3 wire control for 480 V/100A receptacle

[Bolted Short]

I'm having a go around with an engineer over this (The project is running out of money)

We are installing a 3-phase, 480V, 100Amp receptacle in an outdoor location that is also Class 1 Div.1 (It's a long story...... the receptacle is in the classified area, the equipment is not.)

We are using Meltric DS200 pin and sleeve plug/receptacle. The receptacle is horsepower rated, and has two auxiliary pins for interlocking. These pins engage last, after the power pins are connected, and the assembly is designed to suppress arc flash.

The engineers design utilizes a 2 wire control circuit. The aux pins in the plug are shorted together at a terminal block in the portable device.

When the employee seats the plug in the receptacle, the control circuit is completed through the aux pins, and a contactor pulls in to supply power to the receptacle. The employee would be holding the metal case of the plug during this time.

The engineer maintains that since the plug is designed so that the power pins are seated first, and there is no arc flash danger, therefore two wire control is adequate. (like I said, we are over budget and the engineer does not want to spend the money for an explosion proof start/stop station)

My argument is:

1. Every high power cord and plug connected installation I have ever seen has 3-wire control. The employee seats the plug, then presses a start button to energize. Because this adds to cost, it must be a code requirement SOMEWHERE, otherwise no one would do it..

2. In addition to arc flash, there is electrocution danger, especially in an outdoor installation. So the 3-wire start/stop keeps the employee from handling an energized bologna cord. The employee seats the plug, lets go, then presses a start button

3. Since a ground fault in the plug could result in death, we need to reduce the risk as much as possible.

So far, I have looked at NEC, NFPA 79, OSHA and WAC (Washington Administrative Code- basically, local amendments to OSHA). I don't have access to UL 508.

The only thing I can find are OSHA and WAC regulations that say employees shall not handle energized cords with wet hands.

Does anyone know of a chapter and verse in an applicable code that will support my argument? Or is the engineer correct in saying 2-wire control is adequate?

 

[JFletcher]

Welcome to the forum. Are you saying that power will automatically flow once the connection is made due to the aux pins being shorted? 110.3(B) may be what you need.

From Meltric:

http://www.meltric.com/pdfs/insds-dsn_u.pdf

" The DS100 and DS200 are rated as general
purpose switches, but are not horsepower rated
WARNING! If these devices are installed in
motor power supply applications, warning
labels may be required to advise users not to
disconnect the device under load. Labels are
provided in the package, but should only be
used when required."

Would the receptacle/plug NOT being HP rated change the game any? If the mfg warns against disconnecting with a load, there may be similar warnings to not have the plug/receptacle automatically power up upon connection.

 

[drcampbell]

If the auxiliary contacts break first, isn't this moot? The contactor will de-energize the power pins before they disconnect.

 

[Jraef]

The main reason for 3-wire control is so that if power fails, the circuit is de-energized, requiring positive action from the user to re-establish power flow in the form of pressing a button. Ostensibly, this helps to establish and maintain a procedure in which, after a power loss, operators visually inspect a piece of equipment before re-energizing it so as to avoid damage or harm caused by someone having altered the equipment while it was down. There is no NEC requirement for this however, it's an operational safety issue.

 

[kwired]

If you are in a Class 1 Div 1 area, aren't you still going to have a potential arc when that two wire control makes/breaks contact and possible ignition of the material that creates a hazardous location?

Now if you have a switch internal to the enclosure that is either sealed off or is otherwise intrinsically safe then it shoulnd't matter as long as it doesn't allow energizing the receptacle until after contact has been made or de-energizes it before contact is broken.

I am not familiar with what is common in such situations so I can't give comments based on experience here.

 

[augie47]

If you are in a Class 1 Div 1 area, aren't you still going to have a potential arc when that two wire control makes/breaks contact and possible ignition of the material that creates a hazardous location?

Now if you have a switch internal to the enclosure that is either sealed off or is otherwise intrinsically safe then it shoulnd't matter as long as it doesn't allow energizing the receptacle until after contact has been made or de-energizes it before contact is broken.

I am not familiar with what is common in such situations so I can't give comments based on experience here.

I think that's a good point. Regardless of all else, if its not listed for the Classified area I would think it did not belong... is there an exception for this ?

 

[Ingenieur]

If outside are their not mntg height exceptions?
otherwise it needs to be xp

A similar method is common in mining
the long pin is gnd...breaks last, makes first
intermediate are the phases
Shortest are the pilot/interlock...breaks first, makes last

it does not automatically make the circuit
it energizes a ground monitor relay, ie, a good egc
this puts power to the uv latch of the associated cb and allows it to be set (provided other interlocks are satisfied)

what he proposes sounds iffy
the pilot breaks first which is good, prevents being opened under load
If plug set was xp (with unit sealed before pilot makes) might be ok
mining plugs are screwed on and sealed before pins make, ie enclosed and xp

 

[Bolted Short]

The application is a "pop-up" receptacle for the 480 Volt input power to an aircraft ground power unit. The receptacle is stowed in a grade level pit in a "hardstand" area (translate: parking lot) The receptacle is raised out of the pit (out of the classified area) before the plug is inserted. Similar to mining operations, everything is wired with bologna cord.

The input power is 480V/3-phase, 60 Hz, and the output is 208/120, 400Hz.

The unit has a 3-wire ST/SP circuit with an interlock to the aircraft. (The unit must be plugged in to the aircraft to complete the control circuit before it can be started. The aircraft also disconnects if the power is not in voltage/frequency/rotation spec)

The engineer has determined that because of the design of the plug (pin-and-sleeve, ground mates first, then power, and interlock last), and there will be no inrush because the GP unit can not be started until it is plugged into the aircraft; that there is no arc flash danger, so the 2-wire control is sufficient.

My concern is shock. The two interlock pins on the plug are jumpered together, so as soon as the plug is seated, the control circuit is made, and the contactor pulls in while the ramp worker is still holding the metal case of the plug.

While the receptacle is no longer in the classified area, and the design mitigates arc flash, it is still a wet location, and there is a hell of a lot of power. So, even if the possibility of shock is low, I never deal with Sicilians when death is on the line.

 

[Bolted Short]

Welcome to the forum. Are you saying that power will automatically flow once the connection is made due to the aux pins being shorted? 110.3(B) may be what you need.

Thanks.

That may be what I need.

The discussion has been between electricians and engineers, if we get the manufacturer involved, it may resolve the issue.

 

[JFletcher]

The application is a "pop-up" receptacle for the 480 Volt input power to an aircraft ground power unit. The receptacle is stowed in a grade level pit in a "hardstand" area (translate: parking lot) The receptacle is raised out of the pit (out of the classified area) before the plug is inserted. Similar to mining operations, everything is wired with bologna cord.

The input power is 480V/3-phase, 60 Hz, and the output is 208/120, 400Hz.

The unit has a 3-wire ST/SP circuit with an interlock to the aircraft. (The unit must be plugged in to the aircraft to complete the control circuit before it can be started. The aircraft also disconnects if the power is not in voltage/frequency/rotation spec)

The engineer has determined that because of the design of the plug (pin-and-sleeve, ground mates first, then power, and interlock last), and there will be no inrush because the GP unit can not be started until it is plugged into the aircraft; that there is no arc flash danger, so the 2-wire control is sufficient.

My concern is shock. The two interlock pins on the plug are jumpered together,so as soon as the plug is seated, the control circuit is made, and the contactor pulls in while the ramp worker is still holding the metal case of the plug.

While the receptacle is no longer in the classified area, and the design mitigates arc flash, it is still a wet location, and there is a hell of a lot of power. So, even if the possibility of shock is low, I never deal with Sicilians when death is on the line.

You're welcome. The part I bolded, I have a problem with as well. Defeating safety interlocks of any kind is bad juju; if they werent necessary, they wouldnt be there in the first place. Do you have a plant safety officer?

 

[Ingenieur]

Correct me if wrong
the plug/recept are not in a haz loc

even though the plug pilot closes a contactor to the gnd pwr unit the unit must be plugged into the aircraft and turned on the function

is the plug plastic or metal? Looks like plastic and insulated?

Most (almost all)!recept are energized prior to the plug being inserted
this energizes as the plug is inserted
what is the difference?
I doubt the recept must be dead to safely insert the plug
how would that work in a normal application ? Kill cb, insert plug, set cb?

put a time delay on the pilot/contactor ckt
say 5 sec or so, this should ensure plug is fully inserted prior to the recept being energized

 

[Ingenieur]

http://www.meltric.com/pdfs/meltric-decontactors-28-e.pdf
it is metal, but not a concern, plastic is an option but not necessary
safe to remove under load
pushing the release catch withdraws pins prior to removal
looks like a nice device

HP rated
no live part exposure during removal or insertion
arc suppression tubes