Internship Training Report
132kv Grid Substation Kamalabad, P&I
Muhammad Usman Rafiq
A Report submitted to the
Department of P&I, IESCO
in partial fulfillment of the requirements for the
internship of one month for
BACHELOR OF SCIENCE IN ELECTRICAL ENGINEERING
P&I, IESCO Islamabad, 46000 Pakistan
Thanks to almighty Allah, who makes me, grows me and produces such abilities in me so I
become able to produce such report. I want to dedicate “My Little Work” to my parents and
teachers. Parents and teachers have a great role in my life. Both the personalities exert a lot of
efforts on me which make me eligible for making this report.
C E R T I F I C A T E O F A P P R O
V A L
It is certified that the work of report titled “132K Internship Training Report
132kv Grid Substation Kamalabad, P&I IESCO” is carried out by Muhammad Usman
Rafiq, Roll. No F-4068, under the supervision of IESCO, at Islamabad and
Rawalpindi. It is fully adequate, in scope and in quality, as a report for the internship
of one month.
IESCO has been the ideal training institute. Officials deputed are highly trained; their
sage advices, insightful criticisms, and patient encouragement aided this report in
innumerable ways. I would also like to thank trainers which steadfast support of this
report was greatly needed and deeply appreciated.
We hereby declare that this project, neither as a whole nor as a part thereof has been
copied from any source. It is further declared that we have developed this report entirely
on the basis of our personal efforts under the sincere guidance of IESCO officials.
Muhammad Usman Rafiq
Now a day’s everything is depending up on the power. So give the reliable supply to the
consumers. In distribution systems one of the major parts is "SUBSTATIONS".
An electrical substation is a subsidiary station of an electricity, Generation, Transmission
and distribution systems where the voltage is transformed from high to low or reverse using
the transformers .Electric power may flow through several substations between generating
plant and consumer and may be changed in different voltage levels .the equipment used in
substation are Transformer, Lightening arresters, isolator, bus bar, protective devices,
Battery charger, earth switches, earth rods. So for of supply the regular maintenance and
checking is necessary from that we conclude weather it is suitable or not for the desired
TABLE OF CONTENTS
Introduction ....................................................................... 8
1.1 Substation .................................................................... 8
1.2 132Kv single-line grid substation diagram ............................... 9
1.2 Classification of substation ...................................................... 10
1.3 Function of substation ................................................. 11
Equipment Used in a Sub Station …….................................. 12
2.1 Transformer ...................................................................... 12
Types of transformer
2.1.1 Power Transformer ............................................. 12
220.127.116.11 Parts of transformer
1. Conservative Tank .................................. 14
2. Tape Changer .......................... 15
3. Buchholz Relay .......................... 16
4. Silica Gel .......................... 17
2.1.2 Instrument transformer ........................ 19
18.104.22.168 Current transformer ........................ 19
22.214.171.124 Potential transformer ........................ 20
126.96.36.199 Unit Auxiliary transformer ........................ 21
Circuit Breaker …………......................................................... 24
3.1 S F 6 g a s c i r c u i t b r e a k e r ........................... 24
Bus Coupler ................................................... 27
4.1 Bus coupler ....................................... 27
11kv incoming and outgoing Panels ................................................. 29
5.1 11kv incoming Panels .............................................................. 29
5.2 11kv outgoing Panels .............................................................. 31
Batteries and Batteries Charger ................................................. 32
6.1 Station Batteries System ................................................. 32
Electrical Function Numbers ............................................. 36
7.1 Introduction ................................................. 36
7.2 List of device numbers and acronyms ........................................ 36
7.3 Suffixes and prefixes
The present day electrical power system is a.c i.e. electric power is
generated, transmitted and distributed in the form of Alternating current. The
electric power is produce at the power station, which are located at favorable
places. It is delivered to the consumer through a large network of transmission
and distribution. At many place in the line of power system, it may be desirable
and necessary to change some characteristic ( e.g. Voltage, ac to dc, frequency
p.f. etc.) of electric supply. This is accomplished by suitable apparatus called
sub-station for example, generation voltage (11kv or 6.6kv) at the power station
is stepped up to high voltage (Say 220kv to 132kv) for transmission of electric
power. Similarly near the consumer’s localities, the voltage may have to be
stepped down to utilization level. This job is again accomplished by suitable
apparatus called sub-stations.
An electrical substation is a subsidiary station of an electricity
generation, transmission and distribution system where voltage is transformed
high to low or the reverse using transformers. Electric power may flow through
several substations between generation plant and consumer, and may be
changed in voltage in several steps.
The main equipment used in substation are transformers lighting
arresters, Circuit breakers PLCC, isolators, bus bars, protective relays, Battery
1.2 Classification of substations:
Classification of substations based on
(1) Service requirements
(2) Constructional features
1. According to service requirements: According to service requirements
substations are classified into:
i. Transformer Substations
ii. Switching Substations
iii. Power factor correction substations
iv. Frequency changer substations
v. Converting substations
vi. Industrial substations
2. According to construction features: According to constructional features
substations are classified as;
i. Indoor substations
ii. Outdoor substations
iii. Underground substations
iv. Pole mounted substations
1.3 Functions of a Substation:
1 - Supply of required electrical power.
2 - Maximum possible coverage of the supply network.
3 - Maximum security of supply.
4 - Shortest possible fault-duration.
5 - Optimum efficiency of plants and the network.
6 - Supply of electrical power within targeted frequency limits, (49.5 Hz
7 - Supply of electrical power within specified voltage limits.
8 - Supply of electrical energy to the consumers at the lowest cost.
Equipment Used in a Sub-Station
The equipment required for a transformer Sub-Station depends upon the
type of Sub-Station, Service requirement and the degree of protection desired.
TIF Sub-Station has the following major equipments.
Transformer is a static machine, which transforms the potential of
alternating current at same frequency. It means the transformer transforms the
low voltage into high voltage & high voltage to low voltage at same frequency.
It works on the principle of static induction principle.
When the energy is transformed into a higher voltage, the transformer is called
step up transformer but in case of other is known as step down transformer.
TYPES OF TRANSFORMERS
2.1.1 Power Transformer:
It is used for the transmission purpose at heavy load, high voltage
greater than 33 KV & 100% efficiency. It also having a big in size as compare
to distribution transformer, it used in generating station and Transmission
substation at high insulation level. They can be of two types: Single Phase
Transformers and Multi Phase Transformers.
i. Conservator Tank:
This is a cylindrical tank mounted on supporting structure on the
roof the transformer main tank. The main function of conservator tank of
transformer is to provide adequate space for expansion of oil inside the
Function of Conservator Tank of a Transformer:
When transformer is loaded and when ambient temperature rises,
the volume of oil inside transformer increases. A conservator tank of
transformer provides adequate space to this expanded transformer oil. It also
acts as a reservoir for transformer insulating oil.
Construction of Conservator Tank:
This is a cylindrical shaped oil container closed from both ends.
One large inspection cover is provided on either side of the container to
facilitate maintenance and cleaning inside of the conservator.
Conservator pipe, i.e. pipe comes from main transformer tank, is
projected inside the conservator from bottom portion. Head of the
conservator pipe inside the conservator is provided with a cap. This pipe is
projected as well as provided with a cap because this design prevents oil
sludge and sediment to enter into main tank from conservator. Generally
silica gel breather fixing pipe enters into the conservator from top. If it enters
from bottom, it should be projected well above the level of oil inside the
conservator. This arrangement ensures that oil does not enter the silica gel
breather even at highest operating level.
Working of Conservator Tank:
When volume of transformer insulating oil increases due to load
and ambient temperature, the vacant space above the oil level inside the
conservator is partially occupied by the expanded oil. Consequently,
corresponding quantity of air of that space is pushed away through breather.
On other hand, when load of transformer decreases, the transformer is
switched off and when the ambient temperature decreases, the oil inside the
transformer contracts. This causes outside air to enter in the conservator tank
of transformer through silica gel breather.
ii. Tape Changer:
A tap changer is a connection point selection mechanism along
a power transformer winding that allows a variable number of turns to be
selected in discrete steps. A transformer with a variable turns ratio is
produced, enabling stepped voltage regulation of the output. The tap
selection may be made via an automatic or manual tap changer mechanism.
If only one tap changer is required, manually operated tap
points are usually made on the high voltage (primary) or lower current
winding of the transformer to minimize the current handling requirements
of the contacts. However, a transformer may include a tap changer on
each winding if there are advantages to do so. For example, in power
distribution networks, a large step-down transformer may have an off-
load tap changer on the primary winding and an on-load automatic tap
changer on the secondary winding or windings. The high voltage tap is
set to match long term system profile on the high voltage network
(typically supply voltage averages) and is rarely changed. The low
voltage tap may be requested to change positions multiple times each day,
without interrupting the power delivery, to follow loading conditions on
the low-voltage (secondary winding) network.
To minimize the number of winding taps and thus reduce the
physical size of a tap changing transformer, a 'reversing' tap changer
winding may be used, which is a portion of the main winding able to be
connected in its opposite direction (buck) and thus oppose the voltage.
iii. Buchholz Relay:
Buchholz relay in transformer is an oil container housed the
connecting pipe from main tank to conservator tank. It has mainly two
elements. The upper element consists of a float. The float is attached to a
hinge in such a way that it can move up and down depending upon the oil
level in the Buchholz relay Container. One mercury switch is fixed on the
float. The alignment of mercury switch hence depends upon the position
of the float.
The lower element consists of a baffle plate and mercury switch.
This plate is fitted on a hinge just in front of the inlet (main tank side) of
Buchholz relay in transformer in such a way that when oil enters in the
relay from that inlet in high pressure the alignment of the baffle plate
along with the mercury switch attached to it, will change.
In addition to these main elements a Buchholz relay has gas release
pockets on top. The electrical leads from both mercury switches are taken
out through a molded terminal block.
iv. Silica Gel Breather of Transformer:
Whenever electrical power transformer is loaded, the temperature of
the transformer insulating oil increases, consequently the volume of the oil is
increased. As the volume of the oil is increased, the air above the oil level in
conservator will come out. Again at low oil temperature; the volume of the oil is
decreased, which causes the volume of the oil to be decreased which again
causes air to enter into conservator tank.
The natural air always consists of more or less moisture in it and this
moisture can be mixed up with oil if it is allowed to enter into the transformer.
The air moisture should be resisted during entering of the air into the
transformer, because moisture is very harmful for transformer insulation. A
silica gel breather is the most commonly used way of filtering air from moisture.
Silica gel breather for transformer is connected with conservator tank by
means of breathing pipe.
Construction of Silica Gel Breather:
The silica gel breather of transformer is very simple in the aspect of
design. It is nothing but a pot of silica gel through which, air passes during
breathing of transformer. The silica gel is a very good absorber of moisture.
Freshly regenerated gel is very efficient, it may dry down air to a dew point of
below −40°C. A well maintained silica gel breather will generally operate with a
dew point of −35°C as long as a large enough quantity of gel has been used. The
picture shows a silica gel breather of transformer.
Working Principle of Silica Gel Breather:
Silica gel crystal has tremendous capacity of absorbing moisture. When
air passes through these crystals in the breather; the moisture of the air is
absorbed by them. Therefore, the air reaches to the conservator is quite dry, the
dust particles in the air get trapped by the oil in the oil seal cup. The oil in the
oil sealing cup acts as barrier between silica gel crystal and air when there is no
flow of air through silica gel breather. The color of silica gel crystal is dark blue
but, when it absorbs moisture; it becomes pink.
When there is sufficient difference between the air inside the conservator
and the outside air, the oil level in two components of the oil seal changes until
the lower oil level just reaches the rim of the inverted cup, the air then moves
from high pressure compartment to the low pressure compartment of the oil
seal. Both of these happen when the oil acts as core filter and removes the dust
from the outside air.
2.1.2 Instrument Transformers:
These transformers are used for the measurement purposes at that
points where standard voltmeters and ammeters cannot be used. They are of two
188.8.131.52 CURRENT TRANSFORMER:
A current transformer (CT) is used
for measurement of alternating electric currents.
When current in a circuit is too high to apply
directly to measuring instruments, a current
transformer produces a reduced current accurately
proportional to the current in the circuit, which
can be conveniently connected to measuring and
recording instruments. A current transformer
isolates the measuring instruments from what
may be very high voltage in the monitored circuit.
184.108.40.206 POTENTIAL OR VOLTAGE TRANSFORMER:
Voltage transformers (VT) (also called
potential transformers (PT)) are a parallel connected
type of instrument transformer, used for metering
and protection in high-voltage circuits or phase shift
isolation. They are designed to present negligible
load to the supply being measured and to have an
accurate voltage ratio to enable accurate metering. A
potential transformer may have several secondary
windings on the same core as a primary winding, for
use in different metering or protection circuits.
220.127.116.11 Unit Auxiliary Transformers (UAT)
The Unit Auxiliary Transformer is the Power Transformer that
provides power to the auxiliary equipment of a power generating station during
its normal operation. This transformer is connected directly to the generator out-
put by a tap-off of the isolated phase bus duct and thus becomes cheapest source
of power to the generating station.
It is generally a three-winding transformer i.e. one primary and two
separate secondary windings. Primary winding of UAT is equal to the main
generator voltage rating. The secondary windings can have same or different
voltages i.e. generally 11KV and or 6.9KV as per plant layout.
3.1 Sf6 Circuit Breaker:
In such breakers, sulphur hexafluoride gas (SF6) is used as the arc
The sulphur hexafluoride gas (SF6) is an electronegative gas and
has a strong tendency to absorb free electrons. The contacts of the breaker
are opened in a high pressure flow of sulphur hexafluoride (SF6) gas and
an arc is struck between them. The gas captures the conducting free
electrons in the arc to form relatively immobile negative ions. This loss of
conducting electrons in the arc quickly builds up enough insulation
strength to extinguish the arc.
The sulphur hexafluoride (SF6) circuit breakers have been found to be
very effective for high power and high voltage service.
Construction of SF6 Circuit Breaker:
A sulphur hexafluoride (SF6) circuit breaker consists of fixed and
moving contacts enclosed in a chamber. The chamber is called arc
interruption chamber which contains the sulphur hexafluoride (SF6) gas.
This chamber is connected to sulphur hexafluoride (SF6) gas reservoir. A
valve mechanism is there to permit the gas to the arc interruption
chamber. When the contacts of breaker are opened, the valve mechanism
permits a high pressure sulphur hexafluoride (SF6) gas from the reservoir
to flow towards the arc interruption chamber.
The fixed contact is a hollow cylindrical current carrying contact
fitted with an arc horn. The moving contact is also a hollow cylinder with
rectangular holes in the sides. The holes permit the sulphur hexafluoride
gas (SF6) gas to let out through them after flowing along and across the
arc. The tips of fixed contact, moving contact and arcing horn are coated
with copper-tungsten arc resistant material. Since sulphur hexafluoride
gas (SF6) gas is costly, it is reconditioned and reclaimed using suitable
auxiliary system after each operation of breaker.
Working of SF6 CB:
In the closed position of the breaker, the contacts remain
surrounded by sulphur hexafluoride gas (SF6) gas at a pressure of about
2.8 kg/cm2. When the breaker operates, the moving contact is pulled
apart and an arc is struck between the contacts. The movement of the
moving contact is synchronized with the opening of a valve which
permits sulphur hexafluoride gas (SF6) gas at 14 kg/cm2 pressure from
the reservoir to the arc interruption chamber.
The high pressure flow of sulphur hexafluoride gas (SF6) rapidly absorbs
the free electrons in the arc path to form immobile negative ions which
are ineffective as charge carriers. The result is that the medium between
the contacts quickly builds up high dielectric strength and causes the
extinction of the arc. After the breaker operation (i.e. after arc extinction),
the valve is closed by the action of a set of springs.
Advantages of SF6:
Due, to the superior arc quenching properties of sulphur
hexafluoride gas (SF6) gas, the sulphur hexafluoride gas (SF6) circuit
breakers have many advantages over oil or air circuit breakers. Some of
them are listed below Due to the superior arc quenching property of
sulphur hexafluoride gas (SF6), such circuit breakers have very short
Since the dielectric strength of sulphur hexafluoride (SF6) gas is 2
to 3 times that operation due unlike of air, such breakers can interrupt
much larger currents.
The sulphur hexafluoride gas (SF6) circuit breaker gives noiseless
operation due its closed gas circuit and no exhaust to atmosphere unlike
the air blast circuit breaker.
4.1 Bus Coupler:
Bus coupler is a device which is used to couple one bus to the other
without any interruption in power supply and without creating hazardous arcs.
Bus coupler is a breaker used to couple two bus-bars in order to perform
maintenance on other circuit breakers associated with that bus-bar.
It is achieved with the help of a circuit breaker and isolators.
11kv incoming and outgoing Panels
5.1 11kv incoming panel:
11KV Incoming Panel for 20/26MVA power transformer with draw out type
vacuumed circuit breaker, 2500A, 25KA, BIL 95KV, One minute power
frequency withstand voltage 36KV, C.T. Ratio 1600:800/5/5A for metering and
protection purpose having class 0.5 and 5P20 respectively, C.T. Ratio
1600:800/5A having protection class 5P20, P.T. 11000/110V AC, complete with
over current and earth fault relay CDG61, Back up earth fault relay CDG21, DC
supervision relay along with DC healthy lamp, 3 phase 4 wire energy meter for
energy metering, moving iron type ampere meter, moving iron type power
factor meter, Moving iron type volt meter with selector switch, On Off push
button and indication lamps, DP and TP control MCB for AC & DC auxiliary
voltage controls, Trip coil 110 V DC, Closing coil 110 V DC, Anti pumping
feature, Spring charging motor 220 V AC, insulators and 99.9% pure tine coated
copper bus bar arrangement covered with heat shrinkable tubing, Cable
termination pads, Earthing ball arrangement for earthing kit, Explosion vents
and Protection class IP3X
5.2 11kv outgoing panel:
11KV Outgoing Panel with draw out type vacuumed circuit
breaker, 630A, 25KA, BIL 95KV, One minute power frequency withstand
voltage 36KV, C.T. Ratio 400:200/5/5A, complete with over current and earth
fault relay CDG61, DC supervision relay along with DC healthy lamp, 3 phase 4
wire energy meter for energy metering, moving iron type ampere meter, moving
iron type power factor meter, On Off push button and indication lamps, DP and
TP control MCB for AC & DC auxiliary voltage controls, Trip coil 110 V DC,
Closing coil 110 V DC, Anti pumping feature, Spring charging motor 220 V
AC, insulators and 99.9% pure tine coated copper bus bar arrangement covered
with heat shrinkable insulation tubing, Cable termination pads, Earthing ball
arrangement for earthing kit, explosion vents and Protection class IP3X.
Batteries and Batteries Charger
6.1 Station Batteries System
The purpose of station batteries system is to provide safe and liable
power supply to all primary functions. The system is almost independent of all
other power supply and ensures reliable execution of control functions.
Dc batteries (110V) are installed in grid for protection, emergency
power, alarm and indications.
These batteries have their own battery charger. Under normal
conditions, dc supplies are obtaining through AC to DC rectifier but in case of
failure of AC supplies; DC are used to run the system.
Electrical Function Numbers
In the design of electrical power systems, the ANSI standard
device numbers (ANSI /IEEE Standard C37.2 Standard for Electrical Power
System Device Function Numbers, Acronyms, and Contact Designations)
identifies the features of a protective device such as a relay or circuit breaker.
These types of devices protect electrical systems and components from damage
when an unwanted event occurs, such as an electrical fault. Device numbers are
used to identify the functions of devices shown on a schematic diagram.
Function descriptions are given in the standard.
One physical device may correspond to one function number, for
example "29 Isolating Switch", or a single physical device may have many
function numbers associated with it, such as a numerical protective relay. Suffix
and prefix letters may be added to further specify the purpose and function of a
ANSI/IEEE C37.2-2008 is one of a continuing series of revisions
of the standard, which originated in 1928.
7.2 List of device numbers and acronyms:
2 – Time delay Starting or Closing Relay
3 – Checking or Interlocking Relay
4 – Master Contactor
5 – Stopping
6 – Starting Circuit Breaker
7 – Rate of Change Relay
8 – Control Power Disconnecting Device
9 – Reversing Device
10 – Unit Sequence Switch
11 – Multi-function Device
12 – Overspeed Device
13 – Synchronous-speed Device
14 – Underspeed Device
15 – Speed – or Frequency, Matching Device
16 – Data Communications Device
17 – Shunting or Discharge Switch
18 – Accelerating or Decelerating Device
19 – Starting to Running Transition Contractor
20 – Electrically Operated Valve
21 – Distance Relay
22 – Equalizer Circuit Breaker
23 – Temperature Control Device
24 – Volts Per Hertz Relay
25 – Synchronizing or Synchronize-Check Device
26 – Apparatus Thermal Device
27 – Undervoltage Relay
27s - DC under voltage Relay
28 – Flame detector
29 – Isolating Contactor or Switch
30 – Annunciator Relay
31 – Separate Excitation
32 – Directional Power Relay or Reverse Power Relay
33 – Position Switch
34 – Master Sequence Device
35 – Brush-Operating or Slip-Ring Short-Circuiting Device
36 – Polarity or Polarizing Voltage Devices
37 – Undercurrent or Underpower Relay
38 – Bearing Protective Device
39 – Mechanical Condition Monitor
40 – Field (over/under excitation) Relay
41 – Field Circuit Breaker
42 – Running Circuit Breaker
43 – Manual Transfer or Selector Device
44 – Unit Sequence Starting Relay
45 – DC over voltage Relay
46 – Reverse-phase or Phase-Balance Current Relay
47 – Phase-Sequence or Phase-Balance Voltage Relay
48 – Incomplete Sequence Relay
49 – Machine or Transformer, Thermal Relay-OLR
50 – Instantaneous Overcurrent Relay
50G - Instantaneous Earth Over Current Relay (Neutral CT Method)
50N - Instantaneous Earth Over Current Relay (Residual Method)
50BF - Breaker failure
51 – AC Inverse Time Overcurrent Relay
51LR - AC Invers Time overcurrent (locked Roter) protection Relay
51G - AC Inverse Time Earth Overcurrent Relay (Neutral CT Method)
51N - AC Inverse Time Earth Overcurrent Relay (Residual Method)
52 – AC Circuit Breaker
52a - AC Circuit Breaker Position (Contact Open when Breaker Open)
52b - AC Circuit Breaker Position (Contact Closed when Breaker Open)
53 – Exciter or DC Generator Relay
54 – Turning Gear Engaging Device
55 – Power Factor Relay
56 – Field Application Relay
57 – Short-Circuiting or Grounding Device
58 – Rectification Failure Relay
59 – Overvoltage Relay
60 – Voltage or Current Balance Relay.
61 – Density Switch or Sensor
62 – Time-Delay Stopping or Opening Relay
63 – Pressure Switch
64 – Ground Detector Relay
64R - Restricted earth fault
64S - Stator earth fault
65 – Governor
66 – Notching or Jogging Device
67 – AC Directional Overcurrent Relay
67N- Directional Earth Fault relay
68 – Blocking Relay
69 – Permissive Control Device
70 – Rheostat
71 – Liquid Level Switch
72 – DC Circuit Breaker
73 – Load-Resistor Contactor
74 – Alarm Relay
75 – Position Changing Mechanism
76 – DC Overcurrent Relay
77 – Telemetering Device
78 – Phase-Angle Measuring Relay or "Out-of-Step" Relay
79 – AC Reclosing Relay (Auto Reclosing)
80 – Flow Switch
81 – Frequency Relay
82 – DC Reclosing Relay
83 – Automatic Selective Control or Transfer Relay
84 – Operating Mechanism
85 – Communications,Carrier or Pilot-Wire Relay
86 – Lockout Relay/Master Trip
87 – Differential Protective Relay
88 – Auxiliary Motor or Motor Generator
89 – Line Switch
90 – Regulating Device
91 – Voltage Directional Relay
92 – Voltage and Power Directional Relay
93 – Field Changing Contactor
94 – Tripping or Trip-Free Relay( trip circuit supervision Relay)
95 – For specific applications where other numbers are not suitable
96 – Busbar Trip Lockout relay
97 – For specific applications where other numbers are not suitable
98 – For specific applications where other numbers are not suitable
99 – For specific applications where other numbers are not suitable
150 – Earth Fault Indicator
AFD – Arc Flash Detector
CLK – Clock or Timing Source
DDR – Dynamic Disturbance Recorder
DFR – Digital Fault Recorder
DME – Disturbance Monitor Equipment
HIZ – High Impedance Fault Detector
HMI – Human Machine Interface
HST – Historian
LGC – Scheme Logic
MET – Substation Metering
PDC – Phasor Data Concentrator
PMU – Phasor Measurement Unit
PQM – Power Quality Monitor
RIO – Remote Input/Output Device
RTU – Remote Terminal Unit/Data Concentrator
SER – Sequence of Events Recorder
7.3 Suffixes and prefixes:
A suffix letter or number may be used with the device
number; for example, suffix N is used if the device is connected to a
Neutral wire (example: 59N in a relay is used for protection against
Neutral Displacement); and suffixes X, Y, Z are used for auxiliary
devices. Similarly, the "G" suffix can denote a "ground"; hence a
"51G" is a time overcurrent ground relay. The "G" suffix can also
mean "generator", hence an "87G" is a Generator Differential
Protective Relay while an "87T" is a Transformer Differential
Protective Relay. "F" can denote "field" on a generator or "fuse", as in
the protective fuse for a pickup transformer. Suffix numbers are used
to distinguish multiple "same" devices in the same equipment such as
51-1, 51–2. Device numbers may be combined if the device provides
multiple functions, such as the instantaneous/time-delay AC over
current relay denoted as 50/51. For device 16, the suffix letters further
define the device: the first suffix letter is 'S' for serial or 'E' for
Ethernet. The subsequent letters are: 'C' security processing function
(e.g. VPN, encryption), 'F' firewall or message filter, 'M' network
managed function, 'R' rotor, and ‘S’ switch and 'T' telephone
component. Thus a managed Ethernet switch would be 16ESM.