3 phase motor application

[Andy_Sanchez]

I am at a quandry with this. Can an accessory unit capable of being plugged in a 480 circuit be used?

Application:
VFD unit with overload protection.

Accessory: 5HP 480 volt motor with a three pole snap switch.

Situation: The 5HP motor is to be used with the VFD unit which has a 480 Volt plug which mates to the 480 volt receptacle.

Potential: This accessory has the potential of plugging directly to a source voltage, by an end user.

Advice: How can I explain this problem to the personell that there is a potential of Arc Flashing, in event of switch failure, no motor protection and .....

Please advise,

Andy

 

[jim dungar]

Potential: This accessory has the potential of plugging directly to a source voltage, by an end user.

Why not just use a non-standard plug arrangement or one that can be keyed so it only works with it's mating receptacle?

 

[kspifldorf]

It is not recommended to disconnect the load side of most VFD's, any of them in my book.

 

[jatrottpe]

There is a potential safety issue, if the plug and receptacle can be physically disconnected while the unit is energized. Means should be taken to prevent the accidential disconnection of plug and receptacle when energized (usually via an interlocked receptacle of some sort).

If the plug/receptacle does not have any interlocks that prevent the disconnection when energized, The person who is disconnecting (unplugging) the plug from receptacle can be exposed to a potential arc flash.

The incident energy of the arc flash will essentially be the same as it is a the 480V source. Having said that, the mechanic who would be disconnecting the plug shall be protected from the potential arc flash with proper PPE (arc flash suit, hood, gloves, etc).

I am assuming the facility has an arc flash procedure and policy in place and the mechanics are aware of the safety issues associated with arc flash hazards.

 

[480sparky]

Seems to me I saw some sort of receptacle that can act as a disconnect as well. In order to remove the cord, a certain action on your part must be done, and that disconnects the power from the receptacle.

Anyone else remember this, or am I dreaming?

 

[coulter]

There is a potential safety issue, if the plug and receptacle can be physically disconnected while the unit is energized. ... The incident energy of the arc flash will essentially be the same as it is a the 480V source. ....

jatrottpe -
I may need some education. Where are you getting this? My understanding is most ((listed stuff anyway) are rated for disconnect.

Can you cite any UL, mfg literature that says listed plug/receptacle combinations are not rated for load break disconnect.

I agree that I don't think it is good idea to use a cord and plug as a disconnect - but that doesn't mean they are not rated to do it.

It is okay with me if I learn something new here.

carl

 

[andy s.]

I am at a quandry with this. Can an accessory unit capable of being plugged in a 480 circuit be used?

Application:
VFD unit with overload protection.

Accessory: 5HP 480 volt motor with a three pole snap switch.

Situation: The 5HP motor is to be used with the VFD unit which has a 480 Volt plug which mates to the 480 volt receptacle.

Potential: This accessory has the potential of plugging directly to a source voltage, by an end user.

Advice: How can I explain this problem to the personell that there is a potential of Arc Flashing, in event of switch failure, no motor protection and .....

Please advise,

Andy

Switch failure..??? A snap Switch??? Define Switch.

Hey folks, is Article 430 Appropriate here. He made no mention IF this accessory can be used independantly from the VFD, ie a direct 480 volt source. This is a strange one.

 

[scubahigh97]

never seen plug ends on 5 hp motor before why not hard wire and not worry about flashing

 

[rootboy]

It is not recommended to disconnect the load side of most VFD's, any of them in my book.

Yeah, a big blue ugly flash is almost guaranteed. We've done here at work and it's pretty spectacular.


Why not put the VFD after the disconnecting means?

 

[weressl]

It is not recommended to disconnect the load side of most VFD's, any of them in my book.

It used to be true, but most current generation ASD's care less about weather the load is suddeny interrupted or not.

The second thing to remember is that if the disconnecting plug is on the load side of the ASD the arc flash energy may be significantly smaller than on the input side, therefore the current that the operator is trying to interrupt is much smaller than it would be on the line side. Standard plugs and receptacles are not designed for load switching, but interlocked receptacles are available where the plug is mechanically prevented from being withdrawn while the load disconnect switch on the line side is closed. (CH, Hubbel, etc.)

 

[jatrottpe]

Coulter - I AGREE that if the plug and receptacle are UL Listed for load disconnecting AND squelch the arc within the chamber (similar to the Crouse-Hinds Arktite Receptacles/plug combinations) the safety issue I expressed goes away.

The concern here is over the big blue ugly flash (as Rootboy states),

Having said that, Not knowing the manufacturer of the materials involved, and not knowing all of the possible manufacturers, I think it is a stretch to assume that all plugs, receptacles utilized for load break, while meeting the electrical requirements for breaking the load will also automatically mitigate the arc flash hazard that comes with breaking the load.
(not to take a left turn on you but, standard 480V Circuit Breakers in enclosures, MCC's, control panels are rated to switch load, yet they typically offer no protection from arc flash, if circuit breaker is exposed to operator via open door or enclosure, they should be in proper PPE for the incident energy at the breaker before they operate the breaker)

If a manufacturer sends a pre-manufactured package (motor, cord, plug, receptacle), there is no guarantee that the package will meet all required codes, standards, as shipped. (especially if the package is made overseas in areas with less stringent quality control).

I also agree when Weressl states that on the secondary side of th VFD the incident energy COULD be significantly less, however the application of the motor is such that it can plug into ANY 480V source that has the mating receptacle. If you eliminate the VFD the plug/receptacle will have about the same incident energy as the source.

I believe Weressl covered the not designed for load switching part of the question.

 

[plugman]

Alternative Plug and Receptacle

Alternative Plug and Receptacle

Time to put on my saleman hat and recommend our devices. Take a look at Meltric plugs and receptacles. The decontactor series carry full switch ratings along with horsepower ratings. They are designed as a disconnect switch and receptacle in one device.

They also eliminate the arc flash issue since the plug is ejected to a rest position when disconnecting power and full removal of the plug requires an additional operation. When the plug is withdrawn from the receptacle, there is no current flowing and thus no arc flash potential.

 

[Jraef]

Plugman beat me to it. I use Meltric on aggregate machinery control systems exclusively.
http://www.meltric.com/html/mining-aggregates.html

You can also get switch/plug combination units that also lock the plug in place and prevent removal if the switch is on, as well as prevent switch closing if the plug is gone. They are typically more expensive than the Meltric system, however if you want to have the ability to lock off a disconnect without unplugging from the receptacle, they are easier for the layman to understand (compared to Meltric, which many people are unfamiliar with).

 

[weressl]

Time to put on my saleman hat and recommend our devices. Take a look at Meltric plugs and receptacles. The decontactor series carry full switch ratings along with horsepower ratings. They are designed as a disconnect switch and receptacle in one device.

They also eliminate the arc flash issue since the plug is ejected to a rest position when disconnecting power and full removal of the plug requires an additional operation. When the plug is withdrawn from the receptacle, there is no current flowing and thus no arc flash potential.

Preventing the formation of an arc when interrupting full load current and "eliminating arc-flash issue" are two widely distinct features.

If an arc flash develops inside this device due to a short circuit, I have serious doubt that it could safely contain it under all fault levels. Otherwise it would have a maximum I2T limit on the source feeding it.

 

[plugman]

If you separate electrical contacts under load, you are going to draw an arc no matter what. The idea is to separate quick enough and at enough of a distance so that the arc is quickly extinquished. This is the basic theory behind the Meltric devices. Separation of the devices is performed by springs rather than manually withdrawing a plug from a receptacle.

I agree that there are some fault levels that will result in a catastrophic failure of a Meltric device. However, in most electric systems, the Meltric plug and receptacle will not be the weakest link. The Decontactor series have passed 100,000 amp short circuit tests which consist of both withstand and closing tests. These tests were performed with RK1 current limiting fuses.