Are Local Disconnects Needed

krista

Member
Location
Central Florida
Situation: New 30HP &40 HP chiller pumps being controlled through new VFD's. The VFD's are not within site of the pumps. BUT, the breakers in the switchboard feeding the VFD's ARE within site.
Question: Do I need local disconnects at the pump motors? The engineering drawings say I do, but I don't think I do because you can see the breakers from the pumps about 20' away. Need some expert input, please.
Thank you!
 

tom baker

First Chief Moderator
Staff member
Please see the expert opinion (fact) in NEC section 430.102
Basic rule is disconnect within sight of the controller and within sight of the motor but best to review all the rules.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Are the breakers within sight from the VFDs?

If you do install local disconnects, they should have an aux contact that breaks before the power contacts. The AUX contact should be connected to the control circuit of the VFD so that is goes to "coast to stop" when that AUX contact opens.

If this is an industrial occupancy with written safety procedures, part 2 of the exception to 430.102(B) will let you omit the disconnect at the motor. Many of the industrials in my area use this exception.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
Are the breakers within sight from the VFDs?

If you do install local disconnects, they should have an aux contact that breaks before the power contacts. The AUX contact should be connected to the control circuit of the VFD so that is goes to "coast to stop" when that AUX contact opens.

If this is an industrial occupancy with written safety procedures, part 2 of the exception to 430.102(B) will let you omit the disconnect at the motor. Many of the industrials in my area use this exception.
Was just reading up on this the other night because I got to thinking of all the jobs I’ve been on (industrial natural gas facilities). That never had local disconnects, makes sense now.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
In my opinion if the engineering drawings require local disconnects you need to supply them. It's not a code matter necessarily, it's a contract issue. You agreed to supply the equipment in accordance with the contract documents and the drawings are part of the contract documents.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I read it as the breakers are within sight of the motors, but the VFDs are remotely located.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I read it as the breakers are within sight of the motors, but the VFDs are remotely located.
My question is related to the requirement that the disconnect for the VFD be located within sight from the VFD. There is no provision for that disconnect to be remote, even if lockable.
 

electricman2

Senior Member
Location
North Carolina
Occupation
Retired Electrical Contractor
In my opinion if the engineering drawings require local disconnects you need to supply them. It's not a code matter necessarily, it's a contract issue. You agreed to supply the equipment in accordance with the contract documents and the drawings are part of the contract documents.
I agree. Unless you can get a change order you pretty much have to install per drawings.
 

paulengr

Senior Member
Was just reading up on this the other night because I got to thinking of all the jobs I’ve been on (industrial natural gas facilities). That never had local disconnects, makes sense now.
Depending on the area classification (hazardous location), disconnects are a bad idea anyway. Just one more thing to purge and pressurize.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Question: Do I need local disconnects at the pump motors? The engineering drawings say I do, but I don't think I do because ...
Independent of code, the designer said yes, so yes. And, BTW, local disconnects are often specified for local (plant) lockout rules. As I think others have said, be sure they include an early break contact set for the drive enable so a running drive cannot be disconnected.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Independent of code, the designer said yes, so yes. And, BTW, local disconnects are often specified for local (plant) lockout rules. As I think others have said, be sure they include an early break contact set for the drive enable so a running drive cannot be disconnected.
While I think the aux contact interlock is a good idea, disconnect switches are not there to be used as some kind of emergency or other kind of stop switch. Operators (and maybe electricians) need to be trained not to open the local disconnect switch as a way to stop a motor.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
While I think the aux contact interlock is a good idea, disconnect switches are not there to be used as some kind of emergency or other kind of stop switch. Operators (and maybe electricians) need to be trained not to open the local disconnect switch as a way to stop a motor.
Is there a certain size reference at which it becomes impractical to use a local disconnect as means for starting and stopping a motor.

I can imagine Larger motors disconnects having some nasty arcs when opening with hand speed.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Is there a certain size reference at which it becomes impractical to use a local disconnect as means for starting and stopping a motor.

I can imagine Larger motors disconnects having some nasty arcs when opening with hand speed.
They are not intended to be used for that purpose at all. In fact, IIRC, some are not listed to even open under load.

They are supposed to be used to isolate power from the motor. Not as a start/stop switch.

Now there are some manual motor starters that are designed for this purpose and can also be used as a disconnecting means.
 

Dsg319

Senior Member
Location
West Virginia
Occupation
Journeyman Electrician
They are not intended to be used for that purpose at all. In fact, IIRC, some are not listed to even open under load.

They are supposed to be used to isolate power from the motor. Not as a start/stop switch.

Now there are some manual motor starters that are designed for this purpose and can also be used as a disconnecting means.
That makes sense. Let me ask this if it makes any sense at all. At what point and size should one consider using a starter instead of mechanical switch, toggle switch for smaller motors etc..
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't really understand your question. Manual motor starters are typically limited to relatively smaller motors.

If you need a local disconnect just get one and put it out there. Don't try to use the local disconnecting means as a motor controller.
 
Top