Disconnecting DC+ and DC- at combiner box

[arc_flashed]

Hoping someone can help with any code or safety concerns from the following situation. I am with an electrical contractor who was asked by a local utility to make some changes to their combiner boxes. Their existing combiner boxes have a disconnect switch that breaks only the DC+ before it leaves the enclosure. All of the DC- are connected to a multi pole lug only (no means to disconnect). Site maintenance asked if we could redo the combiner boxes so that the panel disconnect could break the positive and negative so they could isolate each array group individually for troubleshoot purposes.
Looking at NEC, I can see where both are disconnected at the invertor, but I'm not sure if it is allowed else where in the circuit.
Any direction on code or standard practice would be much appreciated as PV design is not something I am familiar with and would like to have some educated questions to ask.

 

[jaggedben]

The TL/DR is that what they are asking for is permitted and almost certainly required if the installation is being brought up to current code.

The longer answer...
The code and industry practice on this has changed over the last decade as inverters have changed and as understanding of the needs for safely servicing them have deepened. Until about a decade ago most inverters had a ground-fault fuse that connected one side of the system to ground (usually negative, but not for Sunpower). This was understood to be a 'grounded conductor' and was almost never opened by any disconnecting means. I recall in perhaps the 2011 code, there was a section of article 690 that prohibited disconnecting the 'grounded' conductor but it contained an exception for disconnects for servicing that disconnected all conductors simultaneously. So back then what you are being asked to do was permitted, but rarely done in my experience. However not being able to isolate DC strings on both sides causes lots of problems with troubleshooting, particularly with finding ground faults. And there's no real safety benefit to it. That's why you're being asked to do this.

Nowadays, the code defines those old systems differently, as 'functionally grounded', and 690 requires disconnecting means or 'isolating devices' to disconnect all conductors that are not solidly grounded. It was always very rare to find a solidly grounded system, and that would be pretty non-existent on larger systems and grid-tied systems (A small stand-alone off-grid power source might still be solidly grounded.) Meanwhile most (but not all) newer inverters in the last ten years don't use the 'functional' type of ground reference for ground-fault detection anyway. So nowadays both the DC+ and DC- are usually considered 'ungrounded', or at any rate are not solidly grounded, even if one side used to be considered 'grounded' And then the code requires that disconnecting means and isolating devices are available to disconnect both.

 

[Carultch]

Hoping someone can help with any code or safety concerns from the following situation. I am with an electrical contractor who was asked by a local utility to make some changes to their combiner boxes. Their existing combiner boxes have a disconnect switch that breaks only the DC+ before it leaves the enclosure. All of the DC- are connected to a multi pole lug only (no means to disconnect). Site maintenance asked if we could redo the combiner boxes so that the panel disconnect could break the positive and negative so they could isolate each array group individually for troubleshoot purposes.
Looking at NEC, I can see where both are disconnected at the invertor, but I'm not sure if it is allowed else where in the circuit.
Any direction on code or standard practice would be much appreciated as PV design is not something I am familiar with and would like to have some educated questions to ask.

It used to be the standard (NEC2014 and earlier) that you treat the grounded polarity (usually negative, but could be positive in special cases), as if it were a neutral. The grounded polarity would just be combined on a simple busbar, with no fusing or disconnects. The grounded polarity would be bonded to the EGC and GEC within the inverter through a ground fault fuse. You'd label the inverter, "if a ground fault is indicated, normally grounded conductors may be ungrounded and energized".

If you were to build a system with the inverters of 10 years ago to the current standard, you'd have to disconnect both polarities at every required location for a disconnect. Fuses are only required on one polarity, but disconnecting is now required on both. It's also much more common that inverters of today, don't ground either polarity (i.e. non-isolated inverters).

 

[arc_flashed]

Thank you all for the reply, that helps me get a better idea.

One more question if anyone can help clarify is the number of poles required to break the DC voltage. Current combiner box uses a 4pole switch and the DC+ passes through all 4 poles. Doing some reading it looks like this is to increase the air gap for extinguishing the arc which makes sense, but I cannot find anything detailing how to find/figure out this requirement.
Is it a per manufacturer thing or is there an actual standard/calculation?
I can't see getting an 8 pole switch (if it even exists) to mimic the current set up.

 

[jaggedben]

That's pretty much a manufacturer thing. For example some switches that are good for 600V AC are only good for 600V DC when two poles are used in series. Follow the switch labeling closely.