These are provided over the windings
of the motor .The main function of heaters, is to heat the windings when motor
is in idle condition or kept in storage , in order to prevent moisture or dew settling
over the windings and thereby reducing the insulation resistance CAUTION-Supply
to the heaters must be switched off before switching on the motor. The motors
are provided with space heaters of ratings 25w, 40w, 50w, or 60w 240 V single
phase depending on the frame size. Like thermistors the space heater leads are
also brought in the main terminal box or in a separate
auxiliary terminal box.
THERMISTORS FOR MOTOR PROTECTION
These are semi conductor devices,
which have a property of suddenly
changing their resistance at a definite temperature known as 'curie point'.
Thermistors that may be provided on the motors are those having 'Positive
Temperature Coefficient' (PTC); where the resistance suddenly increases at a
'Curie Point'. The thermistors generally provided are at 130°c (PTC 130) for
Class B temperature rise, 150°C (PTC 150) For class F motors. A combination of
different ratings of thermistors can be provided in same motor for 'Alarm &
Trip' facilities.
MODE OF OPERATION OF
THERMISTORS
Three thermistors connected in series
are placed inside each of the phase windings of the motor. This gives
protections against single phasing and /or over heating due to excess load on
the motor. During normal operation the thermistors carry a current of few mA,
which is sufficient to actuate a relay in control unit. This in turn allows the
contactor operating coil to hold the starter in the 'Run position. If the
winding of the motor heats up to such an extent so as to bring the temperature
of the thermistors up to 'Curie point'. The increase in the resistance causes
the relay to open and the contactor to disconnect the motor supply.
Circumstances that make human electric shock accidents possible
Ø Relatively high fault
current to ground in relation to the area of the grounding system and its resistance
to remote earth.
Ø Soil resistivity and
distribution of ground currents such that high potential gradients may occur at
points at the earth surface.
Ø Presence of a person at
such a point, time, and position that the body is bridging two points of high
potential difference.
Ø Absence of sufficient
contact resistance or other series resistance to limit current through the body
to a safe value under the above circumstances.
Ø Duration of the fault and
body contact and, hence, of the flow of current through a human body for a sufficient time to cause harm at the
given current intensity.
Relative infrequency of
accidents is largely due to the low probability of coincidence of the above un favorable
conditions. To provide a safe condition for personnel within and around the
substation area, the grounding system design limits the potential difference a
person can come in contact with to safe levels.
Substation Grounding System
1. It provides a means of dissipating electric
current into the earth without exceeding the operating limits of the equipment.
2. It provides a safe environment to protect
personnel in the vicinity of grounded facilities from the dangers of electric
shock under fault conditions.
The grounding system includes all of the
interconnected grounding facilities in the substation area, including the
ground grid, overhead ground wires, neutral conductors, underground cables,
foundations, deep well, etc. The ground grid consists of horizontal
interconnected bare conductors (mat) and ground rods. The design of the ground
grid to control voltage levels to safe values should consider the total
grounding system to provide a safe system at an economical cost.
The following information is mainly concerned
with personnel safety. The information regarding the grounding system
resistance, grid current, and ground potential rise can also be used to
determine if the operating limits of the equipment will be exceeded.
Safe grounding requires the interaction of two
grounding systems:
1. Intentional ground, consisting of grounding
systems buried at some depth below the earth’s
surface
2. Accidental ground, temporarily established by
a person exposed to a potential gradient in the vicinity of a grounded facility
It is often assumed that any grounded object can
be safely touched. A low substation ground resistance is not, in itself, a
guarantee of safety. There is no simple relation between the resistance of the
grounding system as a whole and the maximum shock current to which a person
might be exposed. A substation with relatively low ground resistance might be
dangerous, while another substation with very high ground resistance might be
safe or could be made safe by careful design.
There are many parameters that have an effect on
the voltages in and around the substation area. Since voltages are
site-dependent, it is impossible to design one grounding system that is
acceptable for all locations. The grid current, fault duration, soil
resistivity, surface material, and the size and shape of the grid all have a
substantial effect on the voltages in and around the substation area. If the
geometry, location of ground electrodes, local soil characteristics, and other
factors contribute to an excessive potential gradient at the earth surface, the
grounding system may be inadequate from a safety aspect despite its capacity to
carry the fault current in magnitudes and durations permitted by protective relays.
During typical ground fault conditions, unless proper precautions are taken in
design, the maximum potential gradients along the earth surface may be of
sufficient magnitude to endanger a person in the area. Moreover, hazardous
voltages may develop between grounded structures or equipment frames and the
nearby earth.
GAS-INSULATED SUBSTATION
A
gas-insulated substation (GIS) uses a superior dielectric gas, sulfur
hexafluoride (SF6),
at a moderate pressure for phase to phase and phase to ground insulation. The
high-voltage conductors, circuit breaker interrupters, switches, current
transformers, and voltage transformers are encapsulated in SF6 gas
inside grounded metal enclosures. The atmospheric air insulation used in a
conventional, air-insulated substation (AIS) requires meters of air insulation
to do what SF6 can
do in centimeters. GIS can therefore be smaller than AIS by up to a factor of
ten. A GIS is mostly used where space is expensive or not available. In a GIS,
the active parts are protected from deterioration from exposure to atmospheric
air, moisture, contamination, etc. As a result, GIS is more reliable, requires
less maintenance, and will have a longer service life (more than 50 years) than
AIS. GIS was first developed in various
countries between 1968 and 1972. After about 5 years of experience, the user
rate increased to about 20% of new substations in countries where space was
limited. In other countries with space easily available, the higher cost of GIS
relative to AIS has limited its use to special cases.
STATIC ELECTRICITY
Static electricity is a voltage charge which builds up to many thousands of volts between two surfaces when they rub together. A dangerous situation occurs when the static charge has built up to a potential capable of striking an arc through the air gap separating the two surfaces.
Static charges build up in a thunderstorm. A lightning strike is the discharge of the thunder cloud, which might have built up to a voltage of 100 MV, to the general mass of earth which is at zero volts. Lightning discharge currents are of the order of 20 kA, hence the need for lightning conductors on vulnerable buildings in order to discharge the energy safely.
Static charge builds up between any two insulating surfaces or between an insulating surface and a conducting surface, but it is not apparent between two conducting surfaces.
A motor car moving through the air builds up a static charge which sometimes gives the occupants a minor shock as they step out and touch the door handle.
A nylon overall and nylon bed sheets build up static charge which is the cause of the ‘ crackle ’ when you shake them. Many flammable liquids have the same properties as insulators, and therefore liquids, gases, powders and paints moving through pipes build up a static charge.
Petrol pumps, operating theatre oxygen masks and car spray booths are particularly at risk because a spark in these situations may ignite the flammable liquid, powder or gas.
How to reduce Static Electricity?
Bonding surfaces together with equipotential bonding conductors prevents a build-up of static electricity between the surfaces. Use of large-diameter pipes, reduce the flow rates of liquids and powders and, therefore, reduce the build-up of static charge. Hospitals use cotton sheets and uniforms, and use bonding extensively in operating theatres. Rubber, which contains a proportion of graphite, is used to manufacture antistatic trolley wheels and surgeons ’ boots. Rubber constructed in this manner enables any build-up of static charge to ‘ leak ’ away. Increasing humidity also reduces static charge because the water droplets carry away the static charge, thus removing the hazard.
Hazardous area classification & Type of Electrical equipment usage
As per standard, the risk associated with inflammable gases and vapours are classified into 3 classes or zones.
a) Zone 0 is the most hazardous, and is defined as a zone or area in which an explosive gas–air mixture is continuously present or present for long periods. (‘Long periods’ is usually taken to mean that the gas–air mixture will be present for longer than 1000 h per year.)
Ordinary electrical equipment cannot be installed in Zone 0, even when it is flameproof protected. However, many chemical and oil-processing plants are entirely dependent upon instrumentation and data transmission for their safe operation. Therefore, very low-power instrumentation and data transmission circuits can be used in special circumstances, but the equipment must be intrinsically safe, and used in conjunction with a ‘safety barrier’ installed outside the hazardous area. Intrinsically safe equipment must be marked Ex ‘ ia ’ or Ex ’ s ’ , specially certified for use in zone 0.
b) Zone 1 is an area in which an explosive gas–air mixture is likely to occur in normal operation. (This is usually taken to mean that the gas–air mixture will be present for up to 1000 h per year.)
In Zone 1 all electrical equipment must be flameproof and marked Ex ‘ d ’ to indicate a flameproof enclosure.
C) Zone 2 is an area in which an explosive gas–air mixture is not likely to occur in normal operation and if it does occur it will exist for a very short time. (This is usually taken to mean that the gas–air mixture will be present for less than 10 h per year.)
The electrical equipment used in zone 2 will contain a minimum amount of protection. For example, normal sockets and switches cannot be installed in a zone 2 area, but oil-filled radiators may be installed if they are directly connected and controlled from outside the area. Electrical equipment in this area should be marked Ex ‘o’ for oil-immersed or Ex ‘p’ for powder-filled.
If an area is not classified as zone 0, 1 or 2, then it is deemed to be nonhazardous, so that normal industrial electrical equipment may be used.
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