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NAPTIN GRADUATE SKILLS DEVELOPMENT PROGRAMME,ORIENTATION
PROGRAMME
Venue: Regional Training Center, Kainji
Date: Friday, November 29, 2013
Topic: Commercial / Energy Metering Part 1.
Stephen O. Sowemimo (Trainer).
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ENERGY METER
What is an Energy Meter?An electricity meter or energy meter is
a device that measures the amount of electric energy consumed by a
residence, business, industry or an electrically powered
device.
Unit of measurementElectricity meters are typically calibrated
in billing units, the most common one is the kilowatt hour [kWh],
which is equal to the amount of energy used by a load of one
kilowatt over a period of one hour. Periodic readings of electric
meters establishes billing cycles and energy used during a
cycle.
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ENERGY METER ContdDemand is normally measured in kilowatts (KW),
but averaged over a period, most often a quarter or half
hour.Reactive power is measured in "thousands of volt-ampere
reactive-hours", (kvarh). By convention, a "lagging" or inductive
load, such as a motor, will have positive reactive power. A
"leading", or capacitive load, will have negative reactive
power]Kilovolt-amperes (KVA) measures all power passed through a
distribution network, including reactive and actual. This is equal
to the product of root-mean-square volts and amperes.These three
types of power -- true, reactive, and apparent -- relate to one
another in trigonometric form. We call this the power triangle:
(Figure below).
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POWER TRIANGLE
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POWERS KW, KVAR, KVA RELATIONSHIP
Power triangle relating apparent power to true power and
reactive power. Using the laws of trigonometry, we can solve for
the length of any side (amount of any type of power), given the
lengths of the other two sides, or the length of one side and an
angle.
REVIEW: Power dissipated by a load is referred to as true power.
It is symbolized by the letter P and is measured in the unit of
Watts (W).*
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POWERS KW, KVAR, KVA RELATIONSHIP Contd.Power merely absorbed
and returned in load due to its reactive properties is referred to
as reactive power. It is symbolized by the letter Q and is measured
in the unit of Volt-Amps-Reactive (VAR).Total power in an AC
circuit, both dissipated and absorbed/returned is referred to as
apparent power. Apparent power is symbolized by the letter S and is
measured in the unit of Volt-Amps (VA).These three types of power
are trigonometrically related to one another. In a right triangle,
P = adjacent length, Q = opposite length, and S = hypotenuse
length. The opposite angle is equal to the circuit's impedance (Z)
phase angle.
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POWER FACTOR
Distortion of the electric current by loads is measured in
several ways. Power factor is the ratio of resistive (or real
power) to volt-amperes. A capacitive load has a leading power
factor, and an inductive load has a lagging power factor. A purely
resistive load (such as a filament lamp, heater or kettle) exhibits
a power factor of 1. *
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HOW ELECTRICITY METERS OPERATEElectricity meters operate by
continuously measuring the instantaneous voltage (volts) and
current (amperes) and finding the product of these to give
instantaneous electrical power (watts) which is then integrated
against time to give energy used (joules, kilowatt-hours etc.).
Meters for smaller services (such as small residential customers)
can be connected directly in-line between source and customer. For
larger loads, more than about 200 ampere of load, current
transformers are used, so that the meter can be located other than
in line with the service conductors.
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Types of Energy Meter
These can be broadly categorized into two viz Credit Meter and
Prepayment Meter. Credit Meter consumption record is billed after
customer has utilized the energy. The customer is expected to
settle the bill in arrears (post payment), whereas in case of
Prepayment Meter, customer pays for units of electricity before
utilizing them (pre payment)Credit meters could be
electromechanical or electronics (digital).Prepaid meters are
purely electronics and could either be wall mounted or pole
mounted.
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ELECTROMECHANICAL METERThe most common type of electricity meter
is the electromechanical induction watt-hour meter.The
electromechanical induction meter operates by counting the
revolutions of an aluminum disc which is made to rotate at a speed
proportional to the power. The number of revolutions is thus
proportional to the energy usage. The voltage coil consumes a small
and relatively constant amount of power, typically around 2 watts
which is not registered on the meter. The current coil similarly
consumes a small amount of power in proportion to the square of the
current flowing through it, typically up to a couple of watts at
full load, which is registered on the meter.The metallic disc is
acted upon by two coils. One coil is connected in such a way that
it produces a magnetic flux in proportion to the voltage and the
other produces a magnetic flux in proportion to the current. The
field of the voltage coil is delayed by 90 degrees using a lag
coil. *
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ELECTROMECHANICAL METER Contd.This produces eddy currents in the
disc and the effect is such that a force is exerted on the disc in
proportion to the product of the instantaneous current and voltage.
A permanent magnet exerts an opposing force proportional to the
speed of rotation of the disc. The equilibrium between these two
opposing forces results in the disc rotating at a speed
proportional to the power being used. The disc drives a register
mechanism which integrates the speed of the disc over time by
counting revolutions, much like the speedometer in a car, in order
to render a measurement of the total energy used over a period of
time.
The type of meter described above is used on a single-phase AC
supply. Different phase configurations use additional voltage and
current coils.
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PICTURE OF THREE PHASE ELECTROMECHANICAL METER *
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ELECTROMECHANICAL METER Contd.Three-phase electromechanical
induction meter, metering 100 A 230/400 V supply is a shown above.
Horizontal aluminum rotor disc is visible in center of meter. The
aluminum disc is supported by a spindle which has a worm gear which
drives the register. The register is a series of dials which record
the amount of energy used. The dials may be of the cyclometer type,
an odometer-like display that is easy to read where for each dial a
single digit is shown through a window in the face of the meter, or
of the pointer type where a pointer indicates each digit. With the
dial pointer type, adjacent pointers generally rotate in opposite
directions due to the gearing mechanism.The amount of energy
represented by one revolution of the disc is denoted by the symbol
Kh which is given in units of watt-hours per revolution. The value
7.2 is commonly seen. Using the value of Kh , one can determine
their power consumption at any given time by timing the disc with a
stopwatch. *
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ELECTROMECHANICAL METER Contd.If the time in seconds taken by
the disc to complete one revolution is t , then the power in watts
is P = 3600 x Kh t For example, if Kh = 7.2, as above, and one
revolution took place in 14.4 seconds, the power is 1800 watts.
This method can be used to determine the power consumption of
household devices by switching them on one by one.Most domestic
electricity meters are read manually, whether by a representative
of the power company or by the customer. Where the customer reads
the meter, the reading may be supplied to the power company by
telephone, post or over the internet. The electricity company will
normally require a visit by a company representative at least
quarterly in order to verify customer-supplied readings and to make
a basic safety check of the meter.
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Electronic meters (credit meters)
Electronic meters display the energy used on an LCD or LED
display, and can also transmit readings to remote places. In
addition to measuring energy used, electronic meters can also
record other parameters of the load and supply such as maximum
demand, power factor and reactive power used etc. They can also
support time-of-day billing, for example, recording the amount of
energy used during on-peak and off-peak hours.
Remote meter reading is a practical example of telemetry. It
saves the cost of a human meter reader and the resulting mistakes,
but it also allows more measurements, and remote provisioning. Many
smart meters now include a switch to interrupt or restore
service.
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PICTURE OF A TYPICAL ELECTRONIC CREDIT METER*
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Electronic meters (credit meters) Contd.
Electronic meters now use low-power radio, GSM, GPRS, Bluetooth,
IrDA, as well as RS-485 wired link. The meters can now store the
entire usage profiles with time stamps and relay them at a click of
a button. The demand readings stored with the profiles accurately
indicate the load requirements of the customer. This load profile
data is processed at the utilities for billing and planning
purposes.AMR (Automatic Meter Reading) and RMR (Remote Meter
Reading) describe various systems that allow meters to be checked
without the need to send a meter reader out. An electronic meter
can transmit its readings by telephone line or radio to a central
billing office. Automatic meter reading can be done with GSM
(Global System for Mobile Communications) modems, one is attached
to each meter and the other is placed at the central utility
office.
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Basic block diagram of an electronic energy meter
Solid-state design
Basic block diagram of an electronic energy meter
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Electronic meters (credit meters) Contd.
As in the block diagram, the meter has a power supply, a
metering engine, a processing and communication engine (i.e. a
microcontroller), and other add-on modules such as RTC, LCD
display, communication ports/modules and so on.The metering engine
is given the voltage and current inputs and has a voltage
reference, samplers and quantisers followed by an ADC section to
yield the digitised equivalents of all the inputs. These inputs are
then processed using a Digital Signal Processor to calculate the
various metering parameters such as powers, energies etc.RTC and
other add-on modules are attached as slaves to the processing and
communication section for various input/output functions. On a
modern meter most if not all of this will be implemented inside the
microprocessor, such as the Real Time Clock (RTC), LCD controller,
temperature sensor, memory and analog to digital converters.
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Prepayment meters
The standard business model of electricity retailing involves
the electricity company billing the customer for the amount of
energy used in the previous month or quarter. If the retailer
believes that the customer may not pay the bill, a prepayment meter
may be installed. Prepayment meters are commonly used for customers
considered to be a poor credit risk. This requires the customer to
make advance payment before electricity can be used. If the
available credit is exhausted then the supply of electricity is cut
off by a relay.Here in Nigeria, many of our non-MD customers fall
into the category of poor credit risk hence, the recent effort
towards getting all non-MD customers metered with prepayment
meters.
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Picture of a Typical Prepayment Meter
A wall mounted, smart card, split prepayment meter
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Pole mounted Prepayment meters
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However, the problem of some customers tampering with wall
mounted prepaid meters, especially by bypass, is posing a great
challenge. This has prompted the initiative of producing pole
mounted prepaid meters. These are mounted on electric poles out of
customers reach. Customers load cables and communication cables are
extended from the meters on the poles down to supply intake points.
The communication cable is connected to Customer Interface Unit
(CIU) for energy loading and monitoring of consumption.
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Picture of a Single Phase Pole Mounted PPM
A typical pole mounted split single phase prepayment
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Picture of a Three Phase PPM
A typical wall mounted or pole mounted three phase PPM
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Prepaid Meters Energy Purchase / VendingOne system of energy
purchase is the Pay Point network, where rechargeable tokens can be
loaded with whatever money the customer has available. Recently
smartcards are introduced as much reliable tokens that allows two
way data exchange between meter and the utility as we have in
Unistar Prepaid Meters.Another system of recharging prepaid meters
is by entering a unique, encoded twenty digit number using a
keypad. This makes the tokens, essentially a slip of paper, very
cheap to produce, like in Conlog Prepaid Meters.
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Maximum Demand (MD) Meters
When customers load demand is more than 45 KVA that could
conveniently be metered with non-MD three phase meter, an MD meter
is installed, capacity of which depends on the load magnitude. The
meters are in broad categories of:Whole Current, Low Voltage, 415
volts C.T. operated, Low Voltage, 415 volts. C.T and V.T. operated,
High Voltage, 11 Kv.C.T. and V.T. operated, High Voltage, 33
Kv.They can be electromechanical or digital. They have facilities
for measuring Active Energy, KWH and Apparent Energy, KVA used in
billing. Some electromechanical also have portion recording
Reactive Energy, KVAR while the electronic type are designed to
measure many more parameters.
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Metering Current Transformer
Current transformers used as part of metering equipment for C.T.
operated, Low Voltage Maximum Demand Meter. The fourth neutral
cable does not require a current transformer because current cannot
flow in this cable without also flowing in one of the three phase
cables.
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Location of Maximum Demand Meter
The location of an electricity meter varies with each
installation. Possible locations include on a utility pole serving
the property, in a street-side cabinet (meter box) or inside the
premises adjacent to the consumer unit / distribution board.
Electricity companies may prefer external locations as the meter
can be read without gaining access to the premises but external
meters may be more prone to vandalism.
Current transformers permit the meter to be located remotely
from the current-carrying conductors. This is common in large
installations. For example a substation serving a single large
customer may have metering equipment installed in a cabinet,
without bringing heavy cables into the cabinet.
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Metering Current Transformer and Voltage Transformer
Except for 100 Amps Whole Current Maximum Demand meter which is
in use in some specialized areas, all Maximum Demand meters are CT
operated, for Low Voltage MD and CT / VT operated for High Voltage
MD. Because the load current is too high to be fed directly through
the meter, current transformer (CT) is used to step the load
current down to a maximum of 5A. However, when the stepped down
value is fed into the meter, the meter reads it as the real load
current value if the meter current ratio is the same as the CT
ratio. High voltage MD meters also make use of Voltage Transformer
(VT) in addition to CT. The supply/metering voltage is stepped down
to 110v. and fed across the meter
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Multiplying Factor (MF)Multiplying Factor (MF) is the figure by
which consumption recorded on a MD meter is multiplied to reflect
the actual consumption. This is necessary in a situation whereby CT
ratio is not the same as the meter current ratio.MF = CT ratio
Meter current ratio For example, if CT ratio = 500/5A and Meter
current ratio = 300/5A MF = 500 divided by 300 = 500 x 5 = 5 = 1.67
5 5 5 300 3 In High Voltage MD metering, as much as possible the
meter is selected in such a way that its voltage ratio is the same
as the metering VT ratio. However, in an event that the two are
different, necessary adjustment is made with multiplying factor.
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Multiplying Factor (MF)
For example, if CT ratio = 100/5A, VT ratio = 11,000/110v. and
Meter ratio = 50/5A, 33,000/110v. MF = 100 divided by 50 x 11,000
divided by 33,000 5 5 110 110 = 100 x 5 x 11,000 x 110 = 2 = 0.67 5
50 110 33,000 3KWH is the total units of electricity consumed in a
specified period of time, usually one month for billing purpose
while KVA is the maximum level of power demand within the same
period of time.
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Capacity Range of MetersAt distribution level we have credit
meters of following capacity range: Single phase non- MD 230 volts
Three phase non- MD 415 volts Low Voltage MD 415 volts High Voltage
MD 11Kv. High Voltage MD 33Kv.
For now we only have prepayment meters for non-MD customers,
that is, capacity ranges (i) and (ii) above
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Selection of Appropriate Meter Capacity
The major determinant of what capacity of meter is required for
a particular prospective customer is the installed load. Generally,
a customer with installed load below 45 KVA is fed from public
transformer and metered with single phase or three phase non-MD
Meter depending on actual installed load. A prospective customer
with installed load above 45 KVA is required to be on a dedicated
transformer and metered with Maximum Demand (MD) meter, capacity of
which depends on actual installed load and installed transformer
capacity.
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Selection of Appropriate Meter Capacity Contd.Before we can
decide on adequate supply voltage and meter capacity we must be
sure of the prospective customers installed load. We are supposed
to be more knowledgeable than the customer in this regard. Hence,
it is not for the customer to dictate the type of meter he needs
rather it is our duty to advise the customer on supply voltage and
meter capacity required based on the installed load. It is a common
knowledge that someone will just walk into a Power Company office
and say he wants three phase meter not because single phase cannot
cope with his load but because he wants flexibility of changing
from one phase to another. On the other hand, someone whose
installed load requires three phase supply / meter will come to say
he needs single phase meter simply because single phase connection
fee and billing tariff are cheaper than those of three phase.
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Selection of Appropriate Meter Capacity ContdIdeally, when a
prospective customer applies for electricity supply by filling a
Supply Agreement form, we should emphasize on the form being
correctly filled showing details of customers installed load among
other things. Then we have to carry out site technical inspection
or load survey to confirm the information given on the Supply
Agreement form and ascertain whether or not the available network
capacity can still accommodate the intended additional load. At
this stage capacity of meter required can also be confirmed.Based
on positive outcome of load survey, the customer is issued Permit
to Wire. After notification of wiring completion, Supply Authority
carries out installation inspection and tests to ensure that the
installation conforms to required standard. If okay, supply
connection and metering are done. Necessary documentation follows
and billing commences.
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Meter Tampering and Security
Some tampers bypass the meter, wholly or in part. Meters can be
manipulated to make them under-register, effectively allowing power
use without paying for it. This theft or fraud can be dangerous as
well as dishonest.Meter readers are trained to spot signs of
tampering, and with crude mechanical meters, the maximum rate may
be charged each billing period until the tamper is removed, or the
service is disconnected.A common method of tampering on older
meters is to attach magnets to the outside of the meter. These
magnetically saturate the coils or current transformers, preventing
the alternating current from forming eddy currents in the rotor, or
inducing voltages in the current transformer.
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Meter Tampering and Security Contd.We also need to be vigilant
in order to detect the following forms of tampering often done by
dubious customers:Connecting part of the load on direct and leaving
few on the meter to give impression that the customers consumption
is being registered.Dropping meter shunt at certain period of the
month when marketer is not likely to be around for meter reading or
bill distribution and putting the shunt back before the next visit
of the marketer.Bye passing the meter by connecting whole or part
of the load onto the service cable before the metering point, in
hidden places like when service cable passes through ceiling.In
three phase metering, interchanging the supply and load cables on
one phase. The overall effect of this is that only the consumption
on one phase will be effectively registered in a balanced load
situation.
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Meter Tampering and Security Contd.Metering manipulation is
deliberate distortion of meter reading in one or more of the ways
stated above. All of these effects can be detected by the electric
company, and many modern meters. This brings us to the critical
issue of our marketers being adequately knowledgeable in the
expected monthly consumption of each of their customers, given the
level of supply availability during the month and also comparing
with other customers of similar installed load or operation. For
example, a customer with one bathroom water heater, two air
conditioners and other electrical appliances, having meter reading
consumption of 100 units in one month, is questionable. Even if the
marketer cannot detect any shady deal, he or she should report in
the office for proper investigation.
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Meter Tampering and Security Contd.Also a customer that may be,
had record of monthly consumption of 1,000 units in the first six
months of being connected to the network but suddenly the
consumption record drops to about 300 units. The marketer is
expected to highlight such case for proper investigation.The supply
authority normally secures the meter against tampering. Revenue
meters' mechanisms and connections are sealed. One of the measures
against meter tampering is to ensure that under no circumstance
should any customer have access to meter terminal block. The
terminal cover must be fixed and sealed. Any customer that breaks
the seal must be penalized. As a measure to minimize meter bypass,
we must ensure that under no circumstance is service cable allowed
to pass through the ceiling or any other hidden places before
getting to the meter. Experience has shown that many of the meter
bypasses are done in such hidden places.
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Meter Tampering and Security Contd.Newer computerized meters
usually have counter-measures against tampering such as inverted
installation, reversed or switched phases etc.Metering at
Distribution Substation Transformer (statistical metering) with
Automated Meter Reading: The meters are normally of very high
accuracy and they often have sensors that can report opening of the
meter cover, magnetic anomalies, alert the operator to tampering.
Each oversees and records the consumption of the area the
transformer serves. Power companies normally investigate
discrepancies between the total billed and the total generated, in
order to find and fix power distribution problems. These
investigations are an effective method to discover tampering.Energy
theft through metering manipulation is the most dangerous avenue of
non-technical losses. Other forms of non-technical losses are meter
accuracy deterioration, illegal direct connection and application
of wrong (lower) multiplying factor in MD metering.
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Load Factor Analysis for Maximum Demand Customers
Load factor analysis is one of the means of detecting meter
tampering and fraud by MD customers. Meeting meter seals intact
during monthly meter reading is no more a guarantee that tampering
did not take place in-between reading dates. Sealing wire and
sealing lead can be purchased in open market. Experience has shown
that some customers either forge sealing pliers or collude with
dubious staff and get Supply Authority sealing pliers to enable
them break seals at will, tamper with meters (MD and non-MD) and
seal back. For MD meter reading use of numbered seals as against
the conventional sealing lead is strongly recommended.Load Factor =
Energy consumed within a specified period of Time Max. Demand x
Power Factor x the period of Time
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Load Factor Analysis for Maximum Demand Customers Contd.L. F. =
KWH KVA x pf x TimeWhere pf is assumed to be 0.8 and Time is total
number of hours with the period under consideration e. g. 30 x 24
hours = 720 hours in a month or 365 x 24 hours = 8,760 hours in a
year.Ideally, 0 < L.F. < 1. Expected load factor of a company
on regular power supply and operating three shifts is at least
about 0.7. Experience has shown that customers tend to be honest
and law abiding within the first year of operation. It is good to
analyze monthly load factor of each MD customer from inception. Any
sharp drop needs to be investigated to confirm whether it is a
genuine drop due to the customers operational problem / reduced
power supply or a case of fraud.
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Load Factor Analysis for Maximum Demand Customers Contd.
For example, a customer with usual average consumption of
200,000 KWH and Maximum Demand of 500 KVA per month will have
average load factor of 200,000 = 0.69 500 x 0.8 x 720
If suddenly the monthly consumption drops to say 80,000 while MD
remains at 500 KVA, it means the load factor has dropped to 80,000
= 0.28. 500 x 0.8 x 720
Such a case needs to be investigated to confirm the genuiness or
otherwise of the L.F. drop.
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Meter Calibration and Certification Meters are purchased by
Supply Authority from many manufacturers. However, before the
meters are used in the network they are subjected to accuracy tests
in any of the Meter Test Stations in Oshodi (Lagos), Portharcourt
and Kaduna. This is to protect the interests of both Supply
Authority and the customers. Ideally, after fifteen years in
circuit meters are believed to have lost some degree of accuracy
and are therefore expected to be retrieved and sent back to Meter
Test Station for recalibration and recertification. Thereafter they
are sent back to Business Units for reuse.
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ConclusionIn any electricity industry, adequate and accurate
metering of customers is a very crucial business that should not be
viewed with levity if the industry must survive. It needs to be
given the serious attention it deserves at all times. Hence, at the
distribution / marketing level of electricity business, key roles
need to be played in the survival of the Supply Authority and its
workforce.
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THE END
Thank you for your attention
Questions?
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