[RE-wrenches] Megger for array testing

[Drake]

Thank you:

To all Wrenches who responded to the question about Megger testing.

The information discussed here is of major 
importance to photovoltaic installers, and 
clearly has been understood different ways.  The 
List is a great resource which helps increase the 
quality and reliability of solar electric installations.

To briefly sum up the information, as I understand it:
    * Solar arrays can and should be high pot tested
        * Check with module manufacturer before 
proceeding, as warranty issues are possible
    * The UL method of testing requires the 
terminals, of an unilluminated string or array, to be spliced together.
        * High voltage testing is done between the spliced leads and ground
        * This will:
            * totally protect the internal components of the modules
            * give the most uniform results
    * An easy and safe way to test for potential 
ground faults during routine installation is to:
        * Remove the fuses or positive connection to the inverter
        * Remove surge arrestors
        * Connect the Megger negative to ground
        * connect the Megger positive to the negative PV wire
        * Test at either 500 or 1000 V to 100 kOhms
Best regards,

Drake

Drake Chamberlin
Athens Electric
OH License 44810
CO license 3773
NABCEP Certified PV

At 09:11 PM 11/17/2010, you wrote:
>Content-Type: multipart/alternative;
>         boundary="----=_NextPart_000_0062_01CB8682.CEC07160"
>Content-Language: en-us
>
>Meggerers,
>
>Although shorting the terminals to perform the 
>megger tests may provide the most uniform 
>voltage profile, it is difficult to perform on 
>many arrays and adds unnecessary complexity to a 
>test that needs to become routine for all PV installations.
>
>The appropriate way to perform the test at the 
>combiner box level for negatively grounded 
>systems is as follows (voltage would be applied 
>backwards for a positively grounded array):
>
>1.      Open all fuse holders and make sure no 
>fuses are mounted in the holders.
>2.      Remove any connected surge arrestors 
>from the circuits as these will conduct during the test and may fail.
>3.      Remove the grounded feeder conductor so 
>there is no connection to the inverter.
>4.      Connect the negative of the Insulation 
>Tester to the grounding connection in the combiner box.
>5.      Connect the positive of the Insulation 
>Tester to the negative connection in the combiner box.
>6.      Apply either 500V or 1000V to the 
>negative connection and wait for the insulation 
>resistance to achieve at least 100kOhms.
>
>Insulation resistance will vary based upon 
>module construction, moisture on the  modules 
>and conduit systems, and cycle of the moon (oh 
>wait, that’s the PMS procedure­sorry)­actually 
>irradiance and temperature can make a 
>difference. By adding 500V or 1000V to the 
>negative side of the strings, the positive side 
>is going to go to 1000V or 1500V. This does not 
>violate the listing of the product as this is a 
>temporary test. These types of tests are 
>customary for 600Vac wiring systems and they 
>should be mandatory in PV systems of any size 
>operating above 100V. If the module is damaged 
>by this test, it was a faulty module since all 
>module internal wiring is required to withstand 
>2200V to ground throughout the unit­diodes and 
>all. By testing in the normal voltage bias that 
>the product sees during operation, it will more 
>thoroughly stress the higher voltage parts of 
>the array which are more critical for the 
>long-term reliability of the PV system.
>
>If a resistance of 100kOhms cannot be achieved, 
>take all the grounded-side string conductors 
>loose from the combiner block and test each one 
>individually to see if there are variations in 
>resistance. If no variations exist, the modules 
>might be a more leaky variety and a new, lower 
>threshold would need to be established with the 
>help of the module manufacturer.
>
>Happy testing and don’t forget to use your gloves.
>
>Bill.
>
>From: re-wrenches-bounces at lists.re-wrenches.org 
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Hoffer
>Sent: Wednesday, November 17, 2010 4:36 PM
>To: RE-wrenches
>Subject: Re: [RE-wrenches] Megger for array testing
>
>Joel
>
>You are correct about the UL testing requiring 2 
>x system voltage plus 1000 volts. The UL 
>requirement is also to ramp up to the full 
>voltage in 5 sec and held for 1 min, which I 
>believe most meggers are capable of.  The test 
>is considered a failure if greater than 50 uA 
>current is measured.  The difference is that the 
>connection is between the shorted outputs of the 
>module and the ground.  In the case of the 
>required factory tests one end of the high pot 
>testing device is connected to the frame 
>(ground) and one point is connected to both the 
>negative and positive  module terminals shorted 
>together.  In the UL test it is done with the 
>module submerged one terminal of the high pot 
>tester is connnected to a ground plane in the 
>water and the other terminal is connected to the 
>negative and positive terminals shorted 
>together.  In both cases the voltage potential 
>across the module terminals (through the 
>module)  is 0, but between the terminals and 
>ground is the 2 x system voltage plus 1000.  The 
>module never sees 2200 volts potential across 
>the cells and diodes.  The test is normally run 
>again reversing the polarity, but the terminals 
>still are shorted together.  I have performed 
>this test in the factory and observed it being 
>done this way during UL testing.
>
>UL1703 Section 21 Leakage Current Test specifies 
>shorted module output circuits System Voltage is the test level required
>
>21.5 All accessible parts and surfaces are to be 
>tested for leakage current. The positive and negative
>terminals of an unilluminated module are to be 
>connected together and to one terminal of a dc power
>supply. Both polarities of the source connection 
>are to be used, unless it can be shown that one polarity
>will represent both. Leakage currents are to be 
>measured between the part or surface and the other
>terminal of the power supply.  System Voltage is the test level required and
>
>UL1703 Section 26 Dielectric Voltage-Withstand 
>Test does not specify shorted module outputs, 
>but does specify 2 x system voltage plus 1000V
>
>UL1703 27 Wet Insulation Test does specify 
>Shorted output terminals of the module (and only 500vdc)
>27.3 After two min of immersion in the solution, 
>the insulation resistance between the shorted output
>terminals of the module or panel and the 
>solution is to be measured, in both polarities, with an instrument
>having a voltage of 500 Vdc.
>
>43 Factory Dielectric Voltage-Withstand 
>Test  Also doe snot specify shorted out puts
>
>The IEC 61215 is more explicit in all test cases 
>it specifies using the shorted output terminals 
>for all these tests.  IMHO the IEC is the better 
>document to follow (which is what the listing 
>agencies all do when things are not defined well 
>in UL1703).  The industry will be much better 
>off once we standardize with the IEC!
>
>In my experience all three tests for UL listing 
>are performed in the same way on shorted output 
>terminals.  I would recommend that the megger 
>test be performed in the same way especially for 
>field confirmation procedures for commissioning 
>a system.  This will still give you the intended 
>results (identifying a potential short to 
>ground) without the risk of applying unnecessary 
>voltage across the module cells or diodes.  I do 
>not want to be explaining a failed module to the 
>manufacturer or customer after subjecting it to 
>2200 + volts across the cells or diodes!  You 
>would never meg across an inverter input either, 
>even though they are also high pot tested for 
>listing.  Of course standard procedures for 
>megging wire runs without the inverter or 
>modules in the circuit can be done at those higher levels without a problem.
>
>Bill
>
>Bill Hoffer PE
>Sunergy Engineering Services PLLC
>2504 Columbia Ave NW
>East Wenatchee WA 98802-3941
><mailto:SunEngSer at gmail.com>SunEngSer at gmail.com (509)470-7762
>Cell(509)679-6165
>
>
>On Wed, Nov 17, 2010 at 9:05 AM, Joel Davidson 
><<mailto:joel.davidson at sbcglobal.net>joel.davidson at sbcglobal.net> wrote:
>Hello Drake,
>
>I think you should talk to a Sanyo Solar 
>engineer to get accurate information. Every 
>Sanyo and other brand UL listed modules must be 
>dielectric voltage-withstand factory tested to 2 
>times system voltage plus 1,000 volts. I hi-pot 
>test modules in the factory at 3,000 volts so 
>they will meet UL and also IEC standards. I 
>megger field test arrays at 1,000 volts to find 
>module, connector, and wiring problems. Megging 
>at 500 volts is ok, but it may not reveal wire insulation problems.
>
>Joel Davidson
>
>----- Original Message -----
>From: <mailto:drake.chamberlin at redwoodalliance.org>Drake
>To: <mailto:re-wrenches at lists.re-wrenches.org>RE-wrenches
>Sent: Wednesday, November 17, 2010 7:36 AM
>Subject: [RE-wrenches] Megger for array testing
>
>Hello Wrenches,
>
>Looking through the archives on using a 
>Megger  for testing modules, it appears that it 
>is best to keep voltages to 500VDC.  I read all that I could find.
>
>Talking to a Sanyo representative, I was told 
>that keeping test voltage under 600 would not 
>void the warranty or be an issue with UL.  He 
>said they have tested Sanyo modules up to 1000 VDC with no problems.
>
>As a final test on a system, I want to Megger 
>test the array from the terminations at the 
>inverter.  The approach I'd plan to use is:
>Clamp the Fluke 1587 negative to the enclosuer ground.
>Put the positive test lead to the negative conductor from the array and test
>Put the positive test lead to the positive conductor from the array and test
>All tests would be done at 500 V.  The modules 
>are illuminated, but there is no significant 
>reading from either positive or negative terminal to ground.
>The conductors would be removed from their 
>terminals at the inverter and be in free air.
>The conductors would be continuous through the 
>string of 10 Sanyo 210 modules.
>My questions are:
>Does anyone see a problem in this approach?
>Since the array often has 500 VDC open circuit 
>readings and shows no indication of a fault 
>whatever, would this test accomplish anything 
>more than the standard voltage to ground tests I've already done?
>Is there any danger to diodes or other PV components?
>Thanks in advance.
>
>Drake
>
>
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