UL508A and UL1741


Senior Member
I need to have a protective relay panel built to comply with utility interconnection requirements that will interface with a photovoltaic system; it is designed as backup protection to trip the main breakers in the switchgear by sensing voltage and frequency. It does not interface with the PV inverters at all; except by turning off the power. The UL508A panel shop I asked to quote says they cannot build the panel because UL 508A says:

"1.16 Equipment for the control of fuel cells, photovoltaic systems, or utility interactive systems are covered by the Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources, UL 1741."

I am perplexed. I agree that UL 1741 covers the inverter. But, does UL 1741 also cover a protective relay panel that trips a breaker in switchgear? Does this mean I have to go to a UL 1741 certified panel shop to have the panel built? Is there any such thing as a UL 1741 panel shop?

Any one else have any experience with this?


Senior Member
Never heard that before. Go to another shop. There is no such thing as a 1741 listed panel shop that I have heard of. I just work with a protection relay shop, they do the programming, testing, utility verification, etc. and deliver a system ready to be installed. The protection relays I have seen do not interface with the PV system. They monitor the utility POCC and operate a recloser to isolate the site if they pick anything up.
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Senior Member
I recall another thread on this forum that raised this issue. I believe the guy who started that thread called UL and someone quoted him that same language. No real world issue here that anyone should be worried about, just legalistic BS and perhaps a severe case of CYA.
Of course, once a single interactive inverter injects some current into the grid somewhere then that entire grid is subject to UL 1741. Yeah, right. :slaphead:


Staff member
I would also argue that the panel does not control a utility interactive system to any greater degree than a main or branch breaker does. It just protects the wiring and the grid from unwanted exposure.
If it were part of the required anti-islanding system of the inverter it would be a different story. But the inverter anti-islanding is self contained and listed.


Senior Member
Kind of on the same topic, I had an interesting talk with a utility engineer the other day. He was telling me about the problems they had in the past with larger PV systems dropping offline before the protection relay at the POCC would have disconnected the system. Seems that the setpoints for the protection relay were wider than the settings in the inverters for over/under voltage and over/under frequency so the inverters were shutting down early. The utility folks just assumed that the settings in the relay were all that mattered and never even asked about the inverter settings. Caused a lot of problems on the distribution system before someone figured out what was happening.