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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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