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HIPOT Testing

December 1, 2011 29
Comments

What is HIPOT Testing
(Dielectric Strength Test):

Hipot Test is short name of high
potential (high voltage) Teat and It also known as Dielectric Withstand Test.
A hipot test checks for “good isolation.” Hipot test makes
surety of no current will flow from one point to another point. Hipot
test is the opposite of a continuity test.

Continuity Test checks surety of
current flows easily from one point to another point while Hipot Test checks
surety of current would not flow from one point to another point (and turn up
the voltage really high just to make sure no current will
flow).

Importance of HIPOT
Testing:

The hipot test is a nondestructive test
that determines the adequacy of electrical insulation for the normally
occurring over voltage transient. This is a high-voltage test that is applied
to all devices for a specific time in order to ensure that the insulation is
not marginal.

Hipot tests are helpful in finding
nicked or crushed insulation, stray wire strands or braided shielding,
conductive or corrosive contaminants around the conductors, terminal spacing
problems, and tolerance errors in cables. Inadequate creepage and clearance
distances introduced during the manufacturing process.

HIPOT test is applied after tests such
as fault condition, humidity, and vibration to determine whether any
degradation has taken place.

The production-line hipot test,
however, is a test of the manufacturing process to determine whether the
construction of a production unit is about the same as the construction of the
unit that was subjected to type testing. Some of the process failures that can
be detected by a production-line hipot test include, for example, a
transformer wound in such a way that creepage and clearance have been reduced.
Such a failure could result from a new operator in the winding department.
Other examples include identifying a pinhole defect in insulation or finding
an enlarged solder footprint.

As per IEC 60950, The Basic test
Voltage for  Hipot test is the 2X (Operating Voltage) + 1000
V

The reason for using 1000 V as part of
the basic formula is that the insulation in any product can be subjected to
normal day-to-day transient over voltages. Experiments and research have shown
that these over voltages can be as high as 1000 V.

Test method for HIPOT
Test:

Hipot testers usually connect one side
of the supply to safety ground (Earth ground). The other side of the supply is
connected to the conductor being tested. With the supply connected like this
there are two places a given conductor can be connected: high voltage or
ground.

When you have more than two contacts to
be hipot tested you connect one contact to high voltage and connect all other
contacts to ground. Testing a contact in this fashion makes sure it is
isolated from all other contacts.

If the insulation between the two is
adequate, then the application of a large voltage difference between the two
conductors separated by the insulator would result in the flow of a very small
current. Although this small current is acceptable, no breakdown of either the
air insulation or the solid insulation should take place.

Therefore, the current of interest is
the current that is the result of a partial discharge or breakdown, rather
than the current due to capacitive coupling.

Time Duration for HIPOT
Test:

The test duration must be in accordance
with the safety standard being used.

The test time for most standards,
including products covered under IEC 60950, is 1 minute.

A typical rule of thumb is 110
to 120% of 2U + 1000 V for 1–2 seconds.

Current
Setting for HIPOT Test:

Most modern hipot testers allow the
user to set the current limit. However, if the actual leakage current of the
product is known, then the hipot test current can be predicted.

The best way to identify the trip level
is to test some product samples and establish an average hipot current. Once
this has been achieved, then the leakage current trip level should be set to a
slightly higher value than the average figure.

Another method of establishing the
current trip level would be to use the following mathematical formula:
E(Hipot) / E(Leakage) = I(Hipot) /
2XI(Leakage)

The hipot tester current trip level
should be set high enough to avoid nuisance failure related to leakage current
and, at the same time, low enough not to overlook a true breakdown in
insulation.

Test Voltage for HIPOT
Test:

The majority of safety standards allow
the use of either ac or dc voltage for a hipot test.

When using ac test voltage, the
insulation in question is being stressed most when the voltage is at its peak,
i.e., either at the positive or negative peak of the sine wave.

Therefore, if we use dc test voltage,
we ensure that the dc test voltage is under root 2 (or 1.414) times
the ac test voltage, so the value of the dc voltage is equal to the ac voltage
peaks.

For example, for a 1500-V-ac
voltage, the equivalent dc voltage to produce the same amount of stress on the
insulation would be 1500 x 1.414 or 2121 V dc.

Advantage / Disadvantage of use
DC Voltage for Hipot Test:

One of the advantages of using a dc
test voltage is that the leakage current trip can be set to a much lower value
than that of an ac test voltage. This would allow a manufacturer to filter
those products that have marginal insulation, which would have been passed by
an ac tester.

when using a dc hipot tester, the
capacitors in the circuit could be highly charged and, therefore, a
safe-discharge device or setup is needed. However, it is a good practice to
always ensure that a product is discharged, regardless of the test voltage or
its nature, before it is handled.

It applies the voltage gradually. By
monitoring the current flow as voltages increase, an operator can detect a
potential insulation breakdown before it occurs. A minor disadvantage of the
dc hipot tester is that because dc test voltages are more difficult to
generate, the cost of a dc tester may be slightly higher than that of an ac
tester.

The main advantage of the dc test is DC
Voltage does not produce harmful discharge as readily occur in
AC.
It can be applied at higher
levels without risk or injuring good insulation. This higher potential can
literally “sweep-out” far more local defects.

The simple series circuit path of a
local defect is more easily carbonized or reduced in resistance by the dc
leakage current than by ac, and the lower the fault path resistance becomes,
the more the leakage current increased, thus producing a “snow balling” effect
which leads to the small visible dielectric puncture usually observed. Since
the dc is free of capacitive division, it is more effective in picking out
mechanical damage as well as inclusions or areas in the dielectric which have
lower resistance.

Advantage / Disadvantage of use
AC Voltage for Hipot Test:

One of the advantages of an ac hipot
test is that it can check both voltage polarities, whereas a dc test charges
the insulation in only one polarity. This may become a concern for products
that actually use ac voltage for their normal operation. The test setup and
procedures are identical for both ac and dc hipot tests.

A minor disadvantage of the ac hipot
tester is that if the circuit under test has large values of Y capacitors,
then, depending on the current trip setting of the hipot tester, the ac tester
could indicate a failure. Most safety standards allow the user to disconnect
the Y capacitors prior to testing or, alternatively, to use a dc hipot tester.
The dc hipot tester would not indicate the failure of a unit even with high Y
capacitors because the Y capacitors see the voltage but don’t pass any
current.

Step for HIPOT
Testing:

Only electrically qualified workers may
perform this testing.

Open circuit breakers or switches to
isolate the circuit or Cable that will be hi-pot tested.

Confirm that all equipment or Cable
that is not to be tested is isolated from the circuit under test.

The limited approach boundary for this
hi-pot procedure at 1000 volts is 5 ft. (1.53m) so place
barriers around the terminations of cables and equipment under test to prevent
unqualified persons from crossing this boundary.

Connect the ground lead of the HIPOT
Tester to a suitable building ground or grounding electrode conductor. Attach
the high voltage lead to one of the isolated circuit phase
conductors.

Switch on the HIPOT Tester. Set the
meter to 1000 Volts or pre decide DC Voltage. Push the “Test” button on the
meter and after one minute observe the resistance reading.  Record the
reading for reference.

At the end of the one minute test,
switch the HIPOT Tester from the high potential test mode to the voltage
measuring mode to confirm that the circuit phase conductor and voltage of
HIPOT Tester are now reading zero volts.

Repeat this test procedure for all
circuit phase conductors testing each phase to ground and each phase to each
phase.

When testing is completed disconnect
the HIPOT Tester from the circuits under test and confirm that the circuits
are clear to be re-connected and re-energized.

To PASS the unit or Cable under Test
must be exposed to a minimum Stress of pre decide Voltage for 1 minute without
any Indication of Breakdown. For Equipments with total area less than 0.1 m2,
the insulation resistance shall not be less than 400 MΩ. For Equipment with
total area larger than 0.1 m2 the measured insulation resistance times the
area of the module shall not be less than 40 MΩ⋅m2.

Safety precautions during HIPOT
Test:

During a HIPOT Test, There may be at
some risk so to minimize risk of injury from electrical shock make sure HIPOT
equipment follows these guidelines:

The total charge you can receive in a
shock should not exceed 45 uC.

The total hipot energy should not
exceed 350 mJ.

The total current should not exceed
5 mA peak (3.5 mA rms)

The fault current should not stay on
longer than 10 mS.

If the tester doesn’t meet these
requirements then make sure it has a safety interlock system that guarantees
you cannot contact the cable while it is being hipot tested.

For Cable:

Verify the correct operation
of the safety circuits in the equipment every time you calibrate
it.

Don’t touch the cable during
hipot testing.

Allow the hipot testing to complete
before removing the cable.

Wear insulating gloves.

Don’t allow children to use the
equipment.

If you have any electronic implants
then don’t use the equipment.

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About
Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System
Control), B.E(Electrical) from Gujarat University. He has more than 14 years
experience in Power Transmission-Power Distribution-Electrical energy theft
detection-Electrical Maintenance-Electrical
Projects(Planning-Designing-coordination-Execution). He is Presently associate
with one of the leading business group as a Deputy Manager at Ahmedabad,India.
He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of
Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for
"Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger
and Familiar with English, Hindi, Gujarati, French languages. He wants to Share
his experience & knowledge and help technical enthusiasts to find suitable
solutions and updating themselves on various Engineering Topics.

29 Responses to HIPOT Testing

krishanmani says:

December
1, 2011 at 5:06 pm

Rtt Tower & Gbd tower Tell about this topic pls

Reply

Ashirvad says:

December
4, 2011 at 2:30 pm

HEY MR. JIGNESH!! GOOD WORK……KEEP IT UP

CAN YOU PUT UP AN ARTICLE COVERING ALL TESTS ON SWITCHGEAR EQUIPMENTS,

TYPE OF TESTS,
UNDERSTANDING RESULTS WHETHER THE THE EQUIPMENT IS ALRIGHT
OR WHAT MEASURES TO BE TAKEN!!!!!

HOPING IT TO COME SOON!!!!!

Reply

Muhammad Aslam Butt says:

January
16, 2012 at 6:12 am

Please post all tests of Synchronous turbo Generators (stator & rotor)
and Power Transformers

Reply

naresh says:

March
14, 2012 at 2:47 am

Very nice

Reply

M.Sai Phanindra Kumar says:

May
2, 2012 at 8:00 am

I want to know that is there any course for erection, testing and
commissioning of electrical equipments in India? IF so, Please suggest me
which institute is providing this course. As presently I am working in
erection and commissioning of 1x100MW combined cycle power plant, this course
will be helpful to me.

Reply

Scotty Moncrieff says:

May
16, 2012 at 12:24 pm

very good info

Reply

Scotty Moncrieff says:

May
28, 2012 at 7:59 pm

is it possible to test switchgear with a VLF tester?? if not is there any
reason ??

Reply

gurunathan says:

June
19, 2012 at 4:46 am

i had a little confuse tan delta test & polarization index test .Which
is best to identify cable fault by using HV megger or HIPOT Teter

Reply

anup kumar singh says:

July
15, 2012 at 5:49 pm

hi sir
a very nice work out to support electrical engineers. many will
be delighted by it and will be thank full to you for this valuable
support.

Reply

Ramamurthy.c says:

December
27, 2012 at 9:25 pm

Dear Jignesh,
It is a delight to see your your notes.One can feel like
having somebody behind him to support.
Thanx,

Reply

archani says:

February
7, 2013 at 8:30 pm

Hi Sir
Very good job.
What are the limits of leakage current for
cables 6.6 kV and 11 kV?
Thanks

Reply

Parimal Majee says:

July
3, 2013 at 2:18 pm

Dear Sir,
If the normal operating voltage of a turbo generator rotor is
455 V DC and voltage applied during Hi-Pot test is 1500 V AC, what should be
the limit for leakage current? Pl. suggest.

Reply

sam says:

August
28, 2013 at 11:39 am

Are electrical power engineers qualified to carry out hipot testing? or
just electricians? (I’m in Australia)

Reply

JEROME says:

September
5, 2013 at 12:27 pm

THANKS FOR THE INFO, MAY I ASK HOW MANY TIMES CAN WE TEST THE CABLES IN
CASE IF IT IS FAILED ?

Reply

venkatesh says:

September
22, 2013 at 5:54 am

Dear sir,

I have a doubt about the testing please Guide your experience how to test
34.5kV MV cable, i don’t know which test require AC hipot test or DC hipot
test.

venkatesh

Reply

Aldwin
Castillo says:

October
30, 2013 at 7:22 pm

Dear Mr. Jignesh Parmar, My high pot tester is always displaying this OL or
over limit. Is it important or I may say, is it a must to know the leakage
current readings? I’m afraid if our client ask me what is the leakage current
readings, I can’t answer anything. By the way, I am electrician.
Thanks

Reply

Ken Mark M. Carreon says:

November
27, 2013 at 12:05 pm

Dear Sir,

When can I say that Hi pot testing done is okay? Is there limitation for
the leakage current?

Reply

Hafez says:

February
4, 2014 at 12:53 am

Dear Mr Jignesh

I am Hafez from Malaysia. Currently, I am in charge for a commissioning of
MV panel switch gear. My contractor perform a test with 12kV of 6.6kV to a
cable from switch gear to a motor. I would like to ask the acceptance value of
insulation as the value read only 200MOhm, but as I believe it should reach
more than 1GOhm.

Please advise, and cahn I have the reference of IEC standard on this?

Thanks

Reply

manindra nath says:

March
24, 2014 at 4:33 pm

what should be the ideal value of the leakage current during the testing of
11kv bus bar.

Reply

Anil KOndath says:

August
5, 2014 at 4:19 pm

Appreciate if you could tell me what is the reason for choosing DC or AC
voltage for Hipot testing. Thanks

Reply

sasank says:

November
6, 2014 at 3:46 am

what is the range of leakage current should be while testing a 13.8kv cable
with an input of 22 kv

Reply

Khalilzadeh says:

November
22, 2014 at 7:40 am

Dear Mr Jignesh
I am electrical enginer and work in Iran power plant
…….. I have 1 question …what is the maximume I leake for hi pot test for 200mw
15.75 kv synchron generator and has standard ?
tnx very much

Reply

asraf ali says:

December
1, 2014 at 10:19 pm

good job..
i need more detail

Reply

Mohammed_mukry says:

March
24, 2015 at 11:09 am

thank you for this nice article

Reply

servis
alat ukur says:

September
18, 2015 at 12:08 pm

nice…..info

Reply

Tapan Kumar Biswas says:

September
30, 2015 at 2:56 pm

Body earth & Neutral earth for a Power Transformer in a Distribution
Station can be taken from common eatrh mat.

Reply

chandrashekhar says:

March
7, 2016 at 8:17 pm

Is it necessary to check at both the ends of the underground cable for the
fault in the cable or cable check from one end of the cable is
enough

Reply

vinod chandran says:

March
17, 2016 at 11:29 am

it is really useful hats off

Reply

Muhammad Junaid says:

May
3, 2016 at 7:51 am

You are doing a great work. Keep it Up!

Reply

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About Jignesh Parmar:

Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical).
He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He
has more than 14 years experience in Transmission -Distribution-Electrical
Energy theft detection-Electrical Maintenance-Electrical Projects
(Planning-Designing-Technical Review-coordination -Execution). He is Presently
associate with one of the leading business group as a Deputy Manager at
Ahmedabad,India. He has published numbers of Technical Articles in "Electrical
Mirror", "Electrical India", "Lighting India","Smart Energy", "Industrial
Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer
of Advance Excel and design useful Excel base Electrical Programs as per IS,
NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi,
Gujarati, French languages. He wants to Share his experience & Knowledge and
help technical enthusiasts to find suitable solutions and updating themselves on
various Engineering Topics.

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