Using 3-PH MV contactor for single phase loads

[rockman7892]

Is it permissible with using a 3-Ph 4.16kV draw-out contactor to feed a single phase load? A customer has a need to feed a single phase L-N load and wishes to just remove (2) of the fuses from the contractor truck to provide single phase. Obviously the MV MCC would need to contain a neutral from the transformer to be able to accommodate single phase load. I'm assuming that the protective relay would also need to be adjusted to accommodate single phase load.

Do manufacturers of MV draw-out contactors make a version of contactors that is unfused? I know that the fuses are required for interrupting since the contactors interrupting rating is extremely low but in cases where the available fault current is less than the contactor interrupting rating is there an option to have an unfused contactor?

 

[waross]

Why does he want to remove the fuses?
Some breakers will not close unless all three fuses are installed.


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

 

[rockman7892]

These contactors are going to feed temporary test equipment in which the test equipment will be changing over time. In some cases they will need to test 3-Ph equipment and in others just singe phase is needed. So cables will be routed from contactor to load to account for 3-Phase and final connections to test equipment each time will determine if 1-Ph or 3-Ph will be utilized.

In order to eliminate voltage on the ends of cables that weren't connected during single phase testing the thought was to remove the respective fuses in the fuse truck.

 

[waross]

I would run all three phases and neutral to a junction box at the load and then use two conductor cable or three conductor cable for the final connection to the equipment.
Using a three conductor cable sometimes for three phase and sometimes for single phase and changing back and forth at the breaker is an accident waiting to happen.

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

 

[jraef]

You are correct in that in most cases, the fuses are a necessary component in the assembly in that the contactors will not be rated to open under a fault, they rely upon the fuses to clear the fault first.

You can get single pole MV vacuum contactors, but hey will not be connected to "switchgear" however. A bigger potential problem for you however might be the concept of single phase power from a MV source; its not something that people do, so it's hard to find a Neutral bus setup in MV switchgear, especially if the neutral is resistance grounded. Not impossible, just not common.

Assuming that you have 3 phase MV switchgear or an MV MCC to feed power from AND you have access to the Neutral of a 4 wire transformer, there is nothing requiring you to connect all three phases to anything. No need to pull the fuses, just don't connect any wires to the load terminals. That way your other two unused phases on the disconnect are "spare fuse holders".



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Historically, 4160 was derived from the wye connection of 2400 Volt windings.
Sorry if I quote the Canadian Code. It is quite well harmonized with the NEC but not completely.
I always hope that an NEC user will either provide the appropriate NEC rule or point out the difference.
That said:
Under the CEC if a system incorporates a neutral conductor the neutral must be grounded.
Jeff, with a single phase to neutral load I see no issues with a two conductor cable in the field.
Likewise with a three phase load I see no issues with a three conductor cable in the field.
With a three conductor cable and changing between a three phase load and a line to neutral load I can hear someone saying;
"No problem, what could go wrong?"
Plenty.
Mistakes happen and this would be a setup for mistakes.

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

 

[Bill West]

jraef & waross, if I can expand on the sound points you have made:
-it needs to be known if the supply transformer is wye or delta
-it needs to be known if the single phase devices to be tested are L-L, L-N or both
What I quickly think of for 4kv loads are:
-industrial motors
-large commercial 3 phase transformers feeding LV. Single phase loads may show up after them
-utility distribution including single phase branches
So the test subjects sound like they would be 3 phase or single elements of typical 3 phase parts.

For all of these I would take Bill R's connections further. Set up a terminal panel with the 3 phases across and the neutral below, exactly like a 3 phase utility line on the street. Then use R-Y-Bu-W single conductor cables to connect to the test subjects. This arrangement would assert Bill's idea of making the safe handling of L-L and L-N single phase loads obvious. The unused phases could just be left bare inside the panel's cage. Alternatively the cables could start on a stud and when unused the tail could be hung on the stud. The cage could have a green light to confirm that the main 4kv contactor is withdrawn during set up work, no light - no work.

The OP's observation of having to suppress neutral protection during single phase tests remains correct.

Bill

 

[Bill West]

Oops, I should have considered that if the test subjects experience faults, loose single conductor cables will jump.

Bill