Where does a series rating begin?

[curiousinspector]

Curious what would define as a series rated system and where others think a series rated system would start and end.
Example:
Available fault current at a fused service disconnect is 20,000.
The selected fuse limits the fault current on the load side to a value below 10,000 in the first half cycle.
The service disconnects supplys a feeder. The feeder has taps that supply multiple panelboards.
The panel board main breaker is rated for 10,000 as are the branch breakers.

I am aware of the what the code says in 240.86. This section uses the word circuit with no guidance on where the circuit begins. Also no guidance if upstream current limiting devices are considered when determining the available fault current at the line terminals of the downstream device.

Please let me know if you would consider this example a type of series rated system or not and why.
If it is not why would would it be important that a series rated label be required on a series system but nothing would be required for this example.
Thanks for your ideas and opions.

 

[augie47]

We address the AIC from the utilities Point-of-Attachment.
In the example you gave with 10k panelboards and 20k at the fusible service disconnect we would require engineered calculations or manufacturer documentation that the particular panelbords/breakers are series rated with the fuse in the disconnect.

 

[jim dungar]

The selected fuse limits the fault current on the load side to a value below 10,000 in the first half cycle.

This may not be true when circuit breakers and fuses are in series. It is possible for a circuit breaker contact to begin to open at the same time as a fuse element begins to melt. As soon as an arc is struck the increased impedance in the circuit may cause the fuse to operate slower and therefore change its clearing time.

Effectively you can only use the limited fault current of the fuse if you can guarantee that the downstream devices remain close during the fuse clearing time.

 

[curiousinspector]

Thanks for the responses.
I have done some research on the problems there may be from using the fuse chart.
I am looking more for a code answer about where a series system starts and what it considers as the line side current available at the breaker.

 

[augie47]

Since AIC is addressed in 110.10 which covers "electrical installations", I would say it becomes an issues as soon as the NEC applies (end of POCO supply).
Since equipment can have withstand/AIC ratings to 5k (possibly lower), it would apply to the utilization equipment.

Basically from appetite to ** ah! end.

 

[iceworm]

I'm a bit baffled by your queries. Your understanding is likely way past my comments. Adding to Auggie's and Jim's comments:

I have done some research on the problems there may be from using the fuse chart..

What problems would these be? Could you give an example?

The reason I'm asking is 240.86 gives two methods to select a series rated combination. Either there is PE stamped documentation or it is a tested combination. Neither of which mention anything about fuse charts.

Example:
Available fault current at a fused service disconnect is 20,000.
The selected fuse limits the fault current on the load side to a value below 10,000 in the first half cycle.
The service disconnects supplys a feeder. The feeder has taps that supply multiple panelboards.
The panel board main breaker is rated for 10,000 as are the branch breakers.

Please let me know if you would consider this example a type of series rated system or not and why.

I am aware of the what the code says in 240.86.

Then you already know the answer.
Do you have PE stamped documentation specifying the devices?
YES........NO

Is the combination lineside OCPD and the load side circuit breaker tested and marked on the end use equipment?
YES.........NO

Does the motor contribution meet the criteria of 240.86.C?
Yes.......NO

If the system meets these it it is series rated - if not, it's illegal.

I am looking more for a code answer about where a series system starts (and ends)

Again this is in 240.86. It starts with the specified lineside OCPD and ends with the loadside Circuit Breaker.

and what it (the code)considers as the line side current available at the breaker.

It's calculated. One starts with the source Available Short Circuit Current or source impedance, then add all of the distribution system impedance, out the the point of interest, and then calculate the Available SSC at that point.

IEEE 551 gives methods to calculate available short circuit current in nauseating detail

I'm still thinking this did not answer what you are asking.

ice

 

[mpd]

you cannot use the up-over-down method to calculate the amount of fault current at your load side panelboards if that is what your thinking, only tested combination"s can be used, fuse/breaker combination's or breaker/breaker combination's the manufacturer's can supply you with all that info

 

[curiousinspector]

I guess I am not getting across what I am looking for. I have an understanding of what is needed and why. Maybe presenting it this way will help.
Statement: The system listed above is not a series rated system. Because the fuse limits available current to a lower value than the marked interruting rating of the breaker the breaker is not being used on a circuit with a higher available fault current and 240.86 does not apply.
Please comment on that statement.
thanks.

 

[iceworm]

I guess I am not getting across what I am looking for.

I'm sure missing what you are looking for

Statement: The system listed above is not a series rated system.

Because the fuse limits available current to a lower value than the marked interruting rating of the breaker the breaker is not being used on a circuit with a higher available fault current and 240.86 does not apply.

240.86 applies fine. What you describe is not a series rated system because it does not meet 240.86 because:
1. you don't have a tested combination listed on the enduse panel;

2. You don't have PE stamped documentation.

This is not a science or physics issue. Its a legal issue.

Because the fuse limits available current to a lower value than the marked interruting rating of the breaker the breaker is not being used on a circuit with a higher available fault current

Just as an aside, since I don't think this matters a bit: Following jim's earlier comment, unless you have seriously under-represented yourself, you don't have the equipment to be able to tell when the CB is opening as comapred to the fuse completing it's melt. If you don't have the test equipment, how would you know?

 

[ron]

Because the fuse limits available current to a lower value

This is not possible.
Dynamic impedance makes it that you can't say that the fuse lets through anything less than the same value on its line side.

 

[jim dungar]

The fuse does not limit the current below the rating of the circuit breaker, unless you know that the breaker does not begin to open while the fuse is melting.

There is no general procedure for determining available fault current that considers the effects of current limiting protective devices.