Understanding 690.11 (2023)

J

jes25

Senior Member
Location
Midwest
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Electrician
I'm trying to understand this section conceptually. Let me offer a scenario.

I have a PV string circuit over 80V going to a string inverter, I must have a DC Arc Fault protection. So if use an inverter than has Arc fault protection built-in I am compliant. Yes?

If it is correct, I'm struggling a bit with the arc fault protection being on the "load-side" or "down-stream" side of the supply circuit. If the inverter can detect an arc on the dc supply circuit coming to it this would extinguish the arc, but it seems backwards from how I think about protection devices. Am I thinking about this right?
 
I think the question is whether 690.11 is supposed require interruption only of series arcs, or also parallel arcs.

For series arcs in the DC wiring going to the inverter, the inverter can open the circuit to stop the arcing. For parallel arcs, there is still a circuit even when the inverter disconnects.

I guess 690.11's language "arcing faults resulting from a failure in the intended continuity of a conductor" could be taken to mean only series arcs. Although I could argue that it covers parallel arcs as well, as "failure in intended continuity" could mean more continuity (arc to another conductor) as well as less continuity (series arc).

Cheers, Wayne
 
wwhitney said:
I think the question is whether 690.11 is supposed require interruption only of series arcs, or also parallel arcs.

For series arcs in the DC wiring going to the inverter, the inverter can open the circuit to stop the arcing. For parallel arcs, there is still a circuit even when the inverter disconnects.

I guess 690.11's language "arcing faults resulting from a failure in the intended continuity of a conductor" could be taken to mean only series arcs. Although I could argue that it covers parallel arcs as well, as "failure in intended continuity" could mean more continuity (arc to another conductor) as well as less continuity (series arc).

Cheers, Wayne
Click to expand...
Good point
 
Simply, if the protection stops current from flowing in the circuit then it will stop the arcing.

As Wayne alluded to, I think this requirement is mainly aimed at series arcs, such as might occur at poorly mated module connectors. That's probably spelled out somehow in the listing requirements. This requirement showed up around the same time as most steing inverters were going 'ungrounded' and incorporating fancier ground fault detection than a ground fault fuse. Possibly arc fault protection could work in combination with ground fault detection to detect parallel arcs on a functionally grounded system. But the main protection against parallel arcs is still ground fault detection, with the presumption that a first fault to ground will be fixed before an opposite polarity fault to ground creates a short circuit.
 
Side comment I always thought the 80V a odd number, It might be a fine number but I vaguely recall back in 2011 or whenever they were adding this stuff to the NEC they kinda picked this 80V out of a hat, the data studied the joules of energy in a possible arc, not sure if it was series or what, you might access some of the data online or via someone with a university access there was a UL task force on the issue. I think the later IEC 63027 standard that covers the same thing, with some of the same people involved settled on around 750 Joules, and the IEC 62109-1 Decisive Voltage Class (DVC) architecture, which defines 120V DC as the functional boundary for hazardous voltage with some research going back to our Edison DC days. So it could probably be changed to 120 VDC that might help the one guy doing a 96V system? LOL
But it might help keep the cost of North American PV gear down as having fewer regional variations means less cost.
 
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