Sun Safe LLCBlackout Sheet deployed over a shorted module beside the instrument board

The Blackout Sheet

A multi-layer irradiance-suppression cover for PV source circuits. It stops the photocurrent at the source, so a module can be worked on while the sun is up.

PATENT PENDING · US 64/026,462

Solar panels make electricity whenever light hits them, and there is no off switch for that.

Over the years the solar industry has found problems in the field and answered them one at a time with code changes, safety equipment and new standards.

One problem has stayed unsolved: the shorted string. It pushes current whenever the sun is out, and it can’t be safely disconnected under load. Crews either work it live in full PPE or wait for dark to fix it. And because a repair made at night can’t be tested until the sun is back, the circuit has to be revisited the next morning.

Covering the panels has long been an ad hoc field fix, but no cover on the market is designed for full-sun suppression. Off-the-shelf tarps were tested by UL in 2011 and found inconsistent and unreliable. The report concluded: “More research may be needed to find or develop specific foam products or other surface applied film technology that can be proven to be reliable and effective in blocking illumination to PV systems.”

Since then, U.S. solar has grown from about 4 GW to 279 GW, about 70 times larger. Worldwide it has grown from 67 GW to 1,865 GW, nearly 28 times. In that time no one has brought a reusable cover made for PV to market.

After running into this problem again and again through my career, I set out to build that cover, designed for the extremes: keep light from reaching the cells, collapse the photocurrent, and make a failed module far safer to disconnect than it was.

The science
  • Photocurrent. Each cell is a current source. Photons absorbed in the cell free charge carriers, and the short-circuit current rises in direct proportion to irradiance (Isc ∝ G). The test module: 8.71 A at 1,000 W/m².
  • Voltage is there in any light. Open-circuit voltage rises with the logarithm of irradiance, so it stays high even in dim light.
  • Series cells, series modules. Cells and modules in a string carry the same current. Opening one device, or shutting down one section, does not stop the photocurrent in the rest; each lit cell keeps generating.
  • Where shorts come from. See pv magazine USA, “Short circuits: How they happen and how to prevent them,” Aug 28, 2017.
  • Why a short persists. A string’s short-circuit current is only about 10% above its operating current, so a short does not blow the string fuse. Breaking that current under load draws an arc.
  • What rapid shutdown limits. It applies to PV on or in buildings (NEC 690.12), not to ground-mounted utility arrays. It limits conductor voltage: 30 V outside and 80 V inside the array boundary (NEC 2017–2023 690.12(B)). It does not remove the photocurrent inside the module.
  • The only lever left is light. Cut the photons and the current falls with them. Optical density (OD) measures the cut in powers of ten: at OD 2.3, 1,000 W/m² becomes about 5 W/m², and the test module’s 8.71 A becomes about 44 mA.

Formulas, sources and limits: Method.

The problem

In the field

Hioki clamp meter on a shorted stringSeaward Solar Power Clamp reading 4.51 A DC on a shorted stringFluke clamp meter reading 9.7 A DC on a shorted stringHioki clamp meter on a shorted stringFluke clamp meter reading 6.7 A DC on a shorted string
Shorted strings, found over several years. Current still flowing with the sun up, on five different clamp meters. Where the display is legible: 4.51 A to 9.7 A.

What the code and the fire service say

  1. Light means voltage; a disconnect doesn’t change that.
    • Array conductors are “generally energized during daylight hours”; opening a disconnect “provides a false sense of security.” Brooks & Rogers, IAEI 2015
    • “While exposed to the sun, PV systems remain energized, even after a building’s connection to the grid has been disabled.” IEA PVPS T12-09:2017, p. 8
    • NEC 2023 690.13(B): required label: “WARNING — ELECTRIC SHOCK HAZARD — TERMINALS ON THE LINE AND LOAD SIDES MAY BE ENERGIZED IN THE OPEN POSITION.”
  2. The fire service documented the hazard.
  3. The code responded with rapid shutdown, which limits conductors, not modules.
    • NEC 2014 690.12: conductors “more than 1.5 m (5 ft) in length inside a building, or more than 3 m (10 ft) from a PV array” limited “to not more than 30 volts and 240 volt-amperes” “within 10 seconds.” IAEI 2015
    • NEC 2017–2023 690.12(B): 30 V outside / 80 V inside a 1 ft array boundary within 30 s, or a listed hazard control system (UL 3741).
  4. The residual hazard remains at the module.
    • Damaged modules, failed rapid shutdown, pre-2014 systems with none.
    • “The implicit requirement for ‘module-level shutdown’ that results from the 80 V limit has proven to be contentious.” IEA PVPS T12-09:2017, p. 26
    • A shorted string keeps pushing current whenever the sun is up, shutdown or not, and is not safe to disconnect under load.
    • Tarps are inconsistent: 2 of 4 left 1.8–2.1 A, >120 V. UL 2011 §9.1
    • NFPA 70E 2027 first draft, Article 390: “Prior to any work on an energized PV array, a risk assessment shall be performed” (390.3); “No work shall be performed on exposed PV terminals greater than the electric shock or arc flash thresholds” (390.4(A)).
      • Shock threshold (dc): 100 V and 40 mA (draft 390.2(2)).
      • Arc flash threshold: 1.2 cal/cm² (5.0 J/cm²) incident energy, the onset of a second-degree burn; the arc flash boundary is set where exposure reaches this level (NFPA 70E). Fluke
      • Thresholds as written in the draft; confirm section numbers against the issued 2027 edition. Draft: First Revision statements (PDF) · First Draft ballot (PDF)
Fluke clamp meter reading 5.9 A DC on a shorted stringSeaward Solar Power Clamp on a shorted stringFluke clamp meter on a shorted stringHioki clamp meter reading 9.08 A next to a lockout tag: shorted, pushing current when sunny, do not disconnect under loadFluke clamp meter reading 7.2 A DC on a shorted string
Same problem, different sites and meters (Hioki CM4374 and CM4371, Fluke 393 FC and an older Fluke clamp, Seaward Solar Power Clamp). The lockout tag says it plainly: shorted, pushing current when sunny; disconnect at dusk or night at 0 A; do not disconnect under load.

Not an isolated case

  • A shorted string won’t blow its own fuse. A string’s short-circuit current is only about 10% above its operating current; string fuses are there to protect the modules from back-feed, not to clear a short. McNutt, Sekulic & Dreifuerst, Solar Photovoltaic DC Systems: Basics and Safety, NREL, 2018, p. 1. PDF
  • The disconnect didn’t stop it. In a residential fire caused by a miswired string, opening the inverter’s dc disconnect “did not stop the current flow from the array”; the fire went out only after the panels were disconnected. Jackson, “Residential PV System Fire,” IAEI Magazine, May 2016. Article
  • The DC switch won’t clear it. A reversed string in the same MPPT is a direct short that can damage modules and “may cause equipment damage, energy generation reduction or even fire.” “The PV string cannot be turned off by switching off the DC switch”; it is fixed after dark. Solis, “Ensuring DC Polarity is Correctly Connected,” Solis Seminar ep. 46, Jul 19, 2022. Article
  • A utility works at night. To replace two burnt string fuse holders on a 900 V array, a utility crew waited for dark: “Solar panels can’t be turned off when the sun is shining on them.” Blanchard, “The Solar Farm Under the Moon,” Kauaʻi Island Utility Cooperative, Mar 21, 2024. Article
  • How short circuits happen. pv magazine USA, “Short circuits: How they happen and how to prevent them,” Aug 28, 2017. Article

What’s been tried

What I set out to do

Thinking ahead

Plug-in solar: safety first, while it’s still new

Plug-in kits put solar panels straight into homeowners’ hands, on balconies, porches and in yards, with no installer and, so far, no code of their own. The National Electrical Code doesn’t yet define plug-in solar (pv magazine USA, Oct 28, 2026). That makes now the time to set the safety habits, before the market sets them by accident.