Fuses are used to interrupt electric circuit in the event of heavy flow of current to short-circuits or earth faults in a system.  In low voltage system, fuses are widely used as they are cheap, reliable and can be easily replaced.

Low voltage fuses can be classified into
  • Cartridge Fuses where the fuse element or the fuselink is located in a cartridge and
  • Wirable Fuses where the fuselink is a wire that is wired over a ceramic fuse carriers
In cartridge fuses, the fuselink consists of wires which have low resistivity.
  The material used for the fuselinks needs to have low melting and vaporising temperatures and low specific heat.  This ensures that the fuselink acts quickly, melts and interrupts the circuit.  The vast majority of fuselinks used today are alloys which use either copper or silver as the main ingredient in their construction. 

The fuselink is enclosed in a filling material which is usually quartz.  The function of the filling material is to absorb the energy released during fuse operation and to extinguish the arc which may occur.

The cartridge fuse is capped on either side of fuselink bodies. The fuselink bodies are cylindrical in shape and are usually made of ceramic.  The caps serve to  contain the filling material.  They must be robust, have good conductivity and be able to absorb the energy which may be released during fuse operation. 

Cartridge fuses are widely used in industrial low voltage systems. The cartridge fuse is fitted in a fuse holder.  The fuse holder consists of a base to which are connected the incoming and outgoing wires of the circuit.  Mounted on the base is the fuse carrier which holds the fuse.  The fuse carrier is designed in a manner that the fuse can never be touched when the circuit is live.

Rewirable fuses consist of a fuse carrier which carry a piece of wire which acts as the fuse link.  This kind of fuse is not fully enclosed.  These fuses are amongst the earliest fuses. There is a risk of fitting a fuse wire of higher rating in the fuse holder.  They are now being replaced with the cartridge fuses.

In three phase circuits, the use of fuses entails the risk of single phasing.  That is, if the fuse of one phase blows, the motor may be exposed to single phasing which may cause heating and damage the motor.  For these reasons, three phase fuse units are designed to isolate all phases when the fuse on one of the phases is blown.

Bolted Short Circuit refers to a worst case scenario of a short circuit where all the conductors in a system are imagined to be "bolted" together, thus causing the highest possible fault current.  This hypothetical scenario is used to calculate the fault level of an electrical power system.All components of a power system such as switchgear and conductor are designed to withstand this high current.    The impedance of the system is assumed to be zero in this kind of condition.

Switches are used in low voltage for switching power on and off in a system.  These switches find application in industries, buildings and utilities, etc.  The switches are designed to be operated with a handle to interrupt the power supply.  They are sometimes enclosed within a plastic or metallic enclosure(aluminium or steel).

Low voltage switches are available at different breaking capacities. 
This is an important factor which should be taken care of at the design stage.  For high fault level conditions, interruptor switches with high current breaking capacity are ideal.  Where the fault level is moderate, safety switches can be used.  Isolating switches or isolators are used are used to isolate the circuit after the circuit has been opened.

Low Voltage switches have a provision to padlock the switches in the open position. The body is usually made of self-extinguishing material as a precaution against fire.  There are non-fusible and fusible models(with a provision for backup fuse) available.

Low Voltage Switchgear refers to switchgear which is rated for a voltage of 600V and below.  Low voltage switchgear includes switches, fuse gear, enclosures, connection devices and cable distribution cabinets.

The function of any switchgear is Electrical protection, electric isolation and local and remote switching. 

The switchgear typically consists of a section in the front consisting of the circuit breakers, meters, protective relays.  Behind the front section, is the bus section consisting of the busbar and behind it is the cable entry section where the cables are routed.  

Low voltage switchgear can be broadly classified into two types, outdoor types and indoor types.  

Outdoor type switchgear differs from indoor type switchgear due to the presence of a weather-proofing enclosure.  Bus bars are usually made of copper or aluminum.  

Modern low voltage switchgear products consist of integrated modules consisting of switchgear, metering and protective features. These integrated modules are driven by microprocessors.

This article is the first in a series of articles focusing on Low Voltage Switchgear

The exciting current of a transformer refers to the current drawn in order to build the voltage in the secondary.  The current is also known as the magnetizing current.  This current drawn lags behind the applied voltage by 90 degrees.  The exciting current is in phase with the magnetic flux.

The exciting current of a transformer can be measured by measuring the no-load current of the primary when the secondary is kept open.  Any abnormality in the exciting current may indicate shorted turns or defects in the core.

K rated Transformers are Transformers that are specially designed to supply Harmonics generating loads.  K rated Transformers are bigger in size that normal transformers of the same kVA rating.  They also have bigger conductors and are therefore more costly.

Harmonics are generated by non-linear loads.  Most of the electric loads today such as Variable frequency drives, chopper circuits, induction heaters, etc would fall under that category. Harmonics in the power supply leads to heat generation in electric equipment such as transformers and consequently reduce the capacity.

Thus a 100kVA transformer cannot be loaded to its full capacity if the load it is supplying is generating harmonics.  Besides, high harmonics cause insulation ageing and reduce the life of the transformer.

The K-factor refers to the level of harmonics in a particular system (See article). The k-factor exists from 1 to 50.  Thus, a transformer with no harmonics operating at the fundamental frequency is considered to be operating at k-factor of 1.

 As the level of harmonics increases, the k factor too increases.  The k-factor is calculated for loads which are connected to the transformer.

Based on the k-factor of the loads the transformer is selected.  Thus a transformer with a k - rating of 4 will bigger in size over a transformer with k-rating of 1.  It will have its windings made of thicker copper gauge.  In k-rated transformers the thickness of the neutral conductor is double to prevent heating due to the flow of the triplen harmonics.  The k-rated transformer also occupies more space.