Low Voltage Systems Protective Fuses in Commercial Industrial Applications to BS IEC Standard

7/28/2019 Low Voltage Systems Protective Fuses in Commercial Industrial Applications to BS IEC Standard

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LV SYSTEMS JAVED Page 1

LOW VOLTAGE SYSTEMS & PROTECTIVE FUSES IN COMMERCIAL &

INDUSTRIAL SYSTEMS

TO BS & IEC STANDARDS

(S.R. Javed Ahmed)

A Fuse is an important Protective device used to automatically disconnect a live circuit

when a predetermined value of current & or time of predetermined current is exceeded.

The disconnection mean by fuse is based on the applicable Protection philosophies.

Fuses are available in AC & DC circuits from extra low voltages to High Voltages.

A Circuit Breaker (CB) also does similar type of function as a fuse. Unlike a CB, fuse is

self-destructive which requires replacement after it disconnects a circuit. Some CBs

also perform switching function with or without current flowing in the circuit. A hybriddevice is a fused-disconnect switch which has a fuse associated with a load/no-load

disconnecting switch (may be group operated or single pole type).

Some fuses provide additional function such as operation indication (dropout, burnt

mark, a plunger etc), driving an aux contact to close, limiting the peak current etc.

General Protection Philosophies in LV System

Based on the application & philosophies, various Protection means are applied. The

following are general groups on which the Protection systems are designed:

i. Prevention of Electric Shocks

Direct Contact

Indirect Contact

Dealing with Electric shocks due to Direct Contact:

Preventing a current from passing through the human body or any

livestock

Limiting the current which can pass through the human body or any

livestock to lower than the Shock current

Dealing with Electric shocks due to Indirect Contact:

In dealing with protection against indirect contact under single fault

conditions, regulations permits the two methods given above for Direct

contact and additionally Automatic disconnection of supply in a

determined time on occurrence of a fault which is likely to cause a

current to flow through the body in contact with exposed conductive


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parts (where the value of current is equal to or greater than the shock

current).

ii. Prevention against Thermal effects

Preventing ignition of flammable material due to heat or arc

Preventing burns to human or livestock

Preventing degradation or impairment of the Equipment

iii. Prevention against Over Current

Preventing injury to human or livestock or equipment due to thermal as

well as electromechanical stresses due to Overcurrent.

iv. Prevention against Fault Current

Preventing the conductors or any other part carrying Short-circuit from

attaining temperature greater than its design value andElectrodynamic-withstand levels (peak current).

v. Prevention against Over Voltages

Preventing the damage to property, equipment, human or livestock due to:

Over voltages which cause faults between circuits supplied at different

voltages

Over voltages which arise due to switching & atmospheric actions.

1. Fuse Identification

Various Fuses exist in Electric circuits. Our first task is to identify the Fuse

application and type. There are general three types of Fuses found in an Electric

circuit categorized as:

Miniature Fuses

Glass

Ceramic


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These Fuses are identified as type:

FF (Ultra Rapid)

F or QA or QB (Fast Blow)

M or MD (Medium Blow)

T or SB (Slow Blow)

TT (Ultra Slow)

Bottle Fuses

Bottle Fuse

These Fuses are identified as type:

DIAZED 500V Fuses, D1 (E16), D11 (E27), D111 (E33)

NEOZED 380V Fuses, D01 (E14), D02 (E18)

SILAZED Ultra Rapid Fuses, D11 (E27), D111 (E33)

Industrial Fuses

These Fuses are identified as type:aR or gR or uR (Ultra Rapid)

gL or gG (General Line)

gM (Motor rated general Line)

aM (Motor rated)

gF or gTF (Transformer, Cable Protection)

gB (Mining Fuses)


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Knife Blade Fuses

Wedge Fuse


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Typical Street lighting Fuse


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House Service Cut-out Fuse

2. Fuse Current Rating

THE AMPERE RATING

Generally, a Fuse current rating shall not be less than the full load rating of the

circuit it is protecting.

Over loads or Over Currents if occur frequently will degrade the fuse performance

and hence there shall be clear idea of overloads which occur frequently or

infrequently. Over currents typically occur in Motor circuits, charging (energizing) a

reactive equipment like a Capacitor, Shunt Reactor, Transformer etc. Over loads

occur due to diversity in loads (based on max load, connected load or contingency

loads, increased loads, process jams, mechanical failures etc). some Overcurrentsor Overloads are requires the circuits to be disconnected, while others may be

transient requiring fuses to ride through them (in coordination with some other

protective device upstream or downstream). Obviously, upstream device operation

before the fuse is designed to enhance interruption capability.

Two general types are used which are type gG & gM.

Type gG Fuses:

i. For application based on Protection of the cables against Short circuit:

Type gG fuses have characteristics to protect the cables against short circuit

current & Overload conditions when they are selected with a current rating (1N)

is less than or equal to the current rating of the cable (1z).

ii. For Motor circuit application where the Motor Starter will provide Overload

protection.


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In these applications, Fuses provide only against short circuit current.

Fuses for Transformers:

Fuses for Transformer primary side protection typically shall have current rating of at

least twice the nominal Transformer primary current.

Fuses for Fluorescent lighting circuits:

Fuse current ratings normally should be twice the numbers of lights to be

simultaneously switched.

Fuses for PF correction Capacitor circuits:

Fuse current ratings normally should be at least 1.5 times the Capacitor circuit rated

current.

Fuses for Motor circuits:

Motors starting currents are typically larger than their rated (full load) current.

Starting current depends on the type of Motor starting means implemented. Motor

starter Manufacturers recommend coordination based on IEC 60947-4-1.

Type2 coordination with type gG & gM fuses shall be made as per BS88 or IEC

60269.

Most of gG type fuses can be used for Motor circuits but Fuses (& associated

holders) shall be much larger rated current than Motor Full load current (FLA or

motor rated current) to meet the Motor Starting curves.

Type gM fuses however, are rated same as the Motor FLA and fit same holders as

normal gG fuses. gM fuses extend the utilization of standard equipment.

With Motor run-up time of less than 5s, the Table below indicates recommended gG

& gM fuses ratings (with Motor duty infrequent no more than twice per hour). Next

larger size shall be used for demanding applications.


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Fuse rating based on Conductor sizes:

The maximum fuse that can be used depends on the Cable type using an

appropriate K factor.

For PVC insulated cables factor K = 115 (Thermoplastic 70C) and for XLPE

insulated cables K = 143 (Thermosetting 90C).

The following table gives ratings for Fuses for popular Copper conductors (withthermoplastic & thermosetting insulations):


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BS 88 Industrial Fuses:


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Protection against Electric Shocks:

Max Disconnecting time:

For a TN System, a disconnecting time not exceeding 5s is permitted in a

distribution circuit.

Maximum Earth Loop Impedance:

The maximum values of earth loop impedance (Zs) for typical gG 240V fuses to

BS88: Parts 2 & 6 are:


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Fuse derating due to ambient Temperature:

Typically current derating of 0.5% perC above an ambient of 35C is recommended

The rated Continuous Temperature:

Cable circuits are having a typical conductor temperatures rating of 70C for PVC

insulation & 90C for XLPE based insulation.

Fuses, switches and other circuits are thus, generally, based on 70C asallowable conductor temperature.

The rated Short Circuit Temperature of Cables:

Typically 250C is SC temperature ratings for LV cables.

Standard Interruption current withstand rating of Fuses:


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80kA for 415Vac systems

40kA for dc applications

50kA for 240V systems

Fuse to fuse Coordination ratio:

All fuses to BS88: Parts 2 & 6 will give a coordination ratio of 2:1. Meaning an

upstream fuse rated for 200A will coordinate with a downstrean 100A rated fuse.

For most practical situations some Manufacturers recommend a ratio of 1.6:1

Fuse Current & Energy Limitation:

The pre-arcing It values limits shall be as specified in BS88.

Common BS 88 HRC Fuses by various manufacturers


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3. Protection Philosophy

Various philosophies are applicable based on the Protection requirement .

vi. Prevention of Electric Shocks

Electricity regulations mentions of dealing with two types of Electric Shocks:Direct Contact

Indirect Contact

Dealing with Electric shocks due to Direct Contact:

Preventing a current from passing through the human body or any

livestock


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Limiting the current which can pass through the human body or any

livestock to lower than the Shock current

Dealing with Electric shocks due to Indirect Contact:

In dealing with protection against indirect contact under single fault

conditions, Electricity regulatione permits the two methods given above

for Direct contact and additionally Automatic disconnection of supply in

a determined time on occurrence of a fault which is likely to cause a

current to flow through the body in contact with exposed conductive

parts (where the value of current is equal to or greater than the shock

current).

4. Load Diversity

A ratio of Maximum demand to the Connected load defines load diversity of thesystem. This ratio is always less than 1.

It is fairly easy to calculate the connected load by adding all loads. A system

designed to meet the connected load meets all the safety requirements but is

sometimes fairly expensive. Maximum demand evaluation required engineering

knowledge and judgment on part of the designer.

Lighting loads:

Max demands of lighting system load can be calculated based on the luminaries

connected in terms of watts.

Below recommendation is based on the IEE Guidance:


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Below recommendation is based on the IEE Guidance

Heating loads:

Max demands of lighting system load can be calculated based on the connected

load in terms of watts (normally at unity power factor).


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Cookers loads:

Water heating loads:

Motor loads:

Conventional circuit loads:


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Socket loads (other than conventional):

5. System earthing

Value of external earth loop impedance & the magnitude of prospective earth fault

current depends on type of earthing used.

In case of domestic & commercial applications, regulation puts the consumer to be

fully responsible for earthing at his location (not the supplier of Electricity).

Five basic earthing arrangements embodied in systems are identified as:

TN-C, TN-S, TN-C-S, TT & IT.


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Figure 4.2: TN-S system


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Figure 4.3: TN-C-S system


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Figure 4.4: TT System


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Figure 4.5: IT system


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EARTHED EQUIPOTENTIAL BONDING AND AUTOMATIC DISCONNECTION OF

SUPPLY


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6. Mm


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NOTATIONS USED IN WIRING BS/IEC STANDARDS


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Low Voltage Systems Protective Fuses in Commercial Industrial Applications to BS IEC Standard