Bussman_Electrical Protection Handbook.pdf

This handbook is intended to clearly present product data and technical information that will help the end user with design applications. Bussmann reserves the right,

without notice, to change design or construction of any products and to discontinue or limit their distribution. Bussmann also reserves the right to change or update,

without notice, any technical information contained in this handbook.

National Electrical Code ® is a trademark of the National Fire Protection Association, Inc., Batterymarch Park, Quincy, Massachusetts, for a triennial electrical pub-

lication. The term, National Electrical Code as used herein means the triennial publication constituting the National Electrical Code and is used with permission

of the National Fire Protection Association, Inc.

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Bussmann

ELECTRICAL PROTECTION HANDBOOK

Selecting

Protective

Devices Based

On the National

Electrical Code

Fuseology

Circuit Protection

Electrical distribution systems are often quite complicated. They

cannot be absolutely fail-safe. Circuits are subject to destructive

overcurrents. Harsh environments, general deterioration, acciden-

tal damage or damage from natural causes, excessive expansion

or overloading of the electrical distribution system are factors

which contribute to the occurrence of such overcurrents. Reliable

protective devices prevent or minimize costly damage to trans-

formers, conductors, motors, and the other many components and

loads that make up the complete distribution system. Reliable cir-

cuit protection is essential to avoid the severe monetary losses

which can result from power blackouts and prolonged downtime of

facilities. It is the need for reliable protection, safety, and freedom

from fire hazards that has made the fuse a widely used protective

device.

Short-Circuits

Whereas overload currents occur at rather modest levels, the

short-circuit or fault current can be many hundred times larger

than the normal operating current. A high level fault may be 50,000

amperes (or larger). If not cut off within a matter of a few thou-

sandths of a second, damage and destruction can become ram-

pant–there can be severe insulation damage, melting of conduc-

tors, vaporization of metal, ionization of gases, arcing, and fires.

Simultaneously, high level short-circuit currents can develop huge

magnetic-field stresses. The magnetic forces between bus bars

and other conductors can be many hundreds of pounds per linear

foot; even heavy bracing may not be adequate to keep them from

being warped or distorted beyond repair.

Fuses

The fuse is a reliable overcurrent protective device. A “fusible” link

or links encapsulated in a tube and connected to contact terminals

comprise the fundamental elements of the basic fuse. Electrical

resistance of the link is so low that it simply acts as a conductor.

However, when destructive currents occur, the link very quickly

melts and opens the circuit to protect conductors and other circuit

components and loads. Fuse characteristics are stable. Fuses do

not require periodic maintenance or testing. Fuses have three

unique performance characteristics:

1. They are safe. Modern fuses have an extremely “high inter-

rupting” rating–can withstand very high fault currents with-

out rupturing.

2. Properly applied, fuses prevent “blackouts.” Only the fuse

nearest a fault opens without upstream fuses (feeders or

mains) being affected–fuses thus provide “selective coordi-

nation.” (These terms are precisely defined in subsequent

pages.)

3. Fuses provide optimum component protection by keeping

fault currents to a low value. . .They are said to be “current-

limiting.”

Fuses are constructed in an almost endless variety of configurations.

These photos depict the internal construction of Bussmann Dual-

Element and SEMI-TRON ® fuses.

Overcurrents

An overcurrent is either an overload current or a short-circuit cur-

rent. The overload current is an excessive current relative to nor-

mal operating current, but one which is confined to the normal

conductive paths provided by the conductors and other compo-

nents and loads of the distribution system. As the name implies, a

short-circuit current is one which flows outside the normal conduct-

ing paths.

Overloads

Overloads are most often between one and six times the normal

current level. Usually, they are caused by harmless temporary

surge currents that occur when motors are started-up or transform-

ers are energized. Such overload currents, or transients, are nor-

mal occurrences. Since they are of brief duration, any temperature

rise is trivial and has no harmful effect on the circuit components.

(It is important that protective devices do not react to them.)

Continuous overloads can result from defective motors (such

as worn motor bearings), overloaded equipment, or too many

loads on one circuit. Such sustained overloads are destructive and

must be cut off by protective devices before they damage the dis-

tribution system or system loads. However, since they are of rela-

tively low magnitude compared to short-circuit currents, removal of

the overload current within a few seconds will generally prevent

equipment damage. A sustained overload current results in over-

heating of conductors and other components and will cause dete-

rioration of insulation, which may eventually result in severe dam-

age and short-circuits if not interrupted.

The Louisiana Superdome in New Orleans is the world’s largest fully

enclosed stadium. The overall electrical load exceeds 30,000,000 VA.

Distribution circuits are protected with BUSS ® LOW-PEAK ® fuses.

Fuseology

Interrupting Rating - Safe Operation

A protective device must be able to withstand the destructive ener-

gy of short-circuit currents. If a fault current exceeds a level

beyond the capability of the protective device, the device may

actually rupture, causing additional damage. Thus, it is important

when applying a fuse or circuit breaker to use one which can sus-

tain the largest potential short-circuit currents. The rating which

defines the capacity of a protective device to maintain its integrity

when reacting to fault currents is termed its “interrupting rating”.

The interrupting rating of most branch-circuit, molded case, circuit

breakers typically used in residential service entrance panels is

10,000 amperes. (Please note that a molded case circuit breaker’s

interrupting capacity will typically be lower than its interrupting rat-

ing.) Larger, more expensive circuit breakers may have interrupt-

ing ratings of 14,000 amperes or higher. In contrast, most modern,

current-limiting fuses have an interrupting rating of 200,000 or

300,000 amperes and are commonly used to protect the lower

rated circuit breakers. The National Electrical Code, Section 110-9,

requires equipment intended to break current at fault levels to

have an interrupting rating sufficient for the current that must be

interrupted. The subjects of interrupting rating and interrupting

capacity are treated later in more detail.

This photograph vividly illustrates

the effects of overcurrents on

electrical components when

protective devices are not sized

to the ampere rating of the

component.

Considerable damage to electrical

equipment can result if the inter-

rupting rating of a protective

device is inadequate and is

exceeded by a short-circuit

current.

Voltage Rating

Most low voltage power distribution fuses have 250 volt or 600 volt

ratings (other ratings are 125 volts and 300 volts). The voltage rat-

ing of a fuse must be at least equal to or greater than the circuit

voltage. It can be higher but never lower. For instance, a 600 volt

fuse can be used in a 208 volt circuit. The voltage rating of a fuse

is a function of its capability to open a circuit under an overcurrent

condition. Specifically, the voltage rating determines the ability of

the fuse to suppress the internal arcing that occurs after a fuse link

melts and an arc is produced. If a fuse is used with a voltage rat-

ing lower than the circuit voltage, arc suppression will be impaired

and, under some fault current conditions, the fuse may not clear

the overcurrent safely. Special consideration is necessary for semi-

conductor fuse application, where a fuse of a certain voltage rating

is used on a lower voltage circuit.

Fuses are a universal protective device. They are used in power distri-

bution systems, electronic apparatus, vehicles. . .and as illustrated, our

space program. The Space Shuttle has over 600 fuses installed in it pro-

tecting vital equipment and circuits.

Ampere Rating

Every fuse has a specific ampere rating. In selecting the ampere

rating of a fuse, consideration must be given to the type of load

and code requirements. The ampere rating of a fuse normally

should not exceed the current carrying capacity of the circuit. For

instance, if a conductor is rated to carry 20 amperes, a 20 ampere

fuse is the largest that should be used. However, there are some

specific circumstances in which the ampere rating is permitted to

be greater than the current carrying capacity of the circuit. A typi-

cal example is the motor circuit; dual-element fuses generally are

permitted to be sized up to 175% and non-time-delay fuses up to

300% of the motor full-load amperes. As a rule, the ampere rating

of a fuse and switch combination should be selected at 125% of

the continuous load current (this usually corresponds to the circuit

capacity, which is also selected at 125% of the load current).

There are exceptions, such as when the fuse-switch combination is

approved for continuous operation at 100% of its rating.

Fuseology

The table below depicts four different situations involving an

overcurrent device with a normal current rating of 100 amperes

and an interrupting rating of only 10,000 amperes.

Available fault current–50,000 amps

Fuse must have short-circuit

interrupting rating of at least

50,000 amperes.

Circuit with Overcurrent

Circuit

Application

Protective Device

Conditions

And Action

Current Rating= 100A and

of Protective

Interrupting Rating= 10,000A

Device

Normal

Proper

Available fault current–50,000 amps

Circuit breaker must have

capability of interrupting at least

50,000 amperes.

100

Amperes

AMMETER

LOAD

Overload

Proper-Safe

As depicted in the diagram that follows, when using overcur-

rent protective devices with limited interrupting rating, it becomes

necessary to determine the available short-circuit currents at each

location of a protective device. The fault currents in an electrical

system can be easily calculated if sufficient information about the

electrical system is known. See the Point-to-Point Method for short-

circuit calculations. With modern fuses, these calculations normally

are not necessary since the 200,000 ampere interrupting rating is

sufficient for most applications.

Current

Interruption

Greater Than

of Current

Devices

Ampere

Rating

200

Short-Circuit

Proper-Safe

Current

Interruption

Within Device

of Current

Interrupting

Rating

10,000

Short-Circuit

Improper

Current

Explosion or

75,000 Amperes

Exceeds

Rupture

Device

Could Result

50,000

Interrupting

Rating

75,000 Amperes

In the first three instances, the circuit current condition is with-

in the safe operating capabilities of the overcurrent protective

device. However, the fourth case involves a misapplication of the

overcurrent device. A short-circuit on the load side of the device

has resulted in a fault current of 50,000 amperes flowing through

the overcurrent device. Because the fault current is well above the

interrupting rating of the device, a violent rupture of the protective

device and resulting damage to equipment or injury to personnel is

possible. The use of high interrupting rated fuses (typically rated at

200,000 amperes) would prevent this potentially dangerous situa-

tion.

25,000 Amperes

30,000 Amperes

15,000

Amperes

The first paragraph of Section 110-9 requires that the overcur-

rent protective device be capable of interrupting the available fault

current at its line terminals.

Available short-circuit current (indicated by X) at

each panel location must be determined to assure

short-circuit interrupting rating of overcurrent

protective devices is not exceeded.

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