DC PV Arc fault detection Unit

Installation, usage and other information

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1. IntroductionThe National Electrical Code® 2011 states arc-fault protection requirements. The NEC 2011 states that all rooftop

installations with a DC voltage over 80V, needs to be equipped with a means of DC Arc fault protection. The new

NEC 2014 expands this requirement to ground mounted PV installations.

The main reason for this requirement in the NEC is improved safety, the prevention of fires generated by arcing

events. Because of this requirement, Santon has developed the arc fault detection unit (ADU).

The Arc-fault Detection Unit (ADU) is a detection device for detecting series arcing in photovoltaic systems. Arcing

in any system can cause fires. In DC photovoltaic systems the arc will be even more dangerous than in an AC

system as the continuous supply of current, will continue to feed the arc. The arc will become more intensive and

hotter, eventually causing fire or burn damage on other system components if it is not detected and extinguished

by interrupting the current.

This paper describes relevant information on arc fault detection in general and the best practices, usage of the

Santon Arc-fault detection unit.

2. General information - UL standardThe UL standard applicable to the Arc fault circuit interruption as required by the National Electric Code is the UL

1699 - Sub B. This standard embeds the definitions, requirements and testing of Arc fault circuit interruption

solutions.

There are three of devices indicated in the standard. The Arc fault circuit interrupter (PV AFCI) which includes the

detection as well as the interruption of the circuitry, the Arc fault detector (PV AFD) which includes only the

detection and the Arc fault interruption device (PV ID) which includes only the interruption.

Each of the devices mentioned in the standard can be listed as a type 1 device (series arcing) or as a type 2 device

(series and parallel arcing).

2.1 Requirements In the NEC 2011 and NEC 2014, the requirement Arc fault circuit interruption is an UL1699 – sub B type 1.

The UL 1699 – sub B explains that four types of arcs need to be detected.

The arcing conditions that need to be mitigated are:

- 300W within 2.0 seconds with a current of 7A and a gap of 1.6mm

- 500W within 1.5 seconds with a current of 7A and a gap of 4.8mm

- 650W within 1.2 seconds with a current of 14A and a gap of 3.2mm

- 900W within 0.8 seconds with a current of 14A and a gap of 6.4mm

Besides these arcing tests, nuisance tripping testing is also required. However the UL standard does not cover all

the real world conditions to which a device will be subjected.

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3. General information – Arc detection

3.1 Typical system Typical photovoltaic (PV) systems consist of a series connection of multiple solar panels, called a PV string, which

typically have a voltage of up to 1000Vdc. These PV strings can also be connected in parallel, called a PV array, to

increase the current. These PV strings or PV arrays are connected to a PV Inverter which converts the generated

DC power of the PV panels to AC power to feed into the grid. Figure 1 shows a typical setup of a simple PV system.

A lot of different setups are possible.

Figure 1: Typical PV system

3.2 Arc detection performance To understand how the performance of detection devices is influenced throughout the PV installation, it is good

to understand what arcing is and how this can be detected.

3.2.1 What is Arcing

An electrical arc is current that flows from one conductor to another when there is a gap between these two

conductors. The arc generates heat which can create fire.

There are two types of arcing events that can occur in PV installations:

- Series arcing (arcing between two conductors in line with a load)

e.g. this is possible when connectors or other cable connections are not fixed properly.

- Parallel arcing (arcing between two conductors in parallel to a load)

e.g. this is possible when the insulation of two conductors of opposite polarity are damaged and current

starts flowing from one to the other.

3.2.2 How to detect arcing?

All electrical circuits produce unintended frequencies/signals, called noise. This noise can best be described as

random fluctuations of an electrical signal. The noise can be generated by various electronic devices with several

different effects and the noise can vary greatly. All forms of arcing also introduce additional noise in the electrical

circuit.

In a PV system the noise is detectable as different frequencies on top of the normal DC current/voltage. An arc

fault detection/protection unit needs to be able to distinguish the normal “background” noise of the system and

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an arcing event. The technical solution to do this can vary, but basically they all come down to analysis of the

frequencies. In Ffigure 2 it is shown how the frequency spread can be in a simple two string PV installation.

Figure 2: Typical noise in 5kW installation (frequency domain)

3.2.3 Influences on detection

The task of a protection/detection device is to detect the arcing event in the mix of all frequencies in the system.

The inverter generates a lot of noise in a PV system, in the process of constant searching for the Maximum Point

of Power (MPP). The way the inverters search for (optimal) MPP varies from manufacturer to manufacturer.

Next to the MPP tracking process, the inverter also can generate additional excessive noise during start up phase,

or during the self test of the inverter, or during high solar irradiance at/over the maximum input of the inverter,

or during cloudy conditions.

To accurately detect an arcing event all these possible disturbances generated must be considered to avoid false

tripping of a protection/detection device.

Next to the noise generated by an inverter, the detection range (distance) between the arcing event and the

detection device is important. Arc detection works best when there is a high signal-to-noise ratio. The greater the

distance between the arc protection/detection device and the arcing event, the lower the signal-to-noise ratio

will be. The arc will be masked by the “background” noise.

The UL 1699 – sub B has incorporated tests that are intended to determine if the AFCI or AFD is functioning

correctly (detection). The tests consist of detection of the earlier stated arcing wattages near the device as well as

at 66 meters (200 feet) from device. But also tests are included to determine if the device is subject to false

tripping (detection when there is no arcing event). The tests consist of testing the functioning of the device when

installed in combination with an inverter that can create disturbances that could mask the arcing event. But also

the device may not detect the switching of an interrupting device, like a DC isolator.

3.3 Santon Arc Fault detection Unit The Santon arc fault detection unit (ADU) is developed, tested and certified in accordance with the

requirements of the UL1699 – sub B and the NEC 2011/2014. The Santon ADU is UL recognized as a PV Arc

Fault Detection device - type 1.

This means the Santon ADU will detect series arcing. The correct follow up in case of the detection of an

arcing event must consist of disconnecting the DC current flow from the PV modules to the PV inverter.

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4. ADU usage

4.1 Intended use The Santon ADU is intended to be used as a string level detector, but can be used as array and inverter level

detector. Each system will influence the effectiveness of the detection device. Because of this there are some

guidelines to be taken into account when using the ADU in each system. Basically the ADU can be installed in

either the positive (recommended position) or negative conductor. Only one ADU per electrical circuit is needed,

as the frequencies in the system will travel through all connected cabling.

Based on measurements performed by the ADU on this conductor, the unit can determine if an arcing event is

taking place. In case of an arcing event the ADU will trip and give an acoustic signal, a flashing red LED as visual

indication and a relay state change (alarm output). This output can be used to perform the correct follow up of

the arcing event.

Again, please note that it is advised that there is only one ADU needed per MPP tracker. If there is only 1 MPP

tracker in the inverter all the string inputs will be connected internally resulting in the noise signal being present

in all strings as well. Therefore only one detection device is advised in this type of application

4.2 String level detector

4.2.1 Usage

For the usage of the ADU as a string level detector it is advised to install the ADU near the PV inverter. The ADU

will then be accessible in case of an arcing event. Also the follow up signal, in case of an arcing event, can be

easily connected into the inverter or switch off device.

4.2.2 Installation

Typical for this type of installations is a relative low current (approx. maximum 8A).

Figure 3 shows two possible setups with the ADU as string level detector.

- Two string inputs on a 2 MPPT inverter

two separate strings with ADU’s

in each positive conductor.

- Two string inputs on a 1 MPPT inverter

two strings combined in the inverter

with ADU in positive conductor string 1.

Figure 3: Locations for arc fault protection

According to the UL standard the detectable distance is 66 meters (up- or downstream) between the ADU and

the arcing event. If distances in the installation is greater than 66 meters it is allowed to place multiple ADU’s in

series, although this could mean both ADU will detect an arcing event.

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4.3 Array level detector

4.3.1 Usage

For the usage of the ADU as an array level detector it is advised to install the ADU near the PV inverter or in the

unit where the strings are combined (combiner/Junction box). If the ADU is located near the inverter, the ADU will

then be accessible in case of an arcing event. Also the follow up in case of an arcing event can be connected easily

to the inverter. If the ADU is located in the combiner box, the ADU is not always accessible in case of an arcing

event. The follow up must then be handled locally by means of an automated interrupting device or the inverter.

4.3.2 Installation

Typical for this type of installations is a high current (e.g. 16A).

Figure 4 shows some possible setups with the ADU as array level detector.

- Two arrays on a 2 MPPT inverter

two times two strings in parallel connected

to a 2 MPPT inverter. An ADU in each positive

conductor to each MPPT input is required.

- One arrays on a 1 MPPT inverter

four single strings in parallel connected

to a 1MPPT inverter. An ADU in the positive

conductor to the MPPT input is required.

- Two Combiner boxes on a central 2MPPT

inverter

two times two strings in parallel connected

to a 2 MPPT inverter. In each positive

conductor one ADU per MPPT is required.

e.g. ADU’s are integrated in a combiner box.

- One Combiner box on a central 1MMPT inverter

four single strings in parallel connected

to a 1 MPPT inverter. One ADU in the positive

conductor to the MPPT input is required.

e.g. the ADU is integrated in a combiner box. Figure 4: Locations for arc fault detection

As shown in Figure 4 when the current rating of the array is higher than the allowed 40A of the ADU, it is also

allowed to place an ADU in the incoming string before the junction in a junction box. According to the UL standard

the detectable distance is 66 meters (up- or downstream) between the ADU and the arcing event. If distances in

the installation is greater than 66 meters it is allowed to place multiple ADU’s in series, although this could mean

both ADU will detect an arcing event.

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4.4 Inverter level detector

4.4.1 Usage

The ADU can also be used inside the inverter. The inverter manufacturer shall then keep in mind the advices of

one ADU per MPPT and the other technical details as stated in the manual.

4.4.2 Installation

The setup for the ADU inside the inverter is very dependent on the inverter itself.

Figure 5: Locations for arc fault detection

4.5 ADU Operation

4.5.1 Powering up

After switching on power, the Arc fault detection unit will start up with a self test (small beep) and the green LED

(Power) will be blinking. After finishing the self test the green LED will be on continuously.

4.5.2 Normal operation

The arc fault detection unit will monitor the current flow and analyze the signal measured to determine if there is

an arc fault situation. It will do this continuously until an arc fault occurs and/or the DC power supply is removed.

4.5.3 Arcing event detected

In case an arc fault is detected the ADU will signal this with a blinking red LED (fault). At the same time a buzzer

will start beeping for 30 seconds (after 30 seconds the buzzer will only beep once a minute).

The alarm output will change state, ensuring that any connected actuator gets the signal to isolate the DC supply

between the inverter and the solar panels.

4.5.4 Reset of arcing event

The arc fault situation can be resolved by briefly pressing the reset button with a small pin (soft reset).

!! MAKE SURE THAT THE ROOT CAUSE FOR THE ARC IS RESOLVED !!

Once the reset is performed the ADU will go back to normal operation and the alarm output will switch back to

the normal state.

4.5.5 Fault state/initiation ADU self test

In case an ADU error occurs, the unit will indicate this with the blinking green and red LED’s, also the buzzer will

start beeping and the relay output will change state.

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This fault situation can be resolved by pressing the reset button for longer than 5 seconds with a small pin or a

finger nail (hard reset). This will restart the ADU and initiate a self test. The ADU self test will check if the arc

detection unit is still functional. If successful the ADU will return to normal operation, if the self test fails this will

result in the fault state.

Please note that if the ADU is used in as an inverter level detector or installed in a combiner box, the reset button

is sometimes difficult to access. In this case it is allowed to use the removal/interruption of the 24V power supply

for more than 1 second as a reset of an arcing event or reset of a fault state/ initiation of a self test.

5. Validation of the ADU by SantonEach ADU will be assembled and tested during the manufacturing process at Santon. Periodically manufactured

ADU’s will be tested on detection of arc fault events in a real PV system to verify manufacturing process is still in

order.

5.1 Testing of the ADU During the development as well as after, the ADU has been subjected to various tests in the testing facility of

Santon. These tests have been done to confirm the correct functioning of the unit under different conditions.

The conditions that have been tested are:

- Temperature/humidity conditions o Temperatures up to -20degCo Temperatures up to +70degCo High humidity conditions up to 90%o Low humidity conditions up to 0%

- Weather conditions o Sunny, Cloudy and Rainyo Warm, normal and cold temperatures

- Installation conditions o Location of the ADU in the installationo Location of the generated series arc faulto Length of the cablingo Type of cable diametero Parallel connected stringso Impact of switching devices

5.2 Test conclusions Temperature/humidity testing

The tests on the ADU have been performed inside a climate chamber. The ADU has passed all the tests. The unit is

able to detect several different arcing conditions when installed and operated on the maximum of (and

sometimes beyond) the specifications.

Weather condition testing

As all tests have been performed in the test facility of Santon, with real PV power, the tests have been done under

different and fluctuating conditions. The ADU performed optimal under these changing conditions and did not

have any failures due to these conditions.

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Installation conditions

As all ADU’s can be installed in different types of installation the most common situations have been tested. By

installing different cable types, lengths and by varying the location of the generated arc the tests have confirmed

that all arcing events can be detected up to 66 meters(up- or downstream) from an ADU. Also the tests of several

panels in parallel, monitored by one ADU revealed that the ADU will detect arcing events in parallel strings. The

effect of a switching device like the Santon X-type switch in an inverter does not influence the detection of the

ADU.

All the tests performed at Santon during and after the development of the Arc fault detection unit have been

carefully analyzed and the results of all the tests have been included in the UL recognized Santon Arc-fault

Detection unit.

6. Validation of the ADU by Customers

6.1 Testing ADU functioning with real arcing event (non PV setup) The ADU functioning can also be verified by connecting a DC power source (series of Car batteries) and a resistive

load in series with the ADU. When an arc generator setup as described in the UL1699 – Sub B, chapter 23 (see

also below figure) is also included, an arcing event can be made to check the functioning of the ADU.

Figure 6: UL1699 – Sub B Arc generator

Please be aware that there are more requirements to this setup to ensure that the ADU will detect the arc.

Amongst others there needs to be sufficient power (min. Voltage = 100Vdc and min. Current = 1Amp) and the arc

must be sustained for approx. 1 sec. As load there must be a resistive load, an inductive load will not work

properly. If the ADU works correctly, it detects the arcing event. The unit will indicate this with the blinking of the

red LED, the sounding of the buzzer and the change of state of the relay contact.

Please note that the usage of DC power supplies that simulate PV panels have proven to be unreliable due to noise signals generated by the power supply that disturb the arc detection and therefore can cause unwanted detections.

6.2 Testing ADU functioning with real arcing event (PV setup) The ADU functioning can also be verified by connecting a PV source (series of PV panels) to the

Inverter. When an arc generator setup is also included, an arcing event can be created to check the functioning of

the ADU.

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There are requirements to this setup to ensure that the ADU will detect the arc. There needs to be

sufficient power (min. Voltage = 100Vdc and min. Current = 1Amp) and the arc must be sustained for approx. 1

sec. As load an inverter or resistive load can be used.

If the ADU works correctly, it detects the arcing event. The unit will indicate this with the blinking of the red LED,

the sounding of the buzzer and the change of state of the relay contact.

7. Best practices for the ADUAfter the product launch the ADU has been tested and installed in real PV installations. The results from these

installations have led to the following best practices Santon likes to point out.

7.1 Number of ADU’s to be used The number of ADU’s to be used is depending on the type of installation, inverter and distances that are present.

Santon advises to use 1 ADU per MPP Tracker. This ADU will cover the detection up to 66meters from the ADU.

If distances are bigger than 66meters, multiple ADU’s on the same MPP Tracker might be necessary. This could in

case of a strong arcing event lead to the alarm of both ADU’s.

7.2 Reset of multiple ADU’s Larger installations, equipped with multiple ADU’s can be reset all at once. By removing/interrupting the 24Vdc

power feeding these ADU’s for more than 1 second the ADU’s will stop operating. When the 24Vdc power returns

all the ADU’s will start up and perform a self test after which the ADU’s will start normal operation.

7.3 Cabling of the installation The cabling of the installation has a great influence on the signal-to-noise ratio and thus the performance of the

ADU.

7.3.1 Balance positive & negative DC cabling

When there is an unbalance between positive and negative DC cabling there could be cross talk between the two.

As the cross talk could result in a high signal-to-noise ratio in the detection range of the ADU, it could be that the

ADU’s connected trip falsely. It is advised to balance the cabling.

Cable duct

balanced

unbalanced

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7.3.2 AC and DC in same cable duct

Not only cross talk between the positive and negative DC cabling is possible but also cross talk from the AC-grid can

cause false tripping on the ADU. Most likely the AC cabling and DC cabling is routed in the same cable duct. This has

to be prevented.

7.3.3 Excessive cable lengths

As the ADU works best when there is a high signal-to-noise ratio, it is best to minimize the cable lengths. Excessive

cable lengths that are not necessary will damp the noise signals and lower the signal-to-noise ratio. This could

lead to a situation in which an arcing event is not detected.

7.4 Disturbances by other equipment As the detection of arcing event is based on the frequencies and the signal-to-noise ratio, any disturbing equipment

could cause the detection devices to trip falsely. For example if the ADU’s are installed in an industrial environment

where frequency inverters for motor drives are also installed, it is advised to mount the ADU’s in a metal enclosure.

This way the ADU’s are installed in a Faraday cage which minimizes the risk of falsely tripping.

7.5 Parallel arcing Although the Santon ADU is developed, tested and certified as a series arc detection device, in most cases the

Santon ADU will be able to detect parallel arcing as well. However the ADU is not officially certified for detecting

parallel arcing events.

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8. ADU Product details

8.1 Technical specifications String specifications

Maximal string voltage / current 1000VDC / 40A

Number of poles 1 pole (positive or negative)

Minimal cable diameter cage clamp 2,5mm² / AWG14

Maximal cable diameter cage clamp 10mm² / AWG8

Power specifications Power supply 24VDC (± 10%)

Current consumption (passive) 60mA

Current consumption (active) 120mA

Maximal cable diameter connector (2P) 2,5mm² / AWG14

Alarm specifications Alarm relay load (resistive) 0,3A at 125VAC; 1A at 30VDC

Minimal arc detection level 100VDC - 1A

Maximal cable diameter connector (3P) 2,5mm² / AWG14

Arc fault detection (according) UL1699B - PV AFD Type 1

Other specifications Protection degree IP20 / NEMA1

Operating temperature range -20°C till +70°C

Storage temperature range -40°C till +85°C

Humidity (RH) 0% - 90% at 20°C

Weight (approx.) 120 g

8.2 Dimensions and connections

Dimensions are in mm

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9. Troubleshooting - Q & ASingle string inverter set-up Q: I use my one MPPT string inverter with an ADU which generates false alarms.

What is the solution? A: Either you do have an arc in your system or you might have an inverter which generates a frequency

pattern that is disturbing the detection. Santon has tested the ADU with many inverters of different brands, however not all inverter types have been tested. Please contact the Santon main office to find out whether your inverter has been tested in combination with the Santon ADU.

Power optimizers Q: I have a set up with power optimizers at the back of the solar panels. The ADU generates false alarms. A: The power optimizers of some systems are exactly generating noise (frequencies) in the detection area of

the ADU. Please contact the Santon main office for more information.

Multiple MPPT inverters or multiple inverters next to each other Q: My inverter has multiple MPP trackers, how should I connect the ADU? A: Make sure that every MPPT has its own ADU and make sure the plus and the minus of one group of solar

panels is joined at the same MPPT input. As soon as the minus and the plus cables of the various groups of solar panels or inverters are mixed, this generates false tripping.

Q: Is it allowed to join the plus cables of two MPPT’s together before entering the inverter? A: No, that is not.

The two MPPT’s can interfere with each other and thus generate more noise which generate false tripping.

Q: My inverter has multiple MPP trackers and the ADU’s are connected properly. What can be the cause of the false tripping?

A: There are several possible reasons that could be the cause of the false tripping. - There is really an arc in the system - Cabling is not neatly arranged, see chapter 7.3 - Plus or minus of one MPP tracker is accidently connected to another MPP tracker. - Plus or minus of one inverter is accidently connected to another inverter. - Frequency converters are elsewhere in the building generating electromagnetic disturbance.

The solution is to place the ADU within a metal enclosure. - The inverter is of a type which generates a frequency pattern in the detection area of the ADU.

Please contact the Santon main office for more information.

Integration of arc detection units in junction boxes Q: Is the ADU certified to be used with central inverters? A: No, the Santon ADU is only certified for string inverters.

When used in combination with central inverters the test as in chapter 6 has to be fulfilled to establish whether an arc in your PV system can be detected according to the UL standard (UL1699 - sub B). Also false tripping may occur due to different frequency patterns of central inverters compared to string inverters.

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Q: How should the set-up be to secure my field of PV panels? How should it be integrated in junction boxes? A: We advise one ADU per junction box in which all incoming strings are joined into two outgoing cables (plus

& minus). The ADU can be placed in one plus cable of one incoming string. The arcing signal travels through all the connected cables in all directions. Please be aware that the ADU is certified up to a maximum cable length of 66meter. After this a second ADU is advised. In case of multiple junction boxes next to each other, a set up which enables simultaneous resetting is necessary since there is a high chance that two or more ADU’s will give alarm simultaneously since arc signals generally travel a long distance in a copper cable. During the start-up of the ADU a self test generates a frequency pattern which also can trigger a next ADU in the same electrical circuit.

Q: How should I reset multiple ADU’s which are in a solar park? A: All ADU units should be connected to one 24volt cable running through the solar park for feeding the ADU’s

with 24volts. To reset the ADU’s simultaneously remove/interrupt the 24volts of the line and restore the 24volts after approximately one second. During the start-up of the ADU a self test generates a frequency pattern which also can trigger a next ADU in the same electrical circuit.

Laboratory testing

Q: I am testing the arc detection in a laboratory with a DC power supply and I have false tripping. A: The DC power supplies generate frequencies in the detection area of the ADU.

The solution is to test the system with true solar panels.

Contact Santon

Q: A:

I tried all above Q&A’s, however I still have problems Contact our office

Telephone: E-mail:

845-278-5777 beutler@dcsunvolt.com

Please provide following information (if known): - Which ADU is installed (please mention type number and production date)? - How is the ADU used/installed? One ADU per MPPT? - Where is the ADU installed (please mention enclosure type, physical location, surrounding equipment) - What are the ratings of the String/Array in which the ADU is installed? - Which inverter is connected (please mention brand and type code)? - When does the problem with the ADU occur?

Some examples: o During the startup of the ADU.o During start up inverter after installation.o During start up of the installation every morning.o During shut down of the installation every evening.o During the day when there is high irradiance.