against this kind of damage, a non-time delay current limiting fuse
(semiconductor type fuse) must be used. These fuses, under a short circuit
condition, will completely open the faulted circuit in approximately one fourth
(1/4) of a cycle.
The new HDR/Solidstate Controls
power distribution panel will deal properly with both fault conditions. It
contains both a fuse and breaker on each branch circuit. The breaker should be
properly coordinated with the branch circuit rating and will protect the load
during overload conditions. The fuse can be larger since its function is to
clear short circuits. The
fuse could be oversized as much as 300% of the circuit full load rating to
permit motor and power supply start-ups and also allow other large transients
that normally can be handled by the power source. This over sizing should be
reviewed to make sure the source has enough interrupting capacity to allow the
fuse to open in a quarter cycle during a short circuit.
Fast acting fuses as protective
devices for critical loads have more advantages than the exclusive use of
circuit breakers. Semiconductor protection fuses will tend to offer
time-vs-current characteristics which exhibit very fast clearing times which can
be matched to the fault clearing capabilities of the particular type of UPS
system being used (assumed to be of the "On-Line" type) and the “ride through”
capabilities of the individual critical load devices. A fault on a branch
circuit will attempt to pull that circuit and all surrounding circuits to zero
volts until the branch interrupter opens the connection between the source and
the fault. The amount of time we experience this low voltage condition is a
function of the source capacity and the amount of current that is allowed to
flow to the fault (See figure 1). This current not only comes from the UPS but
the fault will actually pull current out of the other loads as the voltage
begins to collapse. Figure 2 shows how the fuse will limit the amount of current
the fault can draw which will protect the other loads. Limiting the current also
shortens the duration of the fault and reduces the time current is going into
the fault. While some loads have “dual” power sources if the secondary is
available, all the loads will experience a fluctuation until transferred and
those that do not have the second power source will be at the mercy of the
faulted system.
Destructive energy impressed upon circuit
components
Normal
Current
Level
Circuit Breaker trips
Fault
occurs
Figure
1. Current build up allowed by a non-current-limiting device releasing a large
amount of destructive short circuit heat energy before
opening.
Normal
Current
Level
Fuse opens and clears short-circuit in less than ½
cycle
Figure
2. High-speed response of the
current limiting fuse does not allow current build up to peak
value
The current limiting capability
of a semiconductor fuse is very important for load protection but is probably
the least understood characteristic of this special type of fuse. Improved overall power distribution in
critical areas does require reviewing of the available current in case a load
device should fail. Properly sized power transformers with shorter runs to motor
control centers have increased the available current to a level that could
damage some or all of the components in the event of a short circuit at a load.
In the opposite extreme, a UPS that does not have the available interrupting
current to operate a semiconductor fuse, such as when the Bypass Source is not
available, could rely on the slower types of fault protection. This could result in all the loads
seeing a reduced voltage longer than can be tolerated for proper critical
applications. The safety side of this issue is to make sure circuit damaging
currents are not allowed to flow from the source to the fault. A current
limiting device such as a semiconductor fuse will only allow current to flow for
a quarter cycle before the fault current reaches the highly destructible level
that might be available. This current limiting feature also will protect lower
limit circuit breakers in series with them. The use of semiconductor fuses can
eliminate the need for special bracing or high withstand ratings of other series
components. Once a short circuit begins, currents could be 40,000 amperes or
more in the first half cycle in circuits not protected by a current limiter.
Faults at this level can generate a tremendous amount of heat and cause severe
insulation damage or even an explosion. This type of fault can also create huge
magnetic forces that could destroy bracing structures or crack insulators. These
examples underscore the need for the selection of a protective device to limit
fault current before it reaches its full potential level.
Properly applied current
limiting fuses will instantaneously (less than ½ cycle) isolate a faulted
circuit and protect the other circuits on the same phase from the effects of a
voltage sag. Proper application means that the available short circuit duty at
the end device is above the current limit threshold of the fuse. Otherwise,
voltage-sensitive equipment will be subjected to low voltage during the extended
interruption interval of the fuse. A short circuit causes the voltage to
collapse to zero at the point of the fault. The voltage at the source is
dependent on the impedance between the fault and the source. If the fault lowers
the source voltage to less than 70% of rated RMS voltage and is allowed to
persist for one cycle or more, voltage-sensitive relays and other devices
(interposing relays, solenoids, SMPS’s, etc.) will
de-energize.
Computer systems, process
controllers, microprocessors, programmable logic controllers, adjustable speed
drives, etc., with switch-mode power supplies have inherent, but limited, stored
energy in the capacitive circuit of the power supply. The stored energy is
expended after about three cycles and is discharged after about one cycle for
very low voltage sags. For example, if the voltage sags to less than 30% of
rated RMS voltage for one cycle, the power supply will be lost. Circuit
breakers, with standard trip ratings (15A, 20A, etc.), do not clear faults fast
enough to prevent voltage sags (and outages) since they frequently trip in their
thermal, time delay region (typically, between 30 and 60 cycles). Even when
operating in the instantaneous region, a circuit breaker may take one to two
cycles to clear a fault. For faults cleared by circuit breakers, a voltage sag
will result on that phase. When voltage sags for more than one cycle motor
contactors will drop out, solenoid valves will close, process controls variables
in volatile memory will be lost and remote I/O will be lost. This may result in
equipment and plant shutdowns. Current limiting fuses are the only protective
devices that operate fast enough to prevent outages. Current limiting fuses, by
definition, will totally clear a fault in less than ½ cycle; given sufficient
fault current, in ¼ cycle or less. The key factor is that the available short
circuit current should be greater than the current-limit threshold of the fuse.
If not, the tripping time of the fuse follows its time-characteristic curve. To
keep current magnitudes high, branch circuit impedance should be
low.
The higher the UPS System's
Inverter and its Automatic Static Switch’s current capacity, the shorter the
time required to open a fuse. Adequate overcurrent ratings are required to allow
proper coordination with the branch circuit fuses to insure clearing of a
downstream fault while maintaining the UPS system on line without a "shut-down"
of the UPS Inverter or system. The faulted load would normally be transferred to
the Bypass Source via the UPS system’s Static Switch. The Bypass Source should
be designed to have extremely high fault clearing capability much higher than
the UPS Inverter. The bypass source is typically derived from a separate, low
impedance utility feed capable of delivering the higher currents for longer
periods of time. There are two key elements to consider: 1) the Bypass Source
has a very high overcurrent rating via the Static Switch to allow faults to
clear without damage or shut down to the system; 2) the Inverter has a high
overcurrent rating in relation to the size of the branch protecting device which
would normally allow it to clear faults in the event that the Alternate, or
Bypass Source is not available during the fault condition. When using AMETEK
Solidstate Control’s Ferroresonant UPS the standard overcurrent ratings of the
system components are Inverter - 560% for 1 cycle, 120% continuous and the
Automatic Static Switch - 1000% for 1 cycle. PWM UPS Systems have a standard
overcurrent rating of: 150% for 1 minute, 125% for 10 minutes, 100% continuous
and the Automatic Static Switch – 1000% for 1 cycle. These ratings need to be
considered when designing the load distribution.
The fault clearing capability
and the overall system reliability is greatly enhanced by the use of
semiconductor fuses as opposed to circuit breakers. The importance of the
overcurrent ratings of the UPS system components relates to the UPS system's
ability to allow proper fault clearing without "interlocking down" (shutting
down to protect the internal components of the system), thereby creating a loss
of primary power to all of the loads. Basically, if the power distribution
system is designed properly, the only primary power loss due to the fault
condition would be to the faulted branch circuit itself.
The only way to effectively
create this higher reliability of overall UPS power continuity to the loads
would be through the use of circuit breaker and fast acting semiconductor fused
distribution connected from an On-Line UPS system with adequate overcurrent
ratings.
Glossary of
Terms
Ampacity
The
current a conductor can carry continuously without exceeding its temperature
rating. Ampacity is a function of cable size, conductor material (ex. Copper vs.
Aluminum), insulation type and the conditions of use.
Ampere
Rating
The
current carrying capacity of a fuse. When the fuse is subjected to a current
above its ampere rating, it will open the circuit after a predetermined period
of time.
Ampere Squared Seconds,
I2t
The
measure of heat energy developed within a circuit during the fuse’s clearing. It
can be expressed as “Melting I2t”, “Arcing I2t” or the sum
of them as “Clearing I2t”. “I” stands for effective let-through
current (RMS), which is squared, and “t” stands for time of opening, in
seconds.
Arcing
Time
The
amount of time from the instant the fuse link has melted until overcurrent is
interrupted, or cleared.
Available Fault
Current
The
maximum short circuit current that can flow in an unprotected
circuit.
Bolt-in
Fuse
A fuse
that is intended to be bolted directly to bus bars, contact pads or fuse
blocks.
Branch
Circuits
The
portion of a wiring system extending beyond the final over current device
protecting the circuit. Typically, individual loads, or sub-feeders, would be
connected to each branch circuit.
Contacts
The
external live parts of a fuse which provide continuity between the fuse and the
balance of the circuit. Also referred to as ferrules, blades or
terminals.
Circuit
Breaker
A
device designed to open and close a circuit by non-automatic means, and to open
the circuit automatically on predetermined overload of current, without injury
to itself when properly applied within its rating.
Clearing
Time
The
total time between the beginning of the overcurrent and the final opening of the
circuit at rated voltage by an overcurrent protective device. Clearing time is
the total of the melting time and the arcing time.
Coordination
The use
of overcurrent devices which will isolate only that portion of an electrical
system which has been overloaded or faulted. See
Selectivity.
Current
Limitation
A fuse
operation relating to short-circuits only. When a fuse operates in its
current-limiting range, it will clear a short circuit in less the ½ cycle. Also,
it will limit the instantaneous peak let-let through current to value
substantially less than that obtainable in the same circuit if that fuse were
replaced wit a solid conductor of equal impedance.
Electrical Load
That
part of an electrical system which actually uses the energy or does the work
required.
Element
A
calibrated conductor inside a fuse which melts when subjected to excessive
current. The element is enclosed by the fuse body and may be surrounded by an
arc-quenching medium such as silica sand. The element is sometimes referred to
as a link.
Fast Acting Fuse
A fuse
which opens on overload and short circuits very quickly. This type of fuse is
not designed to withstand temporary overload currents associated with some
electrical loads.
Fault
An
accidental condition in which a current path becomes available which by-passes
the connected load.
Fault
Current
The
amount of current flowing in a faulted circuit.
Fuse
An
overcurrent protective device with a fusible link that operates and opens the
circuit on an overcurrent condition.
High Speed
Fuses
Fuses
with no intentional time-delay in the overload range and designed to open as
quickly as possible in the short-circuit range. These fuses are often used to
protect solid-state devices or in panels where it is critical to keep the low
voltage attributed to overloads to a very short period of
time.
Interrupting Rating (Breaking
Capacity)
The
rating that defines a circuit breaker or fuse’s ability to safely interrupt and
clear short circuits. This rating is much greater than the ampere rating of a
circuit breaker or fuse.
The NEC® defines Interrupting
Rating as “The highest current at rated voltage that an overcurrent protective
device is intended to interrupt under standard test
conditions.”
Magnetic
Trip
Action is achieved through the
use of an electro-magnet in series with the load current. This provides an
instantaneous tripping action when the current reaches a predetermined
value.
Melting
Time
The amount of time required to
melt the fuse link during a specified overcurrent.
Molded Case
Switch
Switches that are UL 1087
listed devices that have no thermal protection but do have a self protecting
high magnetic trip setting.
Overcurrent
A Condition which exists on an
electrical circuit when the normal load current is exceeded. Overcurrents take
on two separate characteristics – overloads and short
circuits.
Overload
Can be classified as an
overcurrent which exceeds the normal full load current of a circuit. Also
characteristic of this type of overcurrent is that it does not leave the normal
current carrying path of the circuit – that is, it flows from the source,
through the conductors, through the load, back through the conductors, to the
source again.
Peak Let-Through Current,
IP
The instantaneous value of peak
current let-through by a current-limiting fuse, when it operates in its
current-limiting range.
Rejection Fuse
Block
A fuse block which will only
accept fuses of specific UL class. Rejection is a safety feature intended to
prevent the insertion of a fuse with an inadequate voltage or interrupting
rating.
Rejection
Fuse
A current-limiting fuse with
high interrupting rating and with unique dimensions or mounting
provisions.
Resistive Load
An electrical load which is
characteristic of not having any significant inrush current. When a resistive
load is energized, the current rises instantly to its steady-state value,
without first rising to a higher value.
Ride-Through
Time that an electrical circuit
can operate without input voltage by relying on internal stored energy to power
the circuit.
R.M.S.
Current
The R.M.S. (root-mean-square)
value of any periodic current is equal to the value of the direct current which,
flowing through a resistance, produces the same heating effect in the resistance
as the periodic current does.
Selectivity
A main fuse and a branch fuse
are said to be selective if the branch fuse will clear all overcurrent
conditions before the main fuse opens. Selectivity is desirable because it
limits outage to that portion of the circuit which has been overloaded or
faulted. Also called selective coordination.
Semiconductor
Fuses
Fuses used to protect
solid-state devices. See “High Speed Fuses”.
Short
Circuit
Can be classified as an
overcurrent which exceeds the normal full load current of circuit by factor many
times (tens, hundreds or thousands greater). Also characteristic of this type of
overcurrent is that it leaves the normal current carrying path of the circuit –
it takes a “short cut” around the load and back to the source. Another
characteristic of a short circuit that allows this to occur is an extremely low
(zero) impedance or resistance.
Short-Circuit
Rating
The maximum short-circuit
current an electrical component can sustain without the occurrence of excessive
damage when protected with an overcurrent protective
device.
Short-Circuit Withstand
Rating
See Short -Circuit
rating.
Single
Phasing
This condition occurs when one
phase of a three phase system opens, either in low voltage (secondary) or high
voltage (primary) distribution system. Primary or secondary single phasing can
be caused by any number of events. This condition results in unbalanced currents
in polyphase motors and unless protective measures are taken, causes overheating
and failure.
Thermal
Trip
Action that is achieved through
the use of a bimetal heated by the load current. On a sustained overload, the
bimetal will deflect, causing the operating mechanism to deflect, causing the
operating mechanism to trip. Because bimetals are responsive to the heat emitted
by the current flow, they allow a long time delay on light overloads, yet they
have a fast response on heavier overloads.
Threshold
Current
The symmetrical RMS available
current at the threshold of the current-limiting range, where the fuse becomes
current limiting when tested to the industry standard. This value can be read
off of a peak let-through chart where the fuse curve intersects the A-B line. A
threshold ratio is the relationship of threshold current to the fuse’s
continuous current rating.
Time-Delay
Fuse
A fuse with built-in delay that
allows temporary and harmless inrush currents to pass without opening, but is so
designed to open on sustained overloads and short
circuits.
Voltage
Rating
The maximum open circuit
voltage in which a fuse can be used, yet safely interrupt an overcurrent.
Exceeding the voltage rating of a fuse impairs its ability to clear an overload
or short circuit safely.
Withstand
Rating
The maximum current that an
unprotected electrical component can sustain for a specified period of time
without the occurrence of extensive damage.
Bibliography
Protecting Molded Case Circuit
Breakers, An Engineering Solution
Bulletin PMCB11
By
Cooper Bussmann
Branch Circuit Protection for
Uninterruptible Power Supply Systems
By David R. Bratton and Edward E. Cannon
IEEE April 30-May 4, 1973
Bussman Circuit Protection
Solutions
Cooper Industries, Bussman Division 1996
Product Guide, Circuit
Breakers
Eaton-Cutler Hammer 1999
You Too Can Be A Short Circuit
“Expert”
Gould Inc., Electric Fuse
Division