©2007 Thomas & Betts

Section 1

Introduction to the Presentation

• Introduction of T&B Hi-Tech Fuses and our

products

• Expulsion fuses vs. Current-Limiting fuses

• Hi-Tech’s fuse construction & design features

• IEEE standards & testing requirements

• Fuse Application and Coordination

Section 2

Introduction to Hi-Tech Fuses

&

Hi-Tech’s Products

• Established in 1984, in Hickory, NC USA

• Recently acquired by Thomas & Betts (Aug. 2006)

• Focused exclusively on high-voltage, Current-Limiting

fuse technology for power distribution system protection.

• Supplied well over 1 million fuses to the Utility Market

• Global leader in the Current-Limiting fuse market:

– Fuses sold in US, Canada, Latin America and Asia

– Leadership in IEEE/ANSI and IEC fuse standards bodies

– Award winning service levels (9 time ABB outstanding supplier of

the year award winner)

– Active product development efforts

• ISO 9001:2000 certified

Current Product Lines

Trans-Guard OS & OS ShortyTrans-Guard OS & OS Shorty (Oil-Submersible)Backup Type Current-Limiting fuses are primarilyused with bayonet or protective (weak link) typeexpulsion fuses for "two-fuse" protection ofdistribution transformers.

Trans-Guard FX & SX (not shown)Trans-Guard FX & SX (not shown) Full-RangeCurrent-Limiting fuse provides both overload andfault protection for distribution equipment in a singlefuse body.

Trans-Guard EXT Trans-Guard EXT (External) Backup Type Current-Limiting fuses are primarily used for pole mountedtransformer or capacitor protection in series withcutout expulsion fuses.

Fused Loadbreak Elbows

Molded CL Fuses

Molded Fuse Canisters

Current Product Lines

Trans-Guard™ OS & OS ShortyGeneral:Oil-Submersible Backup Type Current-Limiting fuses primarily used withbayonet or protective (weak link) type expulsion fuses for "two-fuse"protection of distribution transformers.

Application:

! Pad mounted distribution

transformers

! Pole-mounted transformers

Other Brands:! Cooper ELSP

! GE OSP

Advantages:

! Highest ratings in a single fuse

body available (less paralleling)

! Shorter or smaller diameter

! Tested to applicable standards

Trans-Guard™ OS ApplicationTrans-Guard OS fuses installed in a 3-Phase

distribution transformer

Trans-Guard™ EXTGeneral:External Backup Type Current-Limiting fuses are primarily used for pole

mounted transformer or capacitor protection in series with cutout expulsion

fuses.

Application:

! Pole-mounted distribution

transformers

! Capacitors

Other Brands:! Cooper NX Companion

! GE ETP

! AB Chance K-mate

Advantages:

! Lowest I2t let-throughs

! Robust and durable design

! Highest ratings available

Trans-Guard™ EXT ApplicationsFor use in high available fault current areas including:

! Pole-mounted transformers

! Overhead capacitors

! Riser pole applications

Trans-Guard EXT w ith Pole-mounted Transformer Trans-Guard EXT w ith capacitorTrans-Guard EXT w ith cutout fuse

Trans-Guard™ FXGeneral:Full-Range Current-Limiting fuse provides both overload and fault protection

for distribution equipment in a single fuse body.

Application:

! Distribution transformers

(in dry-well canisters)

! Switchgear

(clip-mounted)

Other Brands:! Cooper NX, ELX, & X-Limiter

! Eaton CX

Advantages:

! Sealed design

! Damage Sensor

! Tested to latest standards

(includes RMAT testing)

Trans-Guard™ FX Applications! Installed in dry-well canisters for oil filled transformer and

oil/SF6 switchgear protection.

! Clip mounted in live-front switchgear and/or dry-type

transformers

! Externally mounted on overhead distribution systems.

Trans-Guard FX in a clip mountingTrans-Guard FX with a dry-well canister

Trans-Guard™ SXGeneral:Under-oil Full-Range Current-Limiting fuse provides both overload and fault

protection for underground cables and distribution equipment in a single fuse

body.

Application:

! Switchgear

(in wet-well canisters)

Target Market:! Utilities! Switchgear! Manufacturers! Industrial

Other Brands:! Cooper SX-Limiter & ELSG

! AB Chance SL

Trans-Guard™ SX Applications! Installed in wet-well canisters in switchgear

Elastimold® ProductsGeneral:Rubber encapsulated Full-Range Current-Limiting fuse provides dead-front

overload and fault protection for distribution equipment in a single fuse body.

Application:

! Underground vaults

! Dead-front transformers

& switchgear

Section 3

Expulsion Fuses

vs.

Current-Limiting Fuses

Two Main Types of Fuses

1) Non Current-Limiting Fuses

2) Current-Limiting (CL) Fuses

• All fuses, after melting with an overcurrent,

contain an arc that carries the current until

interruption.

• Fuses can be categorized into two main types,

depending on how they interact with relatively

high fault currents:

1) Non Current-Limiting

(Current zero awaiting)

Does not introduce significant

resistance into the circuit after

melting. Requires natural current

zero for interruption.

Types: Expulsion fuse, vacuum fuse, sf6 fuse

(expulsion fuses are the most common)

Common types of Expulsion FusesCommon types of Expulsion Fuses

Bayonet

Expulsion Fuse

Power Fuse

Cutout

Expulsion Fuse

• Very effective at interrupting low fault currents

• Some types can minimize the risk of equipment failure due

to overloading

• Economical replacement

• Provide a variety of time-current curve (TCC)

characteristics

• High continuous current ratings are available

Role of Expulsion Fuses

Expulsion Fuse Selection(Choosing Type)

Types of Bay-O-Net Fuse Available Manufacturer*

" Current Sensing (fault sensing) C, A, E

" Dual Sensing (load sensing) C, A, E

" Dual Element C, A, K

" High Ampere Overload C

*C = Cooper (RTE®), A = ABB, E = ERMCO (GE), K = Kearney

2) Current-Limiting (Current zero shifting)

Introduces significant resistance

into the circuit after melting. At high

current, forces early current zero.

Types: Current-Limiting (CL) Fuse, Fault Limiter

(CL fuses are the most common)

Types of Current-Limiting Fuses

Full-Range

Fuse

External Fuse

and

Oil-

Submersible

Fuse

• Minimizes the risk of eventful/catastrophic failure of distribution equipment

by limiting the peak current and the energy let through during a fault

• Protects distribution equipment especially in areas where available fault

currents exceed interrupting capabilities of other protection devices

• Addresses concerns for potential fire hazards (e.g. grassy areas) or

safety issues associated with populated areas where expulsion gases are

not acceptable

• Enhances overall power quality by reducing “blink” time to fractions

of a cycle

• Improves coordination with source side devices (coordination up to 50kA)

• Alleviates concerns for loud noise (“bang”) during fuse operation

Role of Current-Limiting Fuses

• Backup

• General-purpose

• Full-Range

Three classes of CL fuse:

How well Current-Limiting fuses handle low currents

divides them into three classes:

A fuse capable of interrupting all

currents from the rated maximum

interrupting current down to the

rated minimum interrupting current.

Backup Current-Limiting Fuse

Backup Fuse TCC

OKOK

NO

Minimum I/C Maximum - 50kARated current - IR

Current

Tim

e

1000s

.01s

Fuses cannot interrupt, or may be damaged by, currents in the red zone

A fuse capable of interrupting all

currents from its rated maximum

interrupting current down to the current

that causes melting of the fusible

element in no less than 1 h.

General-purpose Current-Limiting Fuse

General-Purpose TCC

OKOK

NO

Maximum - 50kA

1 Hour

Rated current - IR

Tim

e

Current

Fuses cannot interrupt, or may be damaged by, currents in the red zone

A fuse capable of interrupting all currents from

it’s rated interrupting current down to the

minimum continuous current that causes melting

of the fusible element(s), with the fuse applied at

the rated maximum application temperature

specified by the fuse manufacturer.

Full-Range Current-Limiting Fuse

Full-Range Fuse TCC

OK

50kAIR

1 Hour

25o o

25IR

10,000s

.010sCurrent

Tim

e

Fuse is not damaged by overloads, and can clear any current that causes it tomelt with surrounding temperatures to O

O is RMAT

Two Important Full-Range Fuse

Concepts

1. Fuse can clear any current that melts it

(At ambient temperatures up to its RatedMaximum Application Temperature - RMAT)

2. Fuse is not damaged by overloads, up to thecurrent that causes it to melt (“self-protecting”).

Fundamental Differences

Between

Non Current-Limiting Fuses

(Expulsion Fuses)

&

Current-Limiting Fuses

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

How an expulsion fuse works:

• An expulsion fuse uses a short element

• When the element melts, a low resistancearc produces gas from the fuse liner

• An expulsion action blows the ionized gasout of the fuse

• At a current zero, if the gap is sufficientlyde-ionized, arcing ceases

Expulsion Fuse Design

Short Fuse Element

Gas Evolving liner

Cutout fuse Bayonet fuse

Expulsion Action

Common designs

After melting, arc

produces low resistance

How a Current-Limiting fuse works:

• A Current-Limiting fuse uses a long elemente.g. 3’ (1meter) for a 15.5kV fuse.

• When the element melts, multiple series arcsare produced.

at high fault currents:

Restrictions

initiate arcing

Long elements

are wound on an

Inert core

Current-Limiting Fuse Design

PUNCHED ELEMENT

FILLER

At high currents,

restrictions melt

simultaneously(introducing resistance)

Continued arcing causes

the arcs to lengthen(resistance increases)

Eventually the

whole element

is consumed

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

2. Current Interruption

Expulsion Fuse - Current interruption

Prospective Fault Current

Fuse Current

Current interruption

Fuse Melting Current peak

Current-Limiting fuse - Current interruption

Prospective Fault Current

Fuse Current

Current peak

Fuse Melting

Current interruption

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

2. Current Interruption

3. Energy Let-through

Current let-through

Expulsion Fuse

Current-Limiting Fuse

i2 dt (i2t)Energy

25,000A

7,000A

10,000A rms Symmetrical

Prospective Current

What is I2t ( )?i dt2

!

i

rR

time = t

heat = i 2 R t + i 2 r t

so, i 2 t a energy

T T

Illustration of I2t ( ) Reduction using CL Fusei dt2

!

Fault Current 5000A

rms. symmetricalFirst loop 870,000 A2-sec.

Peak current 12kA

I2t let-through by Hi-Tech 80A = 31,000 A2-sec. (<4%)

Peak current 5 kA (41%)

I

II

rms

I2t is proportional to the

volume of the “box”

t

irms

t

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

2. Current Interruption

3. Energy Let-through

4. Peak Current Let-through

Peak Let-Through Curves

1K 10K 50K 100K

100K

10K

1K

Prospective Fault Current

Amps rms symmetrical

Peak C

urr

ent

Prospective

Current 10,000A

Symmetrical

7,000A

25,000A25K

2.5K

Peak current of

asymmetrical fault

Peak current of CL fuse

7K

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

2. Current Interruption

3. Energy Let-through

4. Peak Current Let-through

5. Sensitivity to Circuit &

Fault Conditions

Prospective Fault Current (as a function of point on wave)

X/R >15

Voltage

The point-on-wave where fault occurs affects asymmetry

Symmetrical Current

Asymmetrical Current

Expulsion Fuse

Operation at High Currents

Current

Voltage

MELTING

TRVExpulsion fuses are sensitive to

circuit Transient Recovery Voltage

Expulsion fuses are

sensitive to circuit X/R

(fault offset)

Fuse waits for current zero

No significant arc voltage

before current zero

Current-Limiting Fuse

Operation at High Currents

Current

Voltage

MELTING

Significant arc voltage (result of

current-limiting action)

Significant Current-Limiting action

Current-Limiting fuses are quite

insensitive to TRV and X/R

Expulsion FuseOperation at Low Currents

Fuse Voltage

Fuse Current

Melt

System Voltage

Backup Current-Limiting FuseOperation at Low Currents

Voltage

Current

Melt

Current switching between multiple elements

Fundamental Differences Between Expulsion Fuses

and Current-Limiting Fuses

1. Construction

2. Current Interruption

3. Energy Let-through

4. Peak Current Let-through

5. Sensitivity to Circuit &

Fault Conditions

6. Performance at Various

Fault Current Levels

Expulsion Fuses

" Must wait until a current zero to interrupt

" Interruption occurs when withstand voltage

exceeds recovery voltage

Expulsion fuses cannot interrupt at high

currents because the voltage withstand never

exceeds the recovery voltage. They

therefore have a maximum interrupting

current

Backup CL Fuse - element melting= Restriction melting

High current

Low current

Fuse does

not interrupt

current

insufficient restrictions melt

Fuse

interrupts

current

all restrictions melt

Minimum I/C

Backup CL fuses cannot interrupt currents less than their

minimum interrupting current

sufficient restrictions melt

Fuse

interrupts

current

Section 4

Hi-Tech’s Fuse Construction

&

Design Features

Machined

brass caps

Low current section(FX Only - includes patented Damage Sensor® ) High current

interruption element

Resin-rich filament

wound glass/epoxy body

Compacted

quartz sand

Welded element

joints

Epoxy jointSoldered electrical

connection and

sealing

Construction of a Hi-Tech Fuse

A. Cost advantages:

! In some applications a single fuse can be used, where parallel fuses

would normally be used

! Minimize the need and extra costs associated with using parallel fuses

B. Shorter overall lengths:

! In many cases, OS Shorty fuses will be shorter or smaller in diameter

than the alternative.

C. Compliance with industry standards:

! All OS designs have been tested to

the applicable IEEE (ANSI) and IEC

standards.

OS & OS Shorty Features

Trans-Guard™ EXT Features

A. Significant design advantages:

! EXT fuses have the lowest let-through energy (I2t) levels in theindustry minimizing the potential for distribution equipment damage.

! Durable design with machined brass end-caps and acrylic paintcoating for minimized UV degradation.

! Largest current ratings available in the industry (65K, 80K and 100K)

B. Wide industry acceptance:

! Our EXT fuses are approved and

widely used throughout the US

utility market.

Trans-Guard™ FX Features

! All FX designs have been tested to the most current IEEE (ANSI) andIEC standards which includes short circuit testing at elevatedtemperatures (RMAT of 140°C for 2” fuses and 71°C for 3” fuses ).

A. Sealed design:

C. Commitment to compliance with industry standards:

! Ensures no gases are discharged during operation! Prevents eventful failure due to leaking dry-well canisters

B. Patented damage sensor :! Significantly reduces the risk of fuse failure should a current surge

damage the fuse elements

E. Rugged machined end caps:! Results in less distortion and secure

attachment in dry-well canisters

D. Low current element in center of fuse:! Eliminates the risk of fuse overheating in

the MCAN or other types of enclosures

Damage Sensor®

Reason for the Damage Sensor

A concern that:

If a surge damages, but does not fully melt, a

Full-Range fuse’s (high current) ribbon

element(s), the element(s) may subsequently

melt with a current too low for the high-current

element(s) to be able to interrupt.

FX Full-Range Fuse Damage Sensor

10000

.010current in amperes

tim

e in s

econds high

current

section

low current

section

damage

sensor

high current section low current section high current section

Damage Sensor®tin

- how it works

Damage Sensor®e D

1. The whole of the fuse’s melting TCC is

generated in the low current section

2. A surge that could damage the ribbon

element will melt, or damage to a greater

extent, the “damage sensor”

3. If the damage sensor subsequently melts at

a low current, the low current section can

interrupt it.

End Cap ComparisonNon Hi-Tech Design

FX Machine Design

Blown Fuse Indication

• Note: This is not available for dry-well

canister applications

Fuse Indicator - before operation Fuse Indicator - after operation

Trans-Guard™ SX Features

! All FX designs have been tested to the most current IEEE (ANSI) andIEC standards which includes short circuit testing at elevatedtemperatures (RMAT of 140°C for 2” fuses and 71°C for 3” fuses ).

B. Commitment to compliance with industry standards:

A. Patented damage sensor :

! Significantly reduces the risk of fuse failure should a current surgedamage the fuse elements

Section 5

IEEE/ANSI Standards

&

Testing Requirements

Latest Applicable IEEE/ANSI Standards

• IEEE C37.40-2003IEEE Standard Service Conditions and Definitions for High-Voltage Fuses, DistributionEnclosed Single-Pole Air Switches, Fuse Disconnecting Switches, and Accessories

• IEEE C37.41-2000IEEE Standard Design Tests for High-voltage Fuses, Distribution Enclosed Single-Pole AirSwitches, Fuse Disconnecting Switches, and Accessories

• ANSI C37.47-2000Specifications for distribution fuse disconnecting switches, fuse supports, and Current-Limiting fuses

• IEEE C37.48-1997IEEE Guide for Application, Operation, Classification, Application, and Coordination ofCurrent-Limiting Fuses with Rated Voltages 1-38kV

Definitions1. Rated Continuous Current - a current they can

carry continuously without damage

2. Rated Maximum Voltage - the maximum voltage against

which they are capable of interrupting current

3. Rated Maximum Interrupting Current - the maximum current

they are capable of interrupting

4. Rated Minimum Interrupting Current – for a backup fuse, the

lowest current the fuse has been shown to be able to interrupt

5. Rated Maximum Application Temperature - the maximum

application temperature at which the fuse is suitable for use

General Test RequirementsAt the Rated Maximum Interrupting Current

At current where approximate maximum arc

energy is absorbed by the fuse

At the Rated Minimum Interrupting Current for

a backup fuse or at the lowest current that can

cause melting for a Full-Range fuse

Repeating of some of the tests above at thethe Rated Maximum Application Temperature

Testing in the “take-over” region where currentinterruption transfers from the low currentelement to the high current element (Full-Range only!)

I1 Testing -

I2 Testing -

I3 Testing -

RMAT Testing -

TCC Testing -

Section 6

Application & Coordination

Coordination of Expulsion Fuse and

Backup CL Fuse

• Hi-Tech Fuses Application Guide FS-10

• This generally follows recommendations in

IEEE Std.C37.48TM application guide.

• Conservative recommendations.

Interrupting Rating Low Current

operationFuse Type

Expulsion

Fuse

Backup

CL Fuse

Poor Excellent

Excellent Poor

Relative Fuse Characteristics

• Combining a Backup Current-Limiting Fuse

with a series Expulsion Fuse

• Full-Range Fuse

Two ways to get Full-Range protection

c

Selecting Oil-Submersible

Backup Current-Limiting Fuse

with Series Expulsion Fuse

Coordination of Expulsion Fuse

and Backup CL Fuse:

There are two types of coordination recommended

by the IEEE (Standard C37.48).

1) Time-current Curve Crossover Coordination (TCC)

2) Match-melt Coordination

– Transformer KVA

– Primary Voltage

– Impedance

– Expulsion Fuse

– Connection

Requirements Needed to Select Fusing

Selecting an Expulsion Fuse

Meeting Temporary Surge Requirements

The expulsion fuse minimum melt curve should be to the right of the

following points:

INRUSH: 12 x IR at 0.1 sec.

25 x IR at 0.01 sec.

COLD LOAD PICKUP: 3 x IR at 10 SEC.

6 x IR at 1 SEC.

Where IR = transformer rated current

1000

100

10

1

0.1

0.01 10 100 1000 10,000

Current in Amperes rms Symmetrical

!

!

!

!

Expulsion Fuse

Minimum Melting TCC

Inrush/cold load pick-up

points

!

Tim

e (

se

c)

The fuse must be able to carry the maximum transformer load current (includingacceptable overload) without melting.

Protective “weak link”, and Current Sensing bayonet fuses are typically pickedto melt at 300% - 400% IR in 300 sec. (600 sec. for fuses rated over 100A).

Dual Sensing, Dual Element and High Ampere fuses respond significantly totransformer oil temperature. They provide transformer overload protection, andtypically allow 200% IR for 2 hours, and 160% IR for 7 hours.

Selecting an Expulsion Fuse

Overload Requirements

Basic Principles of TCC Coordination:

1) Each fuse must protect the other in its region of

non-operation

2) Operation of the expulsion fuse must not melt or

damage the backup fuse (when there is no fault

inside the transformer)

3) Transformer overload must not damage the backup

fuse by exceeding its maximum continuous current

rating

Selecting the Backup Fuse

Requirement 1

Min I/C CL Fuse

50kA

CL Fuse

Minimum

Melt TCC

Ti

me

Current

Max I/C Expulsion Fuse

Expulsion

Fuse Total

Clearing TCC

Region where the

expulsion fuse

cannot interrupt

Max I/C Expulsion Fuse

Expulsion

Fuse Total

Clearing TCC

Region where the

CL fuse cannot

interrupt

Requirement 2 & 3- 25% Margin

Bolted Secondary Fault Current

CL Fuse “No-damage” Curve

100%80%

CL Fuse Minimum Melt CurveExpulsion Fuse

Total Clearing

Curve

Check

Check

Check

100% 120%

Coordination Area

Sample Coordination Tables

@ 2.4%HTSS242125@ 3.8%HTSS2421004000353C1213200

@ 2.5%HTSS242125@ 4.1%HTSS2421004000353C1212470

@ 2.6%HTSS242125@ 4.2%HTSS2421004000353C1212000

Minimum

ImpedanceAlternativesMinimum

ImpedanceAlternativesMinimum

ImpedanceCL FuseLink Cat #

Voltage

L-L

Trans-Guard OS "Shorty" (HTSS------)

Delta Connected Transformers

500 KVA CURRENT SENSING 353C

@ 2.0%HTSS232125@ 3.8%HTSS2321004000353C1213200

@ 2.1%HTSS232125@ 4.1%HTSS2321004000353C1212470

@ 2.2%HTSS232125@ 4.2%HTSS2321004000353C1212000

Minimum

ImpedanceAlternativesMinimum

ImpedanceAlternativesMinimum

ImpedanceCL FuseLink Cat #

Voltage

L-L

Trans-Guard OS "Shorty" (HTSS------)

GRDY-GRDY Transformers - CL Fuse L-N Rated (where possible)

500 KVA CURRENT SENSING 353C

Basic Principles of Match-melt

Coordination:1) Each fuse must protect the other in its region of

non-operation

2) Operation of the expulsion fuse must not melt or

damage the backup fuse (when there is no fault

inside the transformer).

3) Transformer overload must not damage the backup

fuse by exceeding its maximum continuous current

rating.

4) Backup fuse must always let through enough energy

to cause the expulsion fuse to melt open

What is I2t ( )?i dt2

!

i

R

time = t

heat = i 2 R t

so, i 2 t a energy

Done by comparing I2t of fuses

Requirement 4

2 x minimum melt I2t of the backup fuse must be greater than

or equal to the maximum melt I2t of the expulsion fuse.

Current

MELTING

Main benefit of match-melt coordination is that voltage stress is

removed from the backup fuse after interruption

Requirement 4: How to Calculate

2 x minimum melt I2t of the backup fuse must be greater than

or equal to the maximum melt I2t of the expulsion fuse.

Minimum Melt I2t of backup fuse – Published Value

Maximum Melt I2t of expulsion fuse – Must calculate from

expulsion fuse curve

Typically done using the following equations

I.025sec x 1.2 = Imax. melt (Imax-melt )2 x .025sec = Max. Melt I2t

(From min-melt TCC)

Fuse Voltage Rating

Single Phase Transformers:

Fuse voltage rating " maximum applied (L-N) voltage

Three Phase Transformers:

- For GndY-GndY connected transformers having

less than 50% delta connected load

Can typically use L-N rated fuses

- For all other connections (i.e. delta connections)

L-L rated fuses are typically required

Exception

When match-melt coordination is used:

It is usually possible to use L-N rated backup fuses

and L-L rated expulsion fuses

This makes it possible to fuse delta connected

transformers having primary voltages as high

as 34.5kV

Selecting External

Backup Current-Limiting Fuse

with Series Cutout Fuse

• EXT’s have been rated to match-melt

coordinate with a cutout having the same rating

• This means that the backup fuse always allows

enough energy through during a fault to cause

the cutout fuse to drop open

• This removes voltage stress from the backup

fuse & provides visual indication of where the

fault occurred

Selecting a Trans-Guard™ EXT Fuse

• Must always be coordinated with a series

connected cutout expulsion fuse

• Select Voltage Rating– Rated Maximum Voltage of the fuse must be greater than

the maximum system L-N voltage

• Select Current Rating– K rating of backup fuse must be greater than or equal to the

K rating of the cutout fuse

• Select Hardware Configuration

Selecting a Trans-Guard™ EXT Fuse

Trans-Guard™ EXT Hardware

Standard and Offset stud Integral Eyebolt Spade

Parallel Groove Connector Loose Eyebolt Universal Adaptor

Selecting Full-Range

Current-Limiting Fuses

Selecting a Full-Range Fuse

Meeting Temporary Surge Requirements

The expulsion fuse minimum melt curve should be to the right of the

following points:

INRUSH: 12 x IR at 0.1 sec.

25 x IR at 0.01 sec.

COLD LOAD PICKUP: 3 x IR at 10 SEC.

6 x IR at 1 SEC.

Where IR = transformer rated current

1000

100

10

1

0.1

0.01 10 100 1000 10,000

Current in Amperes rms Symmetrical

!

!

!

!

Full-Range CL FuseMinimum Melting TCC

Inrush/cold load pick-up

points

!

Tim

e (

se

c)

The fuse must be able to carry the maximum transformer load current (includingacceptable overload) without melting.

Full-Range fuse are typically selected to allow between 140-200% IR or 200-300% IR.

NOTE: The Fuse’s Melting Time-Current Curve will shift to the left withincreasing oil temperature.

Selecting a Full-Range Fuse

Overload Requirements

Derating of FX fuses

• The fuse melting characteristics move to the

left (on the TCC)

• The fuse’s continuous current to decrease

There are certain conditions that will cause:

Note that an FX fuse maximum continuous

current rating is related to its melting TCC – it

is 95% of the minimum fusing current

(The minimum fusing current is the lowest current that melts

a fuse at a particular ambient temperature, including

manufacturing tolerances)

Derating Factors

1. Derating for the fuse being in an enclosure

(FEP)

2. Derating with an increase in Surrounding

Temperature (Ambient temperature)

There are two factors to consider:

Derating due to use in

a Drywell Canister Submerged in Oil

Reduction of 2.0%

Example: Lowest current to melt a 30A FX fuse, in canister at 250C

IAir = 43 A ICanister = 43A x 0.98 = 42.3A

I = lowest current to melt fuse at 25o C

. ..

(From min-melt TCC)

Derating due to

Elevated Surrounding Ambient Temperature

Reduction of 0.2 % per degree C over 250C

Example: Lowest current to melt a 30A FX fuse, in oil at 1000C

100o - 25

o = 75o 75

o x 0.2% = 15%

100% - 15% = 85% (0.85)

I25Canister = 42.3 A I100 = 42.3A x 0.85 = 36A

I q = lowest current to melt fuse at q o C

. ..

– For single phase applications: The RatedMaximum Voltage of the fuse must begreater than the maximum system L-Nvoltage

– For three phase applications: The RatedMaximum Voltage of the fuse shouldtypically be greater than the maximumsystem L-L voltage (some exceptionsapply)

Fuse Voltage Rating

Selecting an Elastimold Fuse

• Select Voltage Rating– For single phase applications: The Rated Maximum Voltage

of the fuse must be greater than the maximum system L-Nvoltage

– For three phase applications: The Rated Maximum Voltageof the fuse should typically be greater than the maximumsystem L-L voltage (some exceptions apply)

• Select Current Rating– Use continuous current rating

• Must consider derating due to elevatedtemperatures

Derating due to

Elevated Surrounding Ambient Temperature

Reduction of 0.2 % per degree C over 250C

Example: Lowest current to melt a 10A Elbow fuse at 650C

65o - 25

o = 40o 40

o x 0.2% = 8%

100% - 8% = 92% (0.92)

I25 = 15 A I65 = 15A x 0.92 = 13.5A

I q = lowest current to melt fuse at q o

C . ..

(From min-melt TCC)