Mike Sheehan, P.E., works on state level rulemaking and workshops for IREC, and the UEDS is his particular area of expertise. He’s a highly technical guy (he was one of the original members of the IEEE 1547 Committee on Distributed Resources), and exceptionally adept at simplifying those technical details into easily understandable bits for the rest of us. I wanted to understand the issues surrounding UEDS, so I asked him a few simple questions about it. For all of us non-technical types, here’s that dialog:
IREC: Mike, what’s a UEDS? It sounds like some medical device.
MS: A utility external disconnect switch (UEDS) is a device that the utility uses to isolate a renewable energy system to prevent the electric source, usually from a distributed energy source like a PV system, from accidentally sending power to the utility grid during routine or emergency maintenance by utility personnel. Typically, the UEDS is installed in a location that is visible and lockable, yet easily accessible by utility personnel. This has been an ongoing issue for years, since the 1980’s actually.
IREC: That’s a long time to have an issue on the table. Tell me there’s been progress.
MS: Yes, and yes. For years, utilities have historically treated customer-sited generation equipment connected to the grid—like that PV system on the roof of your house– with the same scrutiny as their large central power plants. The National Electric Code® requires that grid interactive inverters meet the safety and operational requirements of Underwriters Laboratories (UL) standard 1741 (which requires a disconnect switch). In response to the NEC Code, the utilities felt they should have their own disconnect switch. In regulatory proceedings, there has been misunderstandings and confusion about the NEC requirement and the utility requirement. Since there is a wide variety of generator types and installations, this dual disconnect switch approach may cause excessive interconnection requirements for small, inverter based generating systems, and even though the requirement has been labeled and addressed as a safety issue, it’s historically has been viewed as a cost barrier by the PV industry. Interestingly, the UEDS switch is never used. In fact, in 2008 over half of the installations did have a UEDS.
IREC: So tell me about the progress.
MS: Only in the past couple of years have we made inroads with this practice. In addition to UL 1741 and the interconnection requirements of Institute of Electrical and Electronic Engineers (IEEE) standard 1547-2003, a trend appears to favor eliminating the requirement for a UEDS on small inverter based systems. In 2007, three California utilities, Pacific Gas & Electric (PG&E), the Sacramento Municipal Utility District (SMUD), and SDG&E voluntarily dispensed with the need for a UEDS on most inverter-based solar systems under 10 kW. In addition, Xcel Energy in Colorado recently announced that it will no longer require a UEDS on PV systems of 10 kW or less, as long as the system uses a UL 1741 certified inverter.
Back in 2005, Chris Cook wrote a great piece for DOE’s Million Solar Roofs program that addressed the UEDS (MSR Case Study: Overcoming Net Metering and Interconnection Objections: New Jersey MSR Partnership DOE/GO-102005-2192 September 2005). For that paper, Cook researched PV systems in the U.S. with external disconnect switches, looking at two primary areas: 1) frequency of use; and 2) safety enhancement. After studying a number of systems and their incident rates, Cook found that none of the external disconnect switches studied had been used by utility maintenance staff. Furthermore, despite their lack of use, no safety incidents had been reported. In fact, he found that the external disconnect switch gave linemen a false sense of security, thereby increasing the probability of human error. Therefore, following standard utility operating procedures and treating the line as live enhanced safety more so than having an external disconnect switch.
IREC: Interesting. Can we deduce that Cook’s findings were solid, and the decision by utilities to do away with their UEDS can be construed as a best practice?
MS: Absolutely. In fact, the recent Connecting to the Grid (6th edition), in the last section called ‘Best Practices,’ we recommend requiring compliance with IEEE 1547 and UL 1741 for safe interconnection.
Our work and those two reports I mentioned earlier have proven influential. Over the past year, Florida and North Carolina both adopted interconnection standards that ensure customers with Inverter based systems under 10 kW will not have to pay an additional cost for installing a UEDS. New York additionally set the threshold at 25 kW for a UEDS and New Hampshire set it at 100 kW. California and Nevada prohibit the use of a UEDS for systems under 1 kW. Between California and New Jersey, these states represent the lion’s share of small 10kW PV systems. So far, eight states have waived the requirement for a UEDS on small inverter based systems. But this trend is not uniform. In 2008, Illinois required the use of a UEDS, even for the smallest systems. So while we’re making progress, we’re really encouraging utilities to follow the lead of other utilities with lots of experience and their best practices when it comes to UEDS.
IREC: I’m sure there’s information about this. Where can folks go for more?
MS: In 2008, two reports were published on this subject. The first was an NREL report titled Utility InterconnectedPhotovoltaic Systems: Evaluating the Rationale for the Utility Accessible External Disconnect Switch. The report concluded that a UEDS is not necessary for inverter-based systems under 10 kW.
The second report, published by the Solar America Board for Codes and Standards: Utility External Disconnect Switch: Practical, Legal and Technical Reasons to Eliminate the Requirement, concluded that half of all photovoltaic installations in the US in 2007 were installed without a UEDS. Additionally, the report showed that the operational histories of these systems demonstrate that a UEDS provides little, if any, additional safety, when PV hardware meets UL and IEEE standards and when the PV is installed in compliance with the requirements of the NEC.