Fuse Servicing 690.16(B) Interrupt Rating vs. Load-Break-Rated

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SolarAlternativesDesign

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For a PV combiner box, does NEC 690.16(B) permit use of a fused combiner box with touch-safe fuse holders, or is a switch type disconnect required? 690.16(B) requires a sufficient interrupting rating (referring to 690.17), but states that "Non-load-break-rated disconnecting means shall be marked "Do not open under load." Does a touch-safe fuse meet the definition of sufficient interrupting rating? What are the definitions of interrupting rating and load-break rating?
 
The load-break rating is based on the ability of the device to interrupt the maximum current imposed on it under normal operating conditions.

Interrupting capacity is the based on the ability of the device to interrupt the maximum current imposed on it during fault conditions.

They're two very different ratings. Off hand, I see no reason why a touch-safe fuse holder wouldn't meet the requirements of 690.19(B) but I'm not a solar guy, so I'll wait for someone else to chime in there.
 
big john said:
The load-break rating is based on the ability of the device to interrupt the maximum current imposed on it under normal operating conditions.

Interrupting capacity is the based on the ability of the device to interrupt the maximum current imposed on it during fault conditions.

They're two very different ratings. Off hand, I see no reason why a touch-safe fuse holder wouldn't meet the requirements of 690.19(B) but I'm not a solar guy, so I'll wait for someone else to chime in there.
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A touch safe fuseholder is not automatically safe for load interruption by its removal. And few automotive fuseholdesr are rated for higher voltage DC.
 
GoldDigger said:
A touch safe fuseholder is not automatically safe for load interruption by its removal...
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No argument, but if he labels it per the requirements of that code section, it appears it would be compliant.
...And few automotive fuseholdesr are rated for higher voltage DC.
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Automotive? I think he's describing standard DIN rail mount touch-safe fusholders which are definitely rated for high voltage AC and DC.
 
SolarAlternativesDesign said:
For a PV combiner box, does NEC 690.16(B) permit use of a fused combiner box with touch-safe fuse holders, or is a switch type disconnect required? 690.16(B) requires a sufficient interrupting rating (referring to 690.17), but states that "Non-load-break-rated disconnecting means shall be marked "Do not open under load." Does a touch-safe fuse meet the definition of sufficient interrupting rating? What are the definitions of interrupting rating and load-break rating?
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Combiner boxes without a disconnect immediately nearby, are no longer a code-compliant solution. They were in 2008, but are no longer in 2014.

Most combiners that will meet this particular requirement, will have an integrated load-break disconnect on the combined output. The fuseholders inside will be touch-safe, but are not load-break rated. You need to go out of your way to open them under load, because the disconnects come with a door interlock, that you cannot open the door without first opening the load-break disconnect.

There is no requirement for a "pre-disconnect" on the input side to a touch-safe fuseholder. Disconnecting the master combined output will suffice.
 
Exactly right. Requirements for dc combiners vary considerably by Code cycle.

Check out is article about The Evolution of DC Combiners:

?The products we use vary substantially based on the Code cycle a particular jurisdiction enforces and whether the application is roof or ground mounted. If an AHJ enforces NEC 2008, we use standard dc combiners in all applications. Where NEC 2011 applies, we deploy roof-mounted systems that provide dc arc-fault protection, but use standard dc combiners in ground-mounted applications. In jurisdictions that have adopted NEC 2014, we provide dc arc-fault protection in all applications and rapid shutdown for roof-mounted systems.?

?Randy Batchelor, Borrego Solar
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See also this article about NEC 690.16(B) Fuse Servicing Requirements.
 
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big john said:
The load-break rating is based on the ability of the device to interrupt the maximum current imposed on it under normal operating conditions.

Interrupting capacity is the based on the ability of the device to interrupt the maximum current imposed on it during fault conditions.

They're two very different ratings. Off hand, I see no reason why a touch-safe fuse holder wouldn't meet the requirements of 690.19(B) but I'm not a solar guy, so I'll wait for someone else to chime in there.
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So, Big John, would we use the PV module's maximum power current to calculate required load break rating and the module's short circuit current to calculate required interrupt rating? The two values would be very similar. Your definition is the best one I've seen so far. Thanks.
 
SolarPro said:
Exactly right. Requirements for dc combiners vary considerably by Code cycle.

Check out is article about The Evolution of DC Combiners:



See also this article about NEC 690.16(B) Fuse Servicing Requirements.
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Those articles are helpful and I read them carefully. I'm working with the 2011 NEC, which is a little ambiguous depending on how load break and interrupt rating are interpreted. In the 2014 NEC DC combiners on roofs will definitely require load break disconnecting means, which will not be a touch safe fuse holder, due to the new 690.15(C). But in the 2011 NEC, as the SolarPro "The New NEC 690.16(B) Fuse-Servicing Requirements and Combiner Boxes" article states, "a load-break-rated disconnect is not specifically required" and different people seem to make different interpretations. The Borrego approach seems wise, but isn't arc fault protection available via the inverter in some newer inverters?
 
Note that if the system uses AFCI inverters, opening a touch-safe fuseholder under load will produce enough of an arc to shut down the inverter.
 
SolarAlternativesDesign said:
So, Big John, would we use the PV module's maximum power current to calculate required load break rating and the module's short circuit current to calculate required interrupt rating? The two values would be very similar. Your definition is the best one I've seen so far. Thanks.
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Be careful, you're overlooking something important...

With a combiner the load break rating is the value for one string whereas the fault current rating needs to be for all the strings paralleled at that combiner.
 
jaggedben said:
Be careful, you're overlooking something important...

With a combiner the load break rating is the value for one string whereas the fault current rating needs to be for all the strings paralleled at that combiner.
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Then the load break rating would be the max power current of one string and the interrupt rating would be the sum of short circuit currents of all the combined strings - so the interrupt rating would be a few times larger than the load break rating. Correct?
I just noticed something that may provide an explicit answer to my original question. in 2011 NEC, 690.16(B) requires "The disconnecting means shall... comply with 690.17." 690.17 requires "The disconnecting means... shall consist of a manually operable switch(es) or circuit breaker(s)" So that does not allow a touch safe fuse holder or pullout regardless of its rating, does it?
 
SolarAlternativesDesign said:
Then the load break rating would be the max power current of one string and the interrupt rating would be the sum of short circuit currents of all the combined strings - so the interrupt rating would be a few times larger than the load break rating. Correct?
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Correct. It could be 20 times larger, or more, if you have a large combiner. It's probably a good idea to use the short-circuit current for one string for the load break rating, because under ideal conditions the max power current may be more than nameplate. This is not likely to require a higher rated device in most cases, I would think.

I just noticed something that may provide an explicit answer to my original question. in 2011 NEC, 690.16(B) requires "The disconnecting means shall... comply with 690.17." 690.17 requires "The disconnecting means... shall consist of a manually operable switch(es) or circuit breaker(s)" So that does not allow a touch safe fuse holder or pullout regardless of its rating, does it?
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I don't know, can a touch safe fuse holder be considered a switch? It meets all the other requirements if you can consider it a switch.

Also note the exception to 690.17. I think that means that if you have a load-break rated disconnect on the output side of a combiner, you can use connectors right outside the combiner (i.e. within sight of and accessible to it) as the disconnecting means on the source side. The connectors are not load break rated which means your servicing method is to disconnect the whole combiner on the output side before disconnecting source circuits. You can do this even on a system that is running and has other combiners. This is a pretty cheap and easy solution, and something I recommend in any case to help commission and service systems. I suppose theoretically if you don't want to work in the combiner 'hot' then you disconnect all the strings before working in the combiner. But if the work is just to replace a fuse in a touchsafe fuse holder... At any rate, I believe such a setup meets the letter of the code.
 
jaggedben said:
Also note the exception to 690.17. I think that means that if you have a load-break rated disconnect on the output side of a combiner, you can use connectors right outside the combiner (i.e. within sight of and accessible to it) as the disconnecting means on the source side. The connectors are not load break rated which means your servicing method is to disconnect the whole combiner on the output side before disconnecting source circuits. You can do this even on a system that is running and has other combiners. This is a pretty cheap and easy solution, and something I recommend in any case to help commission and service systems. I suppose theoretically if you don't want to work in the combiner 'hot' then you disconnect all the strings before working in the combiner. But if the work is just to replace a fuse in a touchsafe fuse holder... At any rate, I believe such a setup meets the letter of the code.
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Well that's an interesting exception. I wasn't aware that connectors could be listed and identified for use as a disconnect. Would that be a special type of connector? Could you provide an example make and model I could look at? Or is this a use that common connectors are listed and identified for?

Thanks for following up.
 
SolarAlternativesDesign said:
Well that's an interesting exception. I wasn't aware that connectors could be listed and identified for use as a disconnect. Would that be a special type of connector? Could you provide an example make and model I could look at? Or is this a use that common connectors are listed and identified for?

Thanks for following up.
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Well, that's a good question with regard to DC connectors that I don't know the answer to. The only connectors I'm aware of having been declared to be listed and identified are Enphase AC connectors.

I'll just add that a touch safe connector seems consistent with the definition of an isolating switch in Article 100.
 
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