Overcurrent Relay as Primary OCPD

[pcanning87]

I'm looking at a 1000Vdc, 1200A circuit design which currently lacks overcurrent protection. My problem is that I've been unable to find a UL listed circuit breaker that's rated for 1000Vdc and 1200A (I have found IEC listed breakers that meet these requirements).

The circuit does currently have a 1200A, 1000Vdc motorized switch at the source of supply. Would a multifunction relay with CTs and appropriate setpoints for instantaneous overcurrent trip meet the NEC 240 OCPD requirements?

Thanks in advance.

 

[GoldDigger]

How fast is the motorized switch?? That is, how long will it take to actually interrupt the circuit if the OC relay is triggered?

 

[pv_n00b]

Interesting problem. Relay/recloser systems are commonly used to provide OCP on MV interconnections so I don't see any reason why they could not be applied to the DC circuit. But it would have to be a listed package. I would not hook up a relay to just any motorized switch and call it OCP. I'm impressed someone sourced a 1,200A 1,000V dc switch. It must be huge.

You could also look into a parallel CB package or perhaps there is a solution using fuses? If all else fails try to get the AHJ to accept an AMM to use the IEC rated CB.

 

[pcanning87]

I'll have to check with Eaton, it looks like somewhere between 40 milliseconds and 150 milliseconds. Is there a standard for the number of cycles it would have to meet to qualify as an OCPD?


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[jaggedben]

If it's DC there are no cycles.

Is this on a PV array output? Is the wiring rated for the total current output of all parallel connected PV sources?

If yes, why would OCP be required?

 

[pcanning87]

Right, of course, no cycles, it was way too late at night to be posting about the NEC.

It is a PV Array output, at the point of supply it's tied into a DC filter and then to the grid. My concern (and I believe the reason the code would require OCP) is that a fault on the conductors could cause a backfeed from the grid side.

As a follow-up, I was looking at 240.15 (A) which specifically says "A combination of a CT and an overcurrent relay shall be considered equivalent to an overcurrent trip unit". My system is ungrounded so 240.15 seems like it would apply. Thoughts?

 

[jaggedben]

There is no inverter?
(Sorry for asking questions instead of being helpful.)

 

[pv_n00b]

I hope there is more than a DC filter between the PV array and the grid, unless this is a DC grid. I'm now really curious about what you are building, at 1,200A in a 1kV array that is about 1MW at mpp.

NEC 240.15 says the relay can act as the trip unit, but it does not say what it is tripping. My thought would be that the relay and switch should be a package that are listed to work together to provide OCP. When I specify OCP for the MV interconnection I get a Cooper relay and recloser together that provide that function.

 

[pcanning87]

Haha yes there's a few other parts between the PV and the grid. I apologize I'm just not supposed to talk about the details too much (corporate bs).

I agree that strictly speaking it should be a single listed unit. I appreciate the input guys.


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