4160V motor feeder

[wolfie1a]

I have 4160v motors that are fed with amored cable. My question is about unwiring the motor. Is it necessary to install personal protective safety ground to unwire the motor? I've heard two different stories.

1. Always ground
2. No need to ground since the armored cable prevent any induced or impressed voltages and any built up capacitve charge is drained thru the motor.

Any input, opinion or thoughts is greatly apprciated.

 

[davidbeach]

At that voltage things exist in one of two states - energized or visibly grounded. No such thing as "deenergized".

 

[DRWeig]

Agree with davidbeach! I think it was one of the original Ten Commandments of Electrical Engineering that went like this:

"Tarry thou not amongst conductors of the thousands of volts if thou hast not witnessed their grounding with thine own eyes, lest thy widow make merry with thy buddies in ways of which thou wouldst not approve."

Goober Dave

 

[LionelHutz]

I believe you'd ensure the motor cables are dead and grounded at the switchgear. This seems to be the accepted safe way to do work on equipment and should be considered as the best practice for any new installation. Some places will just pull the breaker and test the cables for no voltage before working on the equipment.

 

[oldfieldguy]

DRWeig--

Do you have link for the other nine commandments?

old field guy

 

[golftennis]

Hi All,
Dealing with high voltage eqipments you got to follow saferty norms
1)proper Isolating procedures involved you can't make mistke of doing the way you want to do when you follow Isolating procedures. withdrawing the breaker you have to earth the motor tetminals before you can get acess to the motor terminal cover A key is involved,this key makes sure you have grounded the motor terminals before you start working. do not get mixed up with Drives where you have discharing resistors.
High voltage please follw proper procedures
Alex

 

[DRWeig]

Wolfie1A,

Hope you take all of the "safety first" notes to heart. You might save a little time if you can convince yourself that grounding isn't needed, but at what risk?

It's kinda like the people you see trying to beat a train to the crossing. Might save a person five or ten minutes, but actually risking one's life to do so? Not me...

Good on ya,

Goober Dave




Old Field Guy --

I did that one from memory, so it's probably a contortion of some sort. I'll start a search in my archives, will post in Electric Power Engineering forum if I find it.

Good on ya,

 

[Zogzog]

"Tarry thou not amongst conductors of the thousands of volts if thou hast not witnessed their grounding with thine own eyes, lest thy widow make merry with thy buddies in ways of which thou wouldst not approve."

Please do post those, that is the funniest thing ever. (Well, in here anyways).

Seriously, "If it isnt grounded, it isnt dead", our service vans have that bumper sticker.

 

[DRWeig]

Group:

As I stated, I got that commandment kind of twisted up with another one. However, since this is a safety thread, I feel it's appropriate to post links to the originals.

Here's a purist version:

http://www.cyberslayer.co.uk/jokes/joke0636.html

And here's one with a Naval twist:

http://www.goatlocker.org/resources/nav/safety.htm

Hope I don't need to do anything else with the links, this is my first try...

Goober Dave

 

[Laplacian]

In my experience (industrial plant), feeders at this voltage range have insulated or covered (taped, booted, etc.) terminations at the load and at the source.

How do you apply grounds as 5kV motor starters are not typically outfitted with grounding switches?

 

[dpc]

The last time I recall seeing a MV motor unwired, the breaker was opened, racked out and tagged. I don't recall seeing any temporary grounding applied at the motor.

Times may have changed, but in the past, temporary grounding was not routinely done for indoor MV plant distribution systems within one facility where the breaker could be racked out and tagged. For major work on a switchgear bus, a grounding device was generally used.

 

[wolfie1a]

I've contacted the MV starter mfg (AB 1500) and ask them exactly where the best place is to ground the load side of the starter.... still waiting on a reply.

 

[Gianoli]

The application of protective grounds on medium voltage systems in industrial plants can be challenging. Often little consideration was given to this requirement when the systems were designed. However, modern safe work practices dictate the use of having protective grounds installed prior to working on medium and high voltage circuits.

Older motor circuits in some of our facilities seemed to be the most difficult to work on. Often these motors were switched with power circuit breakers at the 6.6kV level. The motor connections in the motor junction boxes were taped with rubber tape and completely insulated. The back of the power circuit breaker cell had fully insulated bus and the cable connections were often taped with rubber tape also. The only answer for these types of installations was to employ what was known as a grounding truck. This device racked into the circuit breaker cell once the truck mounted power circuit breaker was removed from the cell. Once installed the grounding truck engaged the load side tulips in the back of the switchgear cell and shorted them to ground. Effective but one needed a unique truck for every model of switchgear that was on site.

On newer installation we address the problem two ways. All new motor junction boxes are now ordered with large junction boxes that have a small section of bare bus for each phase mounted on insulators inside the box. Motor leads are connected to one end of the bus and feeder cables to the other end. Adequate clearance between bus sections in the junction box means no taping of joints is required and it’s a lot quicker to remove a motor when required. These short sections of bare bus also provide a convnient location to install ground balls (

http://www.hfgp.com/ProductDetail.asp?ProductNumber=21706)

that allow the motor circuit to be grounded. Cable circuits leaving power circuit breaker or contactor cells also have the grounding balls installed on the load side of the breaker /contactor at some point were they accessible once the circuit is isolated and locked out.

I must caution that work on medium and high voltage circuits should only be performed by qualified personnel who are thoroughly familiar with the equipment involved. I cannot stress this enough, I have seen fatalities, injuries and numerous near misses while working on this type of equipment. If you make a mistake while you often don’t get a second chance.