Equipotential Bonding

A pipeline connected to the earthing grid

Equipotential bonding refers to the maintenance of all metallic objects in a vicinity in the same potential.  It is a widely followed practice in earthing.  Equipotential bonding ensures that all metallic objects are at the ground potential.  This eliminates the risk of shock occurring when someone accidentally comes in contact with objects at different potential.  An area where all the objects are kept at the ground potential is called the earthed equipotential zone. 

For instance, in a building there are many metallic fittings which are not part of the electrical distribution system such as bathroom fittings, pipes, metallic supports, steel supports used during construction etc.  Should these fittings become live due to a leak in an electric circuit, they can pose a danger of electrocution.  Hence, it is important that all metallic objects are kept at the ground potential by connecting them to the earthing grid. 

Pipes which are made of plastic or PVC need not be connected to the equipotential network.  In areas, which are likely to be wet such as bathrooms, the fittings are usually connected by an additional link to the equipotential grid.  This is known as supplementary bonding.  

Electronic devices such as computers and telecom equipment sometimes have a separate earthing pit.  This is unsafe as it permits the rise of potential between the main earthing grid and the separate electronic earthing pit in the event of lightning strike.  This can result in damage to the equipment. 

image courtesy : dehn.de

Running Three Phase motors with single Phase supply

A three phase device can be run with a single phase converter by means of a static capacitor phase converter.  The phase converter converts the single phase voltage into three voltages which can be connected to the three phases of the motor. 

The single phase supply is connected to two of the motor phase terminals.  The other terminal is connected to one of the single phase terminals through a capacitor.  The capacitor introduces a phase shift which causes the third phase to be out of phase by 120 degrees.  The produces the rotating magnetic field required for starting and running the three phase motor.    

In motors, the starting current is usually about six times the rated current.  Hence, a bigger value capacitor is usually used as a starting capacitor.  The starting capacitor is kept in line by means of a switch which is opened as the motor picks up speed. 

Phase Sequence Relay for motors

Phase sequence protection is an important safety for motors.  Reversing the phase sequence causes the motor to reverse its direction of rotation.  This can cause serious damage and injury to personnel if for instance, the motor is coupled to cutting equipment, or conveyor belts.  

The phase sequence indicator works by monitoring the phase sequence continually and preventing the motor from starting if the phase sequence has been reversed in the supply. 

Live Tank circuit breakers and Dead tank circuit breakers

Live Tank circuit breakers are circuit breakers in which the interrupting chamber is at the line potential. The interrupting chamber should therefore be provided with insulated supports. The centre of gravity of these circuit breakers is higher, hence live tank circuit breakers need extra support for seismic capability (ability to withstand earthquakes)

In dead tank circuit breakers, the interrupting chamber is at ground potential.  The conductors enter the interrupting chamber through insulated bushings.  Maintenance activities are easier to conduct as the interrupting chamber is at ground level. Seismic capability is higher as the interrupting chambers are at ground level. 

Live Tank Circuit Breaker

Dead Tank Circuit Breaker

Earthing Switches

Earthing switches are safety devices which are integral parts of circuit breakers.  When a circuit breaker is removed and racked out, the sections of the bus bar adjacent to the circuit breaker are automatically earthed by means of these switches.

This protects the maintenance personnel from accidental voltages.    The closing action of the earthing switch is of snap action type.  Earthing switches are usually dimensioned to withstand short circuit currents.  Earthing switches can also be motorised.

Earthing switches are usually used in conjunction with isolators. When the isolator isolates the circuits, the earthing switches make contact with the busbar and discharge any charges which may have accumulated there. 

Can a DC coil of a contactor be used with AC or vice versa ?

Both AC and DC contactors work on the principle of electromagnetic attraction.  However, there are minor constructional differences between them.  AC contactors have a shading coil which is a metallic ring with high remanence which provides magnetism during the zero crossing of the AC voltage.

DC contactors do not have this shading coil.  Hence, if DC coils are powered with an AC voltage, the contactor can chatter as the magnetism becomes zero during the zero crossing of the AC voltage.   Chattering produces an audible noise and can cause the contacts to change state causing interruption in the circuit.  

AC contactors can be used with DC voltage, in theory.  However, the presence of the shading coil in AC contactors can result in a higher drop-off voltage which can cause delay in contact operation.

Wooden Transmission Poles

Wooden Transmission Poles are used in LV and MV power transmission systems.   Wooden Poles have the advantage of being light and cheap. Wooden poles are also aesthetically more pleasing and blend better with the landscape.

Wooden Transmission Poles are generally supported by means of guy wires and usually have a metal cap on the top.  

The wood used to make these poles needs to be properly treated to prevent damage due to pests and decay.  Wooden Transmission Poles are Creosoted which means that they are pressure-treated with creosote, a chemical that provides protection against fungi, insects and marine borers.  

Wooden Poles are made from specific trees such as Pinus Sylvestris, Douglas Fir, southern Yellow Pine, etc.   The disadvantage of wooden poles is that the life of these poles cannot be predicted accurately and thus, they need to be frequently inspected.  Another downside is that these poles sometimes tend to rot at the bottom, especially, in waterlogged locations.

ETAP Software

ETAP is a software that is used for network analysis in Electrical Engineering.  It consists of a number of modules dealing with industrial distribution, transmission, arc flash analysis, etc.  ETAP stands for Electrical Transient Analysis Program. 

ETAP also provides a real time power management software module which offers integrated power monitoring, Load flow and short circuit analysis, etc.  ETAP is particularly used in Transient Analysis which enables engineers to simulate the response of the system to transients. 

A demo version of the software can be found here.  http://www.etap.ca/software-download/ 

Contactors and IEC Classification

Selecting the right contactors and relays for use in motor control and other industrial circuits is extremely important.  It is important that the contactors are chosen keeping in mind the equipment which is to be connected to it and the current it needs to interrupt.

A contactor which is chosen for a heater circuit cannot be used in a motor circuit of the same current rating.

The International Electrotechnical Commission (IEC) has categorized contactors into the following categories.

IEC Categories	Applications
AC1	Non-inductive or slightly inductive rows
AC2	Starting of slip-ring motor
AC3	Starting of squirrel-cage motors and switching off only after the motor is up to speed. This contactor is designed to make Locked Rotor  Current and  Break the Full Load Current.
AC3	Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop. (Make and Break Locked Rotor Current)
AC11	For use in Auxiliary (control) circuits

Duty Cycles of Motors

The relationship between rest and operating periods or repeatable operationg at different loads is known as a duty cycle.  It is important that motors be chosen based on the duty cycle of the equipment they are driving. 

The International Electrotechnical Commission has classified motors into various classes based on duty cycles

S1	Continuous duty	The motor works at a constant load for enough time to reach temperature equilibrium.
S2	Short-time duty	The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature.
S3	Intermittent periodic duty	Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise.
S4	Intermittent periodic duty with starting	Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise.
S5	Intermittent periodic duty with electric braking	Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods.
S6	Continuous operation with intermittent load	Sequential, identical cycles of running with constant load and running with no load. No rest periods.
S7	Continuous operation with electric braking	Sequential identical cycles of starting, running at constant load and electric braking. No rest periods.
S8	Continuous operation with periodic changes in load and speed	Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods.

source : The internet