Littelfuse Fuseology Basics

11
Fuseology Basics
Why Fuses Are Required, How They
Work, and How To Select The Right Fuse
for Your Application

3Confidential and Proprietary | Littelfuse, Inc. © 2020 3
Why Fuses Are Required
A fuse is a thermally responsive device that:
 Protects wires and prevents fires
 Prevents damage to equipment
 Protects equipment and personnel
 Provides overcurrent fault protection

The Two Type of Overcurrents
There are two types of
overcurrents:
1. Short Circuits (top)
2. Overloads (bottom)

What Is a Short-Circuit?
 Flows outside intended path
 > 600% of rated current
 Typically caused by accidents,
dropped tools, bad wiring, or
worn insulation

What Is an Overload Fault?
 Confined to a normal path
 < 600% of rated current
 Typically caused by mechanical
obstructions or jammed equipment

Normal Operating Conditions

Short-Circuit Conditions
Short Circuit Sections

What Is an Overload Fault?
Overload
Section

 NEC® 240.2. The proper localization of a fault condition to
restrict outages to the equipment affected, accomplished
by the choice of selective fault-protective devices.”
 NEC 240.4. Requires the branch circuit, feeder, and
service conductors to be protected against overcurrent in
accordance with their ampacities as specified in 310.15.
Driving Factors for Using OCPDs

Class L
Industrial Fuse Family and Accessories
Class RK1 Class RK5 Class L Class J Class T
Telecom Miscellaneous Fuseblocks & HoldersMidget Class CC/CD Medium Voltage

Indicator
Assembly
Thermal Overload
Element
Precision Formed
Short-Circuit Element
Elastomeric EPR
Overload Module
Dual-element Fuse Design
 No moving parts
 No air chambers
 All sand filled
 Clears faster
 Reliable indication
 Runs cool, cycles longer
Blown Fuse Indication – The Inside Story

Selecting the Proper Fuse
250 V? 600 V? Other?Voltage Rating
1/10 – 6,000 ACurrent Rating
10 – 200 k A.I.R.Interrupting Rating
Time-delay/fast-actingFuse Characteristics
How much space?Physical Size
No? Yes? How much?Current Limiting
Littelfuse InnovationIndication
Selection Considerations

600 VAC
UL Fuse Classes
Class L “Large”
Voltage Rating:
150 – 6000 ACurrent Rating:
200,000A (self-cert @ 300kA)Interrupting Rating:
Time-DelayCharacteristics:
as large as 10-3/4” x 7-1/8”Physical Size:
VeryCurrent Limiting:
NoIndication:
LF Series: KLPC, KLLU, LDC
Typical Applications
Services Entrances
Large motors
Switchboard mains & feeders
General purpose circuits
Back-up protection to circuit breakers
LDC = 600VAC/VDCWhat’s Unique?

250 and 600 VAC
UL Fuse Classes
Class RK1, RK5 “Rejection”
Voltage Rating:
1/10 – 600 ACurrent Rating:
Time-Delay/Fast-ActingCharacteristics:
6 Different Case SizesPhysical Size:
RK1 more than RK5Current Limiting:
Yes (4 of 10 Series)Indication:
LF Series
250V: LLNRK, FLNR, FLNR_ID, KLNR
600V: LLSRK_ID, LLSRK, FLSR_ID, FLSR,
IDSR, KLSR
Services entrances
Switchboard mains and feeders
Circuit with high in-rush currents
(motors, transformers, compressors, pumps)
RK5 = most common fuseWhat’s Unique?

250 and 600 VAC
UL Fuse Classes
(Not For Design-In Purposes)
Class K5 “One-Time”
Voltage Rating:
50,000A (K5)Interrupting Rating:
Same as RK1 / RK5Physical Size:
Yes (K5)Current Limiting:
NoIndication:
LF Series: NLN, NLS
General purpose residential and
commercial circuits with little or no
motor loadNo rejection featureWhat’s Unique?

600 VAC
UL Fuse Classes
Class J “Junior”
Voltage Rating:
Smaller than Class RPhysical Size:
More than Class RK1Current Limiting:
Yes (JTD_ID)Indication:
LF Series: JTD_ID, JTD, JLS
Motor control centers
Transformer protection
Molded case circuit-breaker panels
Anywhere space is at a premium
LDC = 600VAC/VDCWhat’s Unique?

1000 VDC
UL Fuse Classes
Class J (Solar rated)
Voltage Rating:
200kAIR (AC) / Varies (DC)Interrupting Rating:
Same as other Class JPhysical Size:
NoIndication:
LF Series: SPFJ
Inverters
Re-combiner boxes
Anywhere space is a premium in
photovoltaic (PV) systems
Meets UL & IEC StdsWhat’s Unique?

300 and 600 VAC
UL Fuse Classes
Class T “Tiny”
Voltage Rating:
Fast-ActingCharacteristics:
Smallest for 70–1200APhysical Size:
NoIndication:
LF Series: JLLN, JLLS
Individual electric services, meters
Main switch meter stacks
UPS equipment
Equipment containing variable speed
drives, rectifiers, and other surge-
sensitive components
1/3 the size of Class RWhat’s Unique?

600 VAC
UL Fuse Classes
Class CC “Control Circuit”
Class CD (for CCMR 35-60)
Voltage Rating:
1-1/2” x 13/32” (0 – 30A)Physical Size:
ExtremelyCurrent Limiting:
NoIndication:
LF Series: CCMR, KLDR (Time-Delay)
KLKR (Fast-Acting)
CCMR: Motor branch circuits up to
40hp, motor controllers/contactors
KLDR: Transformer, solenoids and
similar inductive components
KLKR: Control panels with solid-state
devices needing fast open
CCMR goes up to 60AWhat’s Unique?

480 and 600 VAC
UL Fuse Classes
Class G “Graduated (length)”
Voltage Rating:
100,000AInterrupting Rating:
Varies x 13/32”Physical Size:
YesCurrent Limiting:
NoIndication:
LF Series: SLC
Cost-effective, space-saving
alternative to other fuse classes
Meets NEC requirement for branch-
circuit protection
Varying length prevents over-fusing
Varying lengthsWhat’s Unique?

125 thru 600VAC
UL Fuse Types
Midget Fuses
Voltage Rating:
Range: 10k – 200kAIRInterrupting Rating:
1-1/2” x 13/32” (10x38mm)Physical Size:
NoCurrent Limiting:
FLA (pin indication)Indication:
LF Series: BLF, BLN, BLS, FLA, FLM, FLQ,
FLU, KLK, KLKD, SPF
UL supplemental protection
Small motors and transformers
Control circuits
Street and HID lighting
Multimeters and similar DC circuits
instruments
BLS is only 1-3/8” longWhat’s Unique?
and solenoids

1000 VDC
UL Fuse Types
Midget Fuses (Solar rated)
Voltage Rating:
1-1/2” x 13/32”Physical Size:
NoCurrent Limiting:
NoIndication:
LF Series: SPF
Combiner boxes
Inverters
Battery charge controllers
Anywhere space is a premium in
photovoltaic (PV) systems
Meets UL & IEC StdsWhat’s Unique?

2,400 thru 38,000VAC
Fuse Types > 600 Volts
Medium Voltage Fuses
Voltage Rating:
2R – 36R / 1/2E – 600ECurrent Rating:
up to 80,000AInterrupting Rating:
up to 30-5/8” x 3” dia.Physical Size:
YesCurrent Limiting:
Some have pin indicationIndication:
E-Rated, R-Rated, Potential Transformer
R-rated for motor protection
E-rated for transformer protection
E-rated PT for protection of potential transformers
Conventional, clip-lock and bolt-in style configurations
Double/Triple barrel designWhat’s Unique?
Approved for use in hazardous environments

170VDC
Fuses Types – Telecom
Telecom Fuses
Voltage Rating:
Various stylesPhysical Size:
ExtremelyCurrent Limiting:
NoIndication:
LF Series: L17T, TLN, TLS
L17T: Telecom circuits & DC power
distrib systems up to 1200A
TLN: Telecom circuits & DC power
distrib systems up to 600A
TLS: Telecom equipment, cables and
components up to 125A
Unique ratings, holderWhat’s Unique?

Current Rating (or Ampere Rating)
 Maximum amperes the fuse is
capable of carrying continuously
under specified conditions
 When selecting a fuse, choose a
current rating as close as possible to
the system’s normal running current
1/10 – 6000A
Available Ratings
Current Rating
Fuse Ratings – Current

 Defines the characteristics of a fuse
 Assists a system designer in selecting the proper
fuse to protect equipment within system
 Types include:
 Time current (T-C) curves plots the fuse’s average melting
or opening time
 Peak let-thru (PLT) curves illustrates the maximum
instantaneous current through the fuse during the total
clearing time, which is the current limiting ability of a fuse
Fuse Characteristic Curve

Start with 1000A
Overcurrent
Go up until
intersects 200A
fuse curve
Results in
average melting
time of ~18 sec.
Fuse Rating: 200A
Reading a Time-Current Curve

Reading a Peak Let-Through Current Curve
Ip
If the available
fault current is
100 kA rms
The instantaneous peak
current can reach 35,000A
The apparent RMS
current is 16,500 A
Fuse Rating: 200A

Let-Through Current Table
Current-Limiting Effects of FLSR and FLSR_ID (600 V) Fuse

 Maximum AC or DC voltage at which
fuse is designed to operate
 Fuse voltage ratings must equal or
exceed the circuit voltage where fuses
will be installed
 Fuses can be used at voltages lower
than maximum design voltage
Example: 250V fuse can be used at
240, 120 or 12 volts…but not 480V
125 – 38,000VAC Voltage Rating
Available Ratings
125 – 1500VDC Voltage Rating
Industry Standards: 250 and 600VAC
DC voltage rating is typically (but not always) ½
of corresponding AC rating.
Fuse Ratings – Voltage

 Maximum current fuse is required to
interrupt under specified conditions
 Standard UL fuse interrupting ratings:
 10,000A (Class H)
 50,000A (Class K5)
 100,000A (Class G)
 200,000A (Class L, R, J, T, CC, CD, Semis)
 300,000A (not a UL Standard)
10kA – 200kAAC Interrupting Rating
Available Ratings
10kA – 50kADC Interrupting Rating
Littelfuse Self-certified for 300kA
Fuse Ratings – Interrupting

Fuse Reaction Times
Fuses are classified based on their operating characteristics and how quickly
they are designed to react.
 Time delay fuses are designed to carry a specific overload for a specified
duration of time
 Fast-acting fuses do not intentionally have time delay feature designed
into them
 Very fast-acting fuses do not intentionally have time delay designed into
them, but are designed to react extremely fast to overcurrent

 Fuses designed to carry specific overload for specified
amount of time before opening.
 UL Standard for most UL Classes: withstand 500% of
fuse’s rated current for minimum of 10 seconds.
(Class CC: 200 % for 12 sec min.)
 If sized properly, Time-Delay fuses are designed to
protect against both overloads and short-circuits.
 Time-Delay fuses are necessary and used for
protection of inductive loads with initial in-rush of
current when energized.
 Inductive loads are generated by equipment with coils.
Time-Delay Fuses
(Class L, RK1, RK5, J, CC, CD and some Midgets)

 Used to protect extremely sensitive equipment
 Used where loads typically have very little or no initial in-
rush of current when energized
 Not designed to carry any specific amount of time-delay
Devices with Non-Inductive Loads
 Resistance Heaters, Ovens
 UPS Equipment
 DC Power Supplies
 Variable Speed Drives
 Diodes & Thyristors
 Triacs & Transistors
 Rectifiers
 Other Solid State Devices
Fast-Acting Fuses
(Class RK1, K5, J, T, CC and some Midgets)

 Dual-element fuse design – a fuse with an internal construction consisting of
separate overload and short-circuit sections.
 Diffusion fuse design – a fuse with an internal construction that combines alloys to
create a desired time-current characteristic.
Fuse Terms – Dual-Element

 A current limiting device
opens and clears a fault
within the first half electrical
cycle (0.00833 seconds)
 To be labeled “current
limiting,” a fuse must mate
with fuse block or holder that
either has rejection feature or
dimensions that reject non-
current-limiting fuses.
Current-Limiting Device – Overview

Short-Circuit Condition – Step 1

Non Current-limiting Fuses or Circuit Breakers
With Current-limiting UL Class RK5 Fuses
With Current-limiting UL Class RK1 Fuses
With Current-limiting UL Class J or T Fuses
Standard Circuit
Breaker
5 electrical cycles
Current-Limiting Fuses Reduce Time and Energy

Current Limitation by Fuse Class

Design in Safety with Current-Limiting Fuses
 Older facilities used UL Class H “renewable” fuses
– No current limitation
– Interrupting rating of only 10,000 A
 Designing in current-limiting fuses provides better protection
Class RK5Class RK1Class J Comparing performance
between the three…
RK5 => 6,400 A
RK1 => 4,100 A
J => 3,500 A

 Lower I2t let-thru energy
 Lower Ipeak (instantaneous peak let-thru current)
 Reduces arc-flash incident energy
 Aids in selective coordination
 Can increase Short-Circuit Current Ratings (SCCR) of industrial
control panels (NEC 409 and UL508A)
Advantages of Current Limitation

Fuse Sizing
When sizing fuses, don’t forget to evaluate…
 Mixed fuses – in 3-phase circuit, all three fuses should be same current
rating, UL Class and manufacturer
 Type of load – need to assess if load requires inductive (time-delay) or
non-inductive (fast-acting) protection
 Ambient temperature – need to look for any abnormal temperatures near
or surrounding the fuses
 Cycling – need to assess if application is requiring repetitive ‘on’ and ‘off’
actions that could prematurely cause nuisance opening
 Codes & Standards – need to assess applicable local and national
standards to confirm compliance

Motor Protection Tables
1/2
3/4
1
1-1/2
2
3
5
7-1/2
10
15
20
25
30
40
50
60
75
100
125
150
200
1
1.4
1.8
2.6
3.4
4.8
7.6
11
14
21
27
34
40
52
65
77
96
124
158
180
240
1-1/4
1-6/10
2-1/4
3-2/10
4
5-6/10
9
12
17-1/2
25
30
40
50
60
80
90
110
150
175
225
300
1-1/8
1-6/10
2
2-6/10
3-1/2
5
8
12
15
20
30
35
45
60
70
80
110
125
175
200
250
1-1/4
1-8/10
2-1/4
3-1/2
4-1/2
6
10
15
17-1/2
30
35
45
50
70
90
100
125
175
200
225
300
1-1/4
1-8/10
2-1/4
3
4
5-6/10
9
15
15
17-1/2
25
35
40
50
60
75
90
125
150
200
225
300
30
30
30
30
30
30
30
30
30
60
60
60
100
100
100
200
200
200
400
400
460 Volt 3 Phase Motors (480 Volt Circuit)
Motor
HP
Full
Load
Amps
Without Overload Relays
S.F. = Less
Than 1.15
Or Temp
Rise More 40C
With Overload Relays
S.F. = 1.15
Or More,
Temp Rise
Not Over 40 C
S.F. = Less
Than 1.15
Or Temp
Rise More 40C
S.F. = 1.15
Or More,
Temp Rise
Not Over 40 C
Switch
or Fuse
Clip
Rating
Class R Fuses

How Are These Products Identified?
…plus labeling requirements!
UL Listed UL Recognized
 UL Classes include: L, RK1, RK5, K5, J, T, G, CC, and CD
 Midget fuses are UL Listed as “Supplementary Overcurrent Protection Fuses”
 High-Speed fuses are UL Recognized as “Special Purpose Fuses”
250V? 600V? Other?
UL Classes Determine a Fuse’s
Selection Considerations
Voltage Rating
1/10 – 6000ACurrent Rating
10k – 200kA.I.R.Interrupting Rating
Time-Delay/Fast-ActingFuse Characteristics
How much space?Physical Size
Yes? No? How much?Current Limiting
Littelfuse InnovationIndication

5454Confidential and Proprietary | Littelfuse, Inc. © 2020
Selective Coordination Overview
 If one circuit is overloaded or faulted, it will not cause a blackout or
other critical circuits to lose power
 Only the protective device that is immediately on the line side of an
overcurrent fault location opens
 Time-current curves provide the amount of time the device will take to
clear a fault at a given value of fault current
 When designing the circuit, place the fastest-clearing devices closest to
the load.

Fuses Simplify Selective Coordination
Fuse Coordination Table
Coordination tables:
 make it easier to
define what will be
selectively coordinated
 prevent the system
designer from needing
to overlay the time-
current curves
 save time

Fuses Simplify Selective Coordination
Example of a selectively coordinated fuse
system

 Number of poles (1, 2, 3 or 4)
 Mounting configuration (Screw-in or DIN-rail mounted)
 Connector type (Screw, Screw w/ Pressure Plate, Box Lug)
 UL Classes R, J, T, G, CC, and CD
 Midgets
 Semiconductors (module design)
 Panel Mount fuseholders
 Dead Front fuseholders
 Inline fuseholders
Available configurations include…
The same factors are also used when selecting the
proper fuseholder or fuse block, plus…
Fuseholder / Fuseblock Selection Considerations

Case Size – The maximum allowable ampere rating of a fuse
with defined dimensions and shape.
0–30 Amps 35–60 Amps 70–100 Amps 110–200 Amps 225–400 Amps 450–600 Amps
Cartridge Design Knife-blade Design
Fuse / Fuseblock Term – Case Size

Distribution Blocks
 Can terminate up to 12 load wires
in single point
 Can be supplied by 1 or 2 line-side
wires
Vs.
Splicer Blocks
 Combines either 1 or 2 load wires
with same number of line wires
LD and LFD Series
Distribution Blocks
Distribution Block vs. Splicer Block
LS and LFS Series
Splicer Blocks

Easy-to-Find Online Resources
Littelfuse University
Fuses White PaperPOWR-GARD Technical
Application Guide
SCCR White Paper
Industrial Fuses Webpage
Motor Protection Guide

Committed to Supporting You
Cross Reference & Technical Support
LuAnn Lange and Melody York
Phone: 1-800-TEC-FUSE (1-800-832-3873)
Email: techline@littelfuse.com
Product Management Support
Craig Greeson
Email: cgreeson@littelfuse.com
Engineering Support
Ryan Blackford
Email: rblackford@littelfuse.com
Marketing Support
Debra Jayson
Email: djayson@littelfuse.com
Mike Sims
Email: msims@littelfuse.com

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Littelfuse Fuseology Basics

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