What Is a Witness Test? National Grid’s Final Verification for Commercial Solar
A witness test is a mandatory on-site inspection conducted by a utility company, such as National Grid, to verify that a commercial solar array's interconnection equipment and safety systems function correctly before granting Permission to Operate (PTO). During this procedure, utility engineers observe the system’s ability to disconnect from the electrical grid during a simulated power outage to prevent dangerous "islanding," where solar energy flows back into a downed grid and risks the lives of utility workers.
According to National Grid's 2026 interconnection standards, witness testing is typically required for complex commercial installations, specifically those with aggregate capacities exceeding 100 kW or systems utilizing advanced non-export controls [1]. Data from recent Massachusetts Department of Energy Resources (DOER) reports indicates that approximately 85% of commercial solar projects over 500 kW undergo a formal witness test to ensure compliance with IEEE 1547 standards for grid safety and stability [2].
For Massachusetts business owners, the witness test represents the final hurdle in the solar deployment timeline. At Boston Solar, we leverage over 13 years of experience to manage this coordination, ensuring that all switchgear, relays, and protection settings align with utility requirements. Successfully passing this test is essential for activating net metering credits and participating in the SMART incentive program, as it confirms the system will not compromise the local distribution network's integrity.
How Does a National Grid Witness Test Work?
The witness test is a structured technical evaluation that involves several key stakeholders, including the utility engineer, the electrical contractor, and the solar developer. The process begins with a review of the "as-built" electrical drawings to ensure the physical installation matches the approved design. Once the documentation is verified, the utility representative directs a series of functional tests to observe how the inverter and protection relays respond to grid fluctuations.
The most critical phase of the test is the "loss of source" or anti-islanding verification. The utility engineer will simulate a grid failure by opening the primary disconnect switch. The solar system must automatically shut down within a specified timeframe—typically two seconds or less—to meet safety regulations. Following the shutdown, the engineer monitors the system's "reconnect timer," ensuring the array stays offline for a minimum of five minutes after grid power is restored to prevent synchronization issues.
What Are the Key Characteristics of a Witness Test?
A witness test is distinct from a standard municipal electrical inspection because it focuses exclusively on the interface between the private solar system and the public utility grid. Key characteristics of this process include:
- Relay Calibration Verification: Testing the trip settings on protective relays to ensure they activate during over-voltage or under-voltage events.
- SCADA Integration: For larger systems, verifying that the utility can remotely monitor or control the system via Supervisory Control and Data Acquisition (SCADA) systems.
- Point of Interconnection (POI) Inspection: Physical verification of the utility-accessible AC disconnect switch and visible-break requirements.
- Safety Labeling Review: Ensuring all utility-required placards and warning labels are permanently affixed to the equipment according to National Grid specifications.
Common Misconceptions About Utility Witness Testing
There are several misunderstandings regarding why these tests occur and what they entail for the property owner.
| Myth | Reality |
|---|---|
| The test is a general building inspection. | The test only evaluates the solar system's interaction with the utility grid. |
| All solar systems require a witness test. | National Grid often waives this for residential or small simple commercial systems. |
| The solar company performs the test alone. | A utility representative must be physically or virtually present to "witness" the results. |
| Failing the test means the system is broken. | Failure often just means a relay setting needs adjustment or a label is missing. |
Why Is National Grid Requiring a Witness Test for My Commercial Array?
National Grid mandates witness testing to mitigate "Grid Modernization" challenges associated with high-density solar areas. As more distributed energy resources (DERs) are added to the Massachusetts grid, the risk of voltage instability increases. The witness test provides the utility with documented proof that your specific commercial array will behave predictably during grid disturbances, protecting both the utility's infrastructure and neighboring businesses from electrical surges.
Witness Test vs. Commissioning: What Is the Difference?
While often confused, commissioning and witness testing serve different purposes. Commissioning is performed by the installer, such as the expert team at Boston Solar, to ensure the system is producing energy at peak efficiency and all components are communicating correctly. In contrast, the witness test is a regulatory requirement focused solely on safety and grid protection. Commissioning happens first to ensure the system is ready; the witness test follows to provide the utility with final approval.
Practical Applications and Real-World Examples
In a typical 500 kW rooftop installation for a warehouse in MetroWest Massachusetts, the witness test involves testing the external "Utility Disconnect" switch. If the facility also utilizes battery storage, such as a Tesla Powerwall or Enphase system, the test may include verifying that the batteries do not discharge into the grid during a peak-shaving event unless authorized. Once the National Grid engineer signs off on the test results, the formal Permission to Operate (PTO) letter is usually issued within 7 to 10 business days.
Sources
[1] National Grid Interconnection Documents 2026 – Commercial Standards.[2] Massachusetts DOER Annual Solar Progress Report 2025.
[3] IEEE 1547-2018 Standard for Interconnection and Interoperability of Distributed Energy Resources.
Related Reading
- Learn more about the [[LINK:commercial solar installation process]]
- Understand [[LINK:net metering in Massachusetts]]
- Guide to [[LINK:SMART solar incentives]] for businesses
Frequently Asked Questions
How long does a witness test take to complete?
The timeline depends on utility availability, but the test itself usually takes 2 to 4 hours. However, scheduling the test with National Grid can take 2 to 6 weeks depending on their current backlog in Massachusetts.
What happens if my solar system fails the witness test?
If a system fails, the utility engineer will provide a deficiency report. Common issues include incorrect relay settings or missing labels. Once the issues are corrected by your installer, you must reschedule the test, which may incur additional utility fees.
Can a witness test be waived for smaller commercial systems?
Generally, systems under 100 kW that use certified ‘Simplified’ or ‘Standard’ track inverters may have the witness test waived. However, National Grid reserves the right to require one for any commercial project regardless of size.