OCPD in a DC circuit

[philly]

I have a question as to weather it is typical to provide OCPD (fuses or breaker) on both positive and negative side of a DC circuit or if it is normally just required on one?

My understanding has always been a matter of how the circuit is grounded and weather the negative side is grounded or the system is floating. For instance with negative side grounded (Ex car battery) i typically only see OCPD on the positive side of circuit whereas as on a floating system (Ex station service batteries) I typically see both positive and negative side of circuit fused.

With a system that is not grounded on the negative side, and is floating, is there a potential for a ground fault to occur on the negative side of the circuit that the OCPD on the positive side would not clear?

 

[don_resqcapt19]

I have a question as to weather it is typical to provide OCPD (fuses or breaker) on both positive and negative side of a DC circuit or if it is normally just required on one?

My understanding has always been a matter of how the circuit is grounded and weather the negative side is grounded or the system is floating. For instance with negative side grounded (Ex car battery) i typically only see OCPD on the positive side of circuit whereas as on a floating system (Ex station service batteries) I typically see both positive and negative side of circuit fused.

With a system that is not grounded on the negative side, and is floating, is there a potential for a ground fault to occur on the negative side of the circuit that the OCPD on the positive side would not clear?

That condition would just create a grounded system and not cause any current flow.

 

[tortuga]

Philly: NEC 240.21 requires a OCPD in each ungrounded conductor, so say I had a 48V DC System and I grounded the + I would need a OCPD in the -. For ungrounded DC you would put a OCPD in each.
DC system grounding requirements are in part 8 of 250

 

[philly]

That condition would just create a grounded system and not cause any current flow.

Philly: NEC 240.21 requires a OCPD in each ungrounded conductor, so say I had a 48V DC System and I grounded the + I would need a OCPD in the -. For ungrounded DC you would put a OCPD in each.

Thanks. I'm trying to follow what happens when a ground fault occurs on an ungrounded system that requires OCPD on both positive and negative.

So lets say we have an ungrounded system and a GF occurs on negative side of circuit. As Don points out above there will be no fault current flowing but rather you just create a grounded system. When a 2nd GF occurs on the system that is when a true "ground fault" would occur on the system with fault current flowing to be interrupted by OCPD?

In the above scenario if the 2nd fault occurs on positive side of circuit then fault current would flow from positive GF location through to 2nd GF location and thus trip the OCPD on the positive side of circuit.

With DC current flowing in only one direction from positive to negative I'm trying to envision a scenario where fault current would flow on the negative that would not be seen by the positive OCPD?

 

[GoldDigger]

Thanks. I'm trying to follow what happens when a ground fault occurs on an ungrounded system that requires OCPD on both positive and negative.

So lets say we have an ungrounded system and a GF occurs on negative side of circuit. As Don points out above there will be no fault current flowing but rather you just create a grounded system. When a 2nd GF occurs on the system that is when a true "ground fault" would occur on the system with fault current flowing to be interrupted by OCPD?

In the above scenario if the 2nd fault occurs on positive side of circuit then fault current would flow from positive GF location through to 2nd GF location and thus trip the OCPD on the positive side of circuit.

With DC current flowing in only one direction from positive to negative I'm trying to envision a scenario where fault current would flow on the negative that would not be seen by the positive OCPD?

Consider an ungrounded DC system whose source is a stack of batteries or PV panels. An internal ground fault within the source combined with a ground fault on one side of the external circuit will cause current to flow only on the faulted conductor. Hence both external conductors must have OCPD.
In the case of a wound DC generator you can still have an internal ground fault as either the first or second fault, with the same result.

 

[philly]

Consider an ungrounded DC system whose source is a stack of batteries or PV panels. An internal ground fault within the source combined with a ground fault on one side of the external circuit will cause current to flow only on the faulted conductor. Hence both external conductors must have OCPD.
In the case of a wound DC generator you can still have an internal ground fault as either the first or second fault, with the same result.

Thanks that example makes sense. Let me see if i'm following correctly...

Lets say I have a wound DC generator connected to an external circuit feeding a load and I have OCPD located on positive and negative after generator.

From what you describe above there could be a first fault within the generator on the positive side that occurs BEFORE the positive OCPD and then a second fault that occurs somewhere on the negative side of the external circuit. In this case with current flowing positive to negative the fault current would from from generator through L-G fault on generator positive through to L-G fault on external negative circuit and back through the OCPD on negative returning to negative side of generator. In this case the positive OCPD would never see fault and only the negative OCPD would be able to detect and interrupt. Without that negative OCPD the fault would continue until it burned clear. Am I understanding this correctly?

How does that change however if the system is intentionally grounded on the negative side? Wouldn't the same example above be able to occur with the only difference being that fault current would flow on first L-G fault in generator as opposed to waiting for 2nd fault (since negative side is already grounded)? Its my understanding that if system is intentionally grounded then only one OCPD on ungrounded portion of circuit is required?