400 PV Low Ampere Fuse
Developing the first UL 248-19 compliant PV chip fuse for solar shingles: through two design pivots, a manufacturing patent, and two years of persistence.
PROJECT DETAILS
Scope
Bespoke product development
Timeline
2018-2020
Location
Philippines
Program/s
Chip Fuse Platform
01 CONTEXT & ROLE
An early-career project that tested every dimension of design engineering
The 400PV Series was one of my early projects at Littelfuse, following the cadetship programme in 2018. The brief came from Zinniatek, a New Zealand solar roofing company developing integrated PV shingles — roof tiles that function simultaneously as weatherproofing and electricity generation, in the same vein as Tesla's solar roof product paired with the Powerwall.
Zinniatek needed a circuit protection solution that met UL 248-19 — the standard governing fuses for photovoltaic power systems — in a compact 2410 surface mount footprint, suitable for embedding directly in each PV shingle cell. As the responsible design engineer, I owned the project end-to-end, from the initial brief through two full design pivots to production launch in 2020.
02 PROBLEM DEFINITION — THE WHY
Two specifications that pulled in opposite directions — within a fixed footprint
The brief set out two requirements that are fundamentally in tension for a low-current fuse: a 10,000A interrupting rating, and a 135% overload rating within one hour. The 375mA current rating and 86VDC voltage rating added further constraint within the 2410 body.
THE CORE TENSION A 375mA fuse requires a very fine element — sub-100 micron wire — to melt at low current. But UL 248-19's 10,000A interrupting requirement means the same element must safely contain the enormous arc energy of a full short-circuit fault. Meanwhile, the 135% overload requirement demands the element also melts reliably at just 1.35x rated current within an hour — a tight thermal window that the element's heat dissipation environment directly governs. |
The solution was wire-in-air: a sub-100um fusible wire suspended in an open cavity inside the fuse body, with no thermal contact to any surrounding medium except air. The air cavity gives molten material space to expand during a fault, enabling clean interruption. The thermal isolation means the wire retains enough heat to melt reliably at 135% overload — it cannot dissipate heat to a substrate or fill material. The architecture was clear. Making it manufacturable at scale was not.
03 DISCOVERY & RESEARCH
Mapping the design space — and identifying why the first solution couldn't scale
Discovery began with existing Littelfuse wire-in-air and open-body fuse designs, mapping which body materials and assembly methods could support sub-100um element handling at production volumes.
UL 248-19 requirements analysis. Worked through the full UL 248-19 standard to map all requirements the design had to satisfy -- not just the headline 10,000A interrupting rating and 135% overload, but environmental, humidity, and solderability tests required for PV certification.
Wire element modelling. Calculated the element diameter range that could satisfy both the 375mA rating and the 135% overload window, while remaining below the maximum safe element mass for open-cavity interruption at 10,000A and 86VDC. This established the sub-100um target.
Assembly feasibility review. Assessed existing wire-threading methods across Littelfuse's manufacturing infrastructure. Confirmed that no existing process could reliably thread sub-100um wire through a small ceramic cavity at the required production volumes without skill-dependent variability.
Body material evaluation. Reviewed alternative fuse body materials to the initial ceramic — evaluating machinability, cavity geometry accuracy, thermal properties, and compatibility with modified threading processes. This research identified the material that enabled the Design v2 approach.
04 SOLUTION & STRATEGIC APPROACH
Wire-in-air architecture — and the manufacturing process that made it viable
The 400PV's core architecture places a sub-100um fusible wire in suspension across an open air cavity inside the fuse body. Two terminations anchor the wire at each end; nothing else contacts the element. This is the wire-in-air principle.
Parameter | 400PV spec | Significance |
Current rating | 375mA | Low-rating fuse for individual PV cell / shingle protection |
Voltage rating | 86VDC | Per UL 248-19 PV circuit requirements |
Interrupting rating | 10,000A | Maximum fault current the fuse can safely interrupt |
Overload rating | 135% within 1 hour | Nuisance blow prevention at normal operating surges |
Footprint | 2410 (6.1 x 2.6mm) | Surface mount, compatible with PCB integration in PV shingles |
Wire element | Sub-100 micron, wire-in-air | Open cavity keeps element thermally isolated — essential for low-current precision |
Compliance | UL/CSA 248-1, UL/CSA 248-19 | Full photovoltaic circuit protection certification |
The wire-in-air approach satisfies both competing requirements: the thermal isolation enables low-current precision, and the open cavity provides the expansion volume for safe high-current interruption. The challenge was never the architecture — it was the process of reliably assembling a sub-100um wire into a small enclosed body at scale.
05 EXECUTION & ITERATION
Two design pivots, a manufacturing patent, and two years from brief to production
Execution was not linear. The project required two fundamentally different design approaches before reaching a manufacturable solution — the second of which led to a patent covering novel thin-wire assembly methods.
Design v1: Manual threading on ceramic body | Design v2: New body material, semi-automated process |
Outcome: unpredictable yield and rising cost — design abandoned |
Outcome: patent granted, production approved 2020 |
Design v1: Why it failed. The manual threading process was accurate in individual assembly, but at production lot sizes the variability emerged: some wires contacted the ceramic cavity wall rather than suspending freely. Wire touching the body creates a thermal bridge that causes overload failures — the element dissipates heat to the ceramic rather than retaining it for the 135% overload threshold. The only detection method was 100% X-ray inspection, which was both slow and expensive. The cost-per-unit made the design commercially unviable.
The pivot conversation. After identifying the process failure, I had to deliver a difficult message to Zinniatek — development delays and a full design restart. That conversation required transparency about the root cause, a credible alternative path, and a revised timeline. Navigating that without losing the customer relationship was as much a part of the project as the engineering.
Design v2: The new body material. Switching to a different fuse body material enabled a semi-automated threading process with significantly tighter dimensional control. The new material's cavity geometry was more amenable to automated alignment of the wire, reducing the probability of wall contact to near zero — eliminating the need for 100% X-ray.
Patent development. The semi-automated process required developing new methods for handling and positioning sub-100um wire within an open-cavity fuse body — methods that did not exist in the industry. These were documented and patented as US 11,355,298 B2, covering various approaches to producing thin-wire fuses in open-cavity configurations.
Iteration to production. Multiple wire materials, diameters, and assembly parameters were iterated before reaching the final production specification. Each iteration required electrical characterization, overload testing, and short-circuit qualification at 10,000A — a non-trivial test that requires specialized high-current laboratory infrastructure.
HONEST REFLECTION The project took approximately two years from brief to production — longer than anticipated, largely because the first design's failure wasn't detected until production lot sizes exposed the process variability. Earlier investment in process capability assessment before committing to Design v1 would likely have surfaced this risk sooner. |
06 RESULTS & IMPACT
The first UL 248-19 compliant PV chip fuse for solar shingle applications
The 400PV Series is listed as the first photovoltaic fuse on the market designed to protect roof shingles from reverse overcurrent. It enables PV shingles to integrate both the roofing material and the PV cell into a single unit, with embedded circuit protection — eliminating the need for separate panel-mounted fusing. The series is available globally through authorized Littelfuse distributors and is listed for photovoltaic shingle and cell applications.
Commercial traction after launch was limited — the solar shingle category remained a niche market despite the activity from Tesla and similar players at the time. But the 400PV development established foundational competencies in thin-wire open-cavity fuse design and manufacturing that directly informed the 806 Series and subsequent work.
PERSONAL IMPACT This was my first major project as a fully independent design engineer. The combination of a novel technical challenge, a customer relationship managed through a significant setback, a manufacturing patent, and a multi-year delivery built the engineering and professional foundations that every project since has drawn on. The habit of recognizing when a process cannot scale — and acting on that signal before sunk cost drives you further down a failing path — is something I learned here and have applied ever since. |
WHAT I'D DO DIFFERENTLY Earlier process capability assessment before committing to Design v1 would have surfaced the manual threading variability at small lot sizes rather than at production scale. Pilot runs at intended production volume — before full design commitment — would be a standard gate I would now insist on for any process-dependent precision assembly. | WHAT I'D REPLICATE The decision to pivot fully rather than iterate on Design v1 was correct. Once the root cause was identified as structural to the process rather than fixable through tolerance tightening, continuing would have extended timeline further and compounded cost. Making the pivot call clearly and early, with transparent customer communication, preserved the relationship and ultimately delivered a better product. |
