NEC 690.12 Rapid Shutdown – More on Conductor Length

Our first article NEC 690.12 Rapid Shutdown for String Inverters on Flat Roofs we explained the basics of implementing a rapid shutdown system using string inverters on a roof.  In that article, we gave a simple example of a single array.

What happens when there are two separate subarrays feeding the same inverter, or the subarrays are greater than 10′ apart?

The code doesn’t clearly state how to approach this scenario. In these types of situations, Pure Power looks at the philosophy of the code and what it is trying to achieve, and develops an interpretation consistent with the intent of the code.

Intent of the Code:

The intent of the rapid shutdown code section is to provide an area of work for emergency responders to safely move around and conduct operations on the roof without the risk of touching an energized conductor.

In the event of an emergency, the emergency responders can activate the rapid shutdown initiation device, then safely move around the roof knowing any areas within 10′ of an array may be energized, but as long as they remain greater than 10′ from a PV array, any solar AC & DC conductors they encounter will be de-energized.

There are a lot of differing opinions on how to interpret 690.12(1), which is the part  that is supposed to define the distances of the controlled conductors. Lets start by looking at the code’s language:

The Code

Requirements for controlled conductors shall apply only to PV system conductors of:

  1. more than 1.5 m (5 ft) in length inside a building or
  2. more than 3 m (10 ft) from a PV array

Here is our breakdown of these two items:

PV system conductors of more than 1.5 m (5 ft) in length inside a building

The first part of 690.12(1) defines the controlled conductor distance for conductors inside a building. This clearly states if you run the conductors in the building, any length of conductors over 5′ is a controlled conductor and subject to the rapid shutdown rules. Less than 5′ and it’s not. This part is pretty black and white. This is more common in residential systems, but usually not encountered on commercial scale flat roof systems.

PV system conductors of more than 3 m (10 ft) from a PV array

The second part of 690.12(1) defines the controlled conductor distance for conductors more than 3 m (10 ft) from a PV array. This part applies to a majority of flat roof systems.

The black and white interpretation of this part is that the conductors are not controlled conductors as long as they dont extend more than 10ft from a PV array. As such, we create a 10’ boundary around each subarray, as explained in our previous article NEC 690.12 Rapid Shutdown for String Inverters on Flat Roofs. Any conductors inside the 10′ boundary are not controlled conductors and not subject to the rapid shutdown rules.

What about larger systems with multiple subarrays, or multiple subarrays feeing the same string inverter, combiner box, or relay contactor?

Adjacent subarrays

A topic that often comes up with AHJs and other industry professionals is whether you can extend a conductor from one subarray’s 10’ zone into an adjacent array’s 10’ zone.  The code does not have any language stating the 10’ is from an individual array, but rather the general statement “from a PV array”.   So, from a black and white code perspective, the conductor won’t be a controlled conductor as long as its 10’ or less from a PV array (it doesn’t need to be a specific array). It would be nice if the code explicitly stated “any array” to avoid doubt, but we still take away the same meaning from a literal interpretation of the current language.

Here is an example of this scenario:

Rapid-Shutdown-Adjacent-Arrays

However, let’s look at this closer and make sure this black and white interpretation isn’t a loop hole that circumvents the intent of the code.  If an emergency responder knows any conductors more than 10’ from an array may be energized, they can stay 10’ away from each and every subarray or PV module and be safe. In the heat of the moment, they are not going to start tracing conduits to determine which subarray the conductors start and stop in.  How can an emergency responder know which subarray is the source of energization? The conductors may even be energized from both sides. There is no practical or reasonable way for an emergency responder to make that determination. Therefore, they must rely on the simple rule that any conductors within 10’ of any and all arrays will not be energized.  The source of the energization, or the routing of the conductors inside that space, is irrelevant with respect to the actions the emergency responder must take.  Therefore, running a conductor from one array’s 10′ zone into an adjacent and continuous 10′ zone is consistent with the intent of the code, consistent with the actions emergency responders need to take, and does not pose additional risk to the emergency responder.

Alternative interpretation – Length of conductors

Over the years, we’ve had discussions on other interpretations and implementation ideas for this article of the code. Some believe the 10’ boundary should be interpreted as conductor length. We disagree with this, because the code doesn’t say anything about length in part (2). More importantly the 10′ conductor length interpretation doesn’t provide a realistic benefit to the emergency responder. Responders need to react quickly and focus on the task at hand, not deciphering complex implementations of the rapid shutdown rule. They are not going to start measuring the lengths of conductors to see if the controlled conductor zone is actually 5’ or 8’ from the array, as it may vary from subarray to subarray.  The 10’ boundary is the worst case scenario and something responders can easily understand and react to without hesitation or distraction. As such, even if a conductor length interpretation was implemented on a system, the responders will still need to assume the 10’ zone is worst case and proceed accordingly.  Therefore, we believe the 10’ length of conductor interpretation should not be used.

Labeling

If solar professionals are confused on how to interpret the rapid shutdown code, I can only imagine how confused the non-technical emergency responders must feel. As such, Pure Power recommends that you add directions on the label.  A directory label is already required to be placed on the rapid shutdown initiation device per 690.12(4), but we believe you need do to more than just call out the name and location of the main disconnect switches. To ensure the safety of the emergency responders, be sure to include directions to this label that concisely explains how to use the rapid shutdown system.

Rapid-Shutdown-Label

I hope this clarifies things. If you have any comments or suggestions Id be happy to discuss them.

NEC 690.12 Rapid Shutdown for String Inverters on Flat Roofs

The 2014 National Electric Code added a new section of code 690.12 requiring “Rapid Shutdown of PV Systems on Buildings”.  Below is the first of 1 of 2 articles we put together to help you understand this code (here is the other: Rapid Shutdown – More on Wire Length).

The Goal of Rapid Shutdown:

In the event of an emergency, the emergency responders can initiate the rapid shutdown device, then safely move around the roof knowing any areas within 10′ of an array may be energized, but as long as they remain greater than 10′ from a PV array any solar AC & DC conductors they encounter will be de-energized.

The Language of the Code:

690.12 Rapid Shutdown of PV Systems on Buildings

PV System circuits installed on or in buildings shall include a rapid shutdown function that controls specific conductors in accordance with 690.12(1) through (5) as follows:

(1)          Requirements for controlled conductors shall apply only to PV system conductors of more than 1.5 m (5ft) in length inside a building or more than 3 m (10 ft) from a PV array.

(2)          Controlled conductor shall be limited on not more than 30 volts and 240 volt-amperes within 10 seconds of rapid shutdown initiation.

(3)          Voltage and power shall be measured between any two conductors and between any conductor and ground.

(4)          The rapid shutdown initiation methods shall be labeled in accordance with 690.56 (B)

(5)          Equipment that performs the rapid shutdown shall be listed and identified.

Explanation of the Code:

This may sound confusing, but its not so bad. Here are the key components:

Controlled Conductors – These are any conductors that extend beyond 10′ from an array.  Any conductors within 10′ of the array are not considered “controlled conductors” and not subject to the requirements in this section (they can stay energized all the time). This applies to both AC and DC conductors, which you will have with string inverters on the roof.

Rapid Shutdown Initiation Device:  This can be a push button or disconnect switch.  Once the switch is thrown, then any controlled conductors (conductors more than 10′ from an array) must be de-energized within 10 seconds. In Pure Power’s designs, we use the main AC disconnect switch as the rapid shutdown device and label the switch as required in (4). This AC switch disconnects and energizes all AC conductors on the roof, easily achieving rapid shutdown for the AC portion of the system.  The DC conductors are not as straight forward, but explained in the next paragraph…

String inverters – For the DC portion of the system, by placing string inverters within 10′ of each array it’s fed from, there will inherently be no controlled conductors further than 10′ from the array.  However, you must be careful that the string inverter make & model you select does not have capacitors on the AC or DC side that may discharge for several minutes after the switch in thrown which will energize the conductors for more than the 10 second limit in (2).

Here is a sample drawing of a rapid shutdown compatible system:

rapid shutdown 690.12(d) As you can see, the DC conductors are all kept within 10′ of the array, so there are no “controlled conductors” in this example. In this case, achieving rapid shutdown is straightforward with no additional equipment or provisions necessary.

However, its not always so cut and dry. What happens when you have multiple subarrays instead of just one connected to an inverter?   Then the code can get a little more confusing, but we have a straightforward philosophy for solving this situation. Read more on this scenario in our other blog article here .

Other notes:

Use of contactor relays: Where the conductors must extend beyond 10’ from a PV Array, we can use contactor relays to de-energize the controlled conductors. The contactors are kept in the closed state by voltage sourced from the Solar’s AC system. Once the Solar Generator Disconnect Switches are switched to the open position, AC is removed and the contacts open, de-energizing any conductors outside the controlled conductor zone.

Central Inverter: If you had the same array on the roof but were using a central inverter on the ground, this method wouldn’t work exactly the same. The good news is, there is no AC on the roof to worry about. The bad news is, the DC is not as straightforward and requires additional expense to implement.  You can place the combiner box within 10′ of the array, but you would still need to have a disconnect switch or contactor inside the combiner box that disconnects the DC output feeder when the rapid shutdown device is initiated.  Otherwise, the live DC conductors would extend beyond 10′ from the array. Additional control wiring is necessary to control the switch in the combiner box to disconnect and de-energize the feeder running to the edge of the roof and down to the inverter on the ground.

For more discussion on Rapid Shutdown, read our next article: Rapid Shutdown – More on Wire Length

Photo of the Month – January

Here is something that’s crazy, and yet we completely understand why they did it. Some evil genius put duct tape around the vents of a NEMA 3R transformer. If this transformer is heavily loaded it runs the risk of overheating, but at least snow cant drift under the vent hoods and cause a fault such as this one: Faulted transformer from snow drift .

Transformer1

Transformer2

I thought this was great, but technically I cant endorse doing this on any project we engineer.

For more info on selecting the correct enclosure rating for an outdoor dry type transformer, see our January 2016 post 480 to 208V transformers – Caution when mounting outdoors.

Now Hiring: Solar Engineers

Pure Power is now accepting applications from experienced candidates for the position of Solar Project Engineer.

Job Description

Pure Power is seeking experienced engineers to design and create construction documents for commercial and utility scale solar PV systems. Join the team known for producing best looking, most value engineered and constructable drawings in the industry. You will be joining a culture of constant education, improvement, and teamwork. As a Pure Power engineer, you will challenge yourself every day to make each design and drawing set the best one yet .

Responsibilities

  • Design and engineer commercial PV solar systems between 50kW and 5MW in size.
  • Develop construction documents using AutoCAD.
  • Travel to project sites from time to time to gather site-specific data, as required.
  • Site assessment and analysis both from on-site data collection and network tools.
  • Contributing to or writing technical papers.
  • Site specific production analysis of PV arrays using existing industry models.
  • Develop specifications, collect data, and complete utility interconnection applications and building permit applications.
  • Develop detailed equipment specifications.
  • Coordinate activities between clients, utilities, permit agencies, contractors, and other engineering firms.
  • While most of this work will be carried out in Pure Power’s office in Hoboken NJ, some travel to sites will be expected.
  • Roof surveying using total stations and other professional land surveying equipment.
  • Integrating energy storage systems into the solar PV system.
  • Mentor and train new and junior engineering personnel.

Required Qualifications

  • 2+ years’ experience designing commercial solar PV systems.
  • 4+ years’ experience working with AutoCAD with an understanding of layer control, dimensioning and scaling, sheet sets, blocks, xrefs, line weights, plot files, and file transmission.
  • Highly knowledgeable about commercial and utility scale PV installations.
  • Ability to communicate among construction, technical and non-technical personnel, both internally and outside the company.
  • Detail oriented, thorough, with excellent planning, process, and project management skills.
  • Strong mathematical and quantitative skills.
  • Strong verbal and written communication skill.

Preferred Qualifications 

  • Experience in traditional building design with an MEP firm.
  • Medium Voltage engineering experience.
  • Experience designing energy storage systems
  • Land surveying experience.
  • Hands on experience commissioning, testing, and troubleshooting solar PV systems.
  • NABCEP certification.
  • BS in electrical engineering, structural engineering, construction management, or other related discipline.

Compensation and Benefits
We offer a competitive salary commensurate with experience.  We pay 100% of our employee benefits costs and offer healthcare plans, long-term and short-term disability, life insurance, dental, vision and 401K.

Location
Pure Power is in the exciting township of Hoboken, with plenty of public transportation options. We also provide a parking spot for those that wish to drive to the office.

Pure Power strives for a multicultural work environment; diversity is a core value. AA/EOE.

Please send your resume to careers@pure-ps.com

2 Important settings for Thermal (IR) Cameras

Thermal (IR) cameras are a great tool for preventative maintenance and inspection of your PV system. With a little thermography “know-how” and some image focusing, problems can be discovered quickly before they create a fault or safety hazard in the PV system. Below are issues that can lead to a system fault that the infrared camera can expose:

  • Hot spots near lugs from poor wire terminations and untorqued lugs.
  • Hot spots on modules- indicating damaged cells.
  • Excess heat on feeders created by unbalanced loads

No two sites are alike; therefore, we must calibrate our camera to site specific conditions. Below are two important settings that are often overlooked when using an infrared camera.

Emissivity
Emissivity is how well an object reflects radiation. Very reflective surfaces such as shiny metals have low emissivity. Absorbent surfaces such as rubber and electrical tape have a high emissivity value. Knowing this, the infrared camera will have to be calibrated when measuring bus bars and recalibrated when measuring insulated cable. Unfortunately, frequent recalibration is time consuming but very necessary to obtain accurate measurements. So how do we adjust and maintain two objects with different emissivity properties in one image- like a shiny aluminum mechanical lug and the XHHW jacketed feeder? You could take two different images with respective emissivity settings for each item or you could exploit your knowledge on emissivity and keep both items in one image. I will describe the procedure below, but first, it should be made known that this procedure incorporates contact with mechanical lugs and only qualified persons with de-energizing and system testing training should be performing this procedure:

  • Wire insulation has emissivity values of 0.95- equivalent to black electrical tape. Set the infrared camera emissivity level to 0.95 and apply black electrical tape onto the lug. Give the tape a few seconds to adjust to the temperature of the lug. This “surface” will now reflect both the temperature of the lug and also possess the emissivity properties of the wire insulation. Without the tape, the shiny surface would have registered much lower temperatures (due to low emissivity) and presented false results.
  • Safety first! Do not apply tape to any exposed components while the system is energized! Even though the system is off, always be mindful of line and load sides!
  • Here is a link of various materials and their respective emissivity levels, courtesy of Fluke. http://www.emlab.com/m/store/Fluke%20Thermal%20Camera%20Emissivity%20Values.pdf
    If you are using a model by Fluke or Flir, it’s likely the emissivity values are already programmed into the camera and you just have to select the proper one.

Background Temperature
Think of “background temperature” as “reflected temperature.” It is the infrared energy of our surroundings that is being reflected off the object that we are trying to measure. A real world example: Have you ever seen your IR reflection in the image? I tend to see this occur when capturing bus bar images in switchgear or panelboards. It also tends to be more dramatic on colder days when your body heat is much higher relative to the surface temperature of the measured object. This can cause false alarm when looking over the captured image as the alleged “hot spot,” which in actuality is a portion of your reflected body heat, is being registered by the camera lens. Be mindful of this concept. If you see a hot spot on your display, see if simply changing your position will fix this anomaly. If you find yourself in a small or cramped room and hot/cold background objects cannot be avoided from the image, you will have to calibrate your camera and use a curtain. The simplest way is to scan the room and note the average temperature. Adjust for this value in the background temperature settings. Then, place a “curtain” (I use a sheet of cardboard) between the background images and the camera- basically, right behind you. This will prevent any background heat from being reflected off the measured image and reaching the lens.
Thermal Background Temperature

Scary Photo of the Month – September

On this system, the installer stacked ballast so high that it shades the modules at noon of each day. Why would anyone use a hollow block for ballast?
Module interrow Shading

At the same site but on a different roof… There was ample space on the roof, but the installer placed the modules right up against the parapet wall where it would it would be heavily shaded in the afternoons year round.  Module Shading 2
Module shading 1

Mounting String Inverters on a Roof

This photo is an excellent example of an inverter installation on the roof. Keeping the inverters next to the array allows the unfused string wires to be kept at a minimum length, increasing safety. Notice how the inverters are tilted to the north so they absorb a bit less direct sun at that angle (reverse of the reason modules are tilted south to take on more sun).
Inverters-on-Roof

Scary Photo of the Month – August

We were called out to investigate a performance issue at a site, and were shocked to see this wire management. We suspect the AHJ made this contractor install physical protection after the strings were installed, which is why the contractor cut the PVC conduit in half, wrapped it around the wires, and zip-tied it back together. I don’t know how this could have passed inspection…

Wire-Management

However, the string wiring is still running unsecured on the roof. Ice and snow will lock up around the string wire and pull hard on the cables until something fails.

Wire-Management2

Wire-Management4

Even if you considered this adequate physical protection (of course its not), it’s not going to last long when it’s unsecured as shown by these photos here. In the rare instances where they used caddy blocks, they didn’t even secure the conduit so of course they started to fall off!

Wire-Management5

Wire-Management6

Wire-Management7

Scary Photo of the Month – May

I appreciate Shoal’s new arc fault technology as shown in this advertisement, however I was shocked to see the photo of a man standing on PV modules.

This is a reminder that you can not stand on solar PV modules like this if you expect them to last 25 years. Standing on the modules creates micro fractures on the cells that will lead to premature failure. If you were ever in a difficult situation where you needed to step on a module, at least be sure all your weight it applied to the frame and not on the module surface (but even that is risky).

standing-on-modules