Conductor ampacity adjusted for pickup

[Mbrooke]

Does relay pickup or fuse TCC require consideration in conductor sizing? My fear arises from the potential of having a fault above the feeder STE (Short Term Emergency rating) rating, but below the 200-250% pickup.

 

[waross]

Overload versus overcurrent.
A molded case breaker has a thermal trip that will trip after a time delay at 200% current.
It has an instantaneous trip that will trip instantly at from 500% to 1000% of rated current.
On a relay, one setting for overload andd a second setting for an over current.
These settings may be on separate relays.
eg. A motor starter. The instantaneous Trip Breaker provides over-current protection and the Overload Relay provides overload protection.
Fuses?
Dual element fuses address this issue. The melting alloy element provides overload protection and the fusing element provides over current protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Right, but overload for a fuse does not start until 220-240% of its listed rating. Similar for a Hydraulic Recloser.

 

[waross]

It depends on the application.
Respectfully, I don't share your concern.
That is not to say that I would not consider it an issue for investigation.
If I were to investigate, I would dig into the catalogue and compare the ampacity of a number of typical feeder cables with the damage curve.
I suspect that your concern may be manifest in only rare and unusual installations.
But now that you bring it up, I am mildly interested and will be following this thread.
Please share the results of your investigation.
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

While I have the damage curve for URD and XLPE cables, I don't have the damage curve of tree wire or spacer cable.

 

[Mbrooke]

@Warros: Here is one typical example of a fault being below the pickup setting of a device:

https://www.facebook.com/wethersfieldCTPD/videos/861672144279489/

The problem is worsened due to the drying of the earth around the fault. It can continue indefinitely until manually de-energized.

 

[bacon4life]

Are you talking thermal damage from a through fault? Or a conductor burndown situation where the arc is anchored to a single spot resulting in rapid local heating. EPRI's free report on conductor burndown of small wire and treewire is available at

https://www.epri.com/#/pages/product/1017839/?lang=en-US.

EPRI found much shorter burndown times than earlier research. Glad you posted this question, as finding the EPRI report this morning may change our fusing practices for small wire. Also, the EPRI report found that Arc Protective Devices dramatically improve the burndown characteristics of tree wire.

 

[Mbrooke]

Conductor/Insulation damage across the whole feeder from an uncleared fault being above the conductor's rating but below the relay pickup.