MEGGER TEST – PREVEZA- LEFKAS – IONIAN ISLANDS
FOR SHIPS, BOATS, YACHTS
ELECTRICAL ENGINEER
Mob. 6946 024
022
PREVEZA - GREECE
For any electrical
installation. For your ship, your boat, your
yacht…
The device is being used since
1889, popularity raised during 1920s, since long back devise is same in its
uses and purpose of testing, few real improvement appeared in recent years with
its design and quality of tester. Now high quality option are available which
are easy to use and quite safe. The Megger test is a method
of testing making use of an insulation tester resistance meter that
will help to verify the condition of electrical insulation.
Insulation resistance quality
of an electrical system degrades with time, environment condition i.e.
temperature, humidity, moisture and dust particles. It also get impacted
negatively due to the presence of electrical and mechanical stress, so it’s
become very necessary to check the IR (Insulation resistance) of equipment at a
constant regular interval to avoid any measure fatal or electrical shock.
The IR gives a measure of the
enduring power of an insulator to bear the service voltage without any current
leakage path. It gives an idea of an insulator’s condition. It is measured
using an instrument named Megger capable of impressing D.C. voltage
between its two probes, automatically calculating and then displaying the IR
value.
Megger being so popular that
“Insulation Resistance” and “Megger” are synonymously used.
Why Megger Testing is
Done?
Insulation resistance quality
of an electrical system degrades with time, environment condition i.e.
temperature, humidity, moisture and dust particles. It also get impacted
negatively due to the presence of electrical and mechanical stress, so it’s
become very necessary to check the IR (Insulation resistance) of equipment at a
constant regular interval to avoid any measure fatal or electrical shock.
Another scenario is if your
home has just experienced a fire and the fire department has left the scene.
The Electric Company has turned off your gas and electric and you are in the
dark. By the grace of God all that is damaged is your home and you need to
start the rebuilding process. Your Insurance Company tells you that local
jurisdiction, or the Insurance Company themselves, require a “Megger Test” to
verify the integrity of the wiring system in your home.
When a fire or other high heat
event (lightning, explosion, etc.) occurs, wiring and its corresponding
elements (insulation, etc.) are subjected to high levels of heat. All
metals and physical compounds have a melting point. In the course of some fire
events this melting point is reached and the current carrying integrity of the
wiring is compromised. The insulation may have melted internally or both the
wire and insulation may have been melted. When this occurs you have a pocket of
resistance that forms as the electrical current tries to flow across that
melted area. As current flow increases, to try to traverse the pocket, it
creates heat. That heat may create enough temperature to actually cause another
fire. Just what you don’t need! The scary part about these compromised wires
is you may have no idea that this has occurred since the wire may be compromised
behind the walls or in your attic
Megger testing does not cause
any damage, making it a good option when someone does not want to put holes in
walls to test electrical insulation for any problems or issues. The
testing device only goes between 500 and 1,000 volts, which is relatively low.
Due to the low voltage, some punctures in insulation go undetected. It
generally provides information about the leakage current and whether insulation
areas have excessive dirt or moisture as well as the amount of moisture,
deterioration and winding faults.
What is Done During Megger
Testing?
We can test your circuits
for the existing connections and melted fault areas that may have occurred
during a fire event. These results are then analysed and specific circuits
can be isolated and replaced to insure that there are no further problems on
your affected circuits. If you have had a fire talk to your Adjuster and see if
megger testing is required. It is normally covered by Insurance since the last
thing they want to do is pay another claim a month after you are able to
reclaim your domicile.
Carelabs has the
equipment on hand and the expertise to perform your Megger Testing and get
those results logged and filed with your insurance carrier as well as the Local
Building Department. We are here to help you make sure your existing wiring
is safe, and of course, to install new wiring as needed as well. We are here
for all of your electrical needs.
How Megger Testing is
Performed?
A multimeter is used as an
insulation tester in some conditions and mostly a continuity test only is performed.
But for detecting and testing for a leakage current during a normal or
overloaded condition, a special instrument known as the Insulation tester is
used.
We measure electrical leakage
in wire and results are very reliable as we shall be passing electric
current through device while we are testing. We verify the electrical
insulation level of any device such as motor, cable, generator winding, or
general electrical installation. This is a very poplar test being carried out
since very long back. Not necessary it shows us exact area of electrical
puncture but shows the amount of leakage current and level of moisture within
electrical equipment/winding/system.
The procedure of insulation
resistance test or megger test are given below:
·
First we will disconnect all the line and neutral
terminals of the transformer.
·
Megger leads are connected to LV and HV bushing studs
to measure Insulation Resistance IR value in between the LV and HV
windings.
·
Megger leads are connected to HV bushing studs and
transformer tank earth point to measure Insulation Resistance IR value in
between the HV windings and earth.
·
Megger leads are connected to LV bushing studs and
transformer tank earth point to measure Insulation Resistance IR value in
between the LV windings and earth.
The empirical relation as
below gives the recommended minimum value for IR, its unit being mega ohm
(MΩ). . The value measures gives us the idea about the insulation strength
of the cable and whether it is deteriorated or not.
IRmin (in MΩ) = kV + 1
Where kV = rated service
voltage in kV
There are instances when the
measured IR is almost 10 to 100 times more than the IRmin found from above
equation.
General measurement procedure
consists of measuring IR between the three phases and also between individual
phase and earth. IR is also measured for equipment body. The procedure
varying from equipment to equipment. There are different voltage levels
which are applied to cable based on their rating and size. For doing
a Megger of a 33kV HT cable. The voltage level applied is of 5000V and the
value of IR can be anywhere between 1GigaOhm to 200GigaOhm.
When we are using a
multimeter, we are measuring a resistance, voltage, and current. Based on
this, I hope we are familiar with the term Insulation. This means that a
current cannot pass or leak through a certain conducting wire when it is
properly insulated or protected. These wires may be inside a building,
appliances or an electric motor.
You are basically testing the
resistance on a wire. For instance if you want to see if a motor is bad, you
will “megger” it, testing each of the three phases of motor to ground and to
each other to see if it is shorted to either ground or itself.
Working Principle of Megger
·
Voltage for testing produced by hand operated megger
by rotation of crank in case of hand operated type, a battery is used for
electronic tester.
·
500 Volt DC is sufficient for performing test on
equipment range up to 440 Volts.
·
1000 V to 5000 V is used for testing for high voltage
electrical systems.
·
Deflecting coil or current coil connected in series
and allows flowing the electric current taken by the circuit being
tested.
·
The control coil also known as pressure coil is
connected across the circuit.
·
Current limiting resistor (CCR and PCR) connected in
series with control and deflecting coil to protect damage in case of very low
resistance in external circuit.
·
In hand operated megger electromagnetic induction
effect is used to produce the test voltage i.e. armature arranges to move in
permanent magnetic field or vice versa.
·
Where as in electronic type megger battery are used to
produce the testing voltage.
·
As the voltage increases in external circuit the
deflection of pointer increases and deflection of pointer decreases with an
increases of current.
·
Hence, resultant torque is directly proportional to
voltage and inversely proportional to current.
·
When electrical circuit being tested is open, torque
due to voltage coil will be maximum and pointer shows ‘infinity’ means no
shorting throughout the circuit and has maximum resistance within the circuit
under test.
·
If there is short circuit pointer shows ‘zero’, which
means ‘NO’ resistance within circuit being tested.
VASILIOS GKIKAS
ELECTRICAL ENGINEER
Mob. 6946 024 022
PREVEZA - GREECE
