Grounding v/s Sheilding

Bering C Sparky

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Mar 2014
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Bering Sea, Alaska
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Hello,
Not really PLC related but I know alot of you out there also have industrial backgrounds, so here goes.

We are going to be installing new electric winches to replace existing Hyd. winches.
I am just finishing my cable list to make an order and one question I asked back to the engineer at the winch company is "why do you only speck ground cable be run with the feeder cables from the distribution panel to the VFD cabinets and not from the VFD cabinets to the motors.
(220 kw motors 440vac 3ph)

His reply was the sheilding on the power cables is used as the ground wire. (!!!!WHAT!!!!)
(We are using Lapp FD90CY single conductor cables run in parallel)

To borrow a phrase from Forest Gump "I am not a smart man" and I dont claim to be able to resite the NEC or other code, nor do I have an engineering degree but I simply do not believe that this is correct.

I have done some google'ing and brushed through 250 in NFPA70 but have not yet found anything to say specifically that this is right or wrong, but common sense alone says this cant be the correct way.
(I can find lots of articals that somewhat say you cant do this, like conductor size for PE, min size wire allowed to be paralleled, etc, but nothing specific about using the sheild as ground conductor.

I am going to install seperate grounding conductors anyway so I am not really worried what he says, but I am currious if anyone else knows if this is a standard practice or not.
If it is not does anyone know if it is even legal, where can I find something in writing to say this is right or wrong.
If this is a correct and legal use, I will eat my proverbial hat.
I will keep looking but I cant waste too much time on it.

Note: Winch Company and Engineer are not based in USA.

Thanks for any input.
BCS
 
I dont know your NEC laws
As far as using the cable shield as a ground,
this is not as uncommon as you may think.
I did say CABLE SHIELD not shielding for noise.
so long as it is grounded at either end and has the fault carrying capacity it becomes a suitable ground (we call it an Earth)

this is common place with HV cables

I stress this is not used for signal noise reduction
 
Hi iant,
Thank you for your reply,
I respect what you are saying but these cables will be connected to VFD'S (Big one's) so the cable is shielded to contain the noise caused by the VFD.
To use this same shield as ground for such a large circuit just does not sit well with me.
Min size ground conductor for this circuit would be #2 awg, so if you figure circular mils of all the shields and add them together will you have what is equivalent to a #2 awg? o_O Maybe, I have not checked that, nor have I found anything that says if it is, it would be ok to use this for equipment grounding.
Even if I did find something then you are still paralleling these conductors, NEC says min size wire that can be paralleled is 1/0 and the sheilding on each of these cables added together probably would not be 1/0.

Am I being nit picky? Maybe
We have very little regualatory oversight on these vessels and I see guys doing some crazy stuff that turns what hair I have left white, but to have an electrical engineer tell me that, well, I was put back a bit by the statement.

Thanks again for your insight though, always open minded to how others do things and was curious if this was an exceptable practice in other parts of the world.

EDIT: Here is a link to see the cable I am talking about, you can clearly see the shielding in question.
http://products.lappgroup.com/onlin...s-applications-approved/oelflex-fd-90-cy.html

Regards,
BCS
 
Last edited:
Thanks for the link - I had already googled it

I agree 100% with you
I have installed 45Kw motors (90mm2) 3 core +ecc (R.W.B.+ E) Shielded
so the shielding is for noise only And only grounded at one end.
the 'E' is the ground, Bonded at both ends.
 
Our spec from the engineer for this project is to take sheild to earth on both sides of the cable for 440v power. (At Motor side & at VFD side)
But only earth one side for the smaller control/signal cables.

This is SOP for us on all the smaller drives we use onboard also. (Around 75 of them) Mostly Siemens & some A.B and ABB
(Shield both sides of power (440) and only one side of control/I/O)

I have heard people saying that you only connect the shield on one side, and for control/I/O cables I would aggree.

I may be very wrong in my belief (See quote from Forest Gump in post #1)
But all data I have seen from the Siemens VFD's we use says to sheild both sides of the power cable feeding the motor & only VFD side of the control and I/O.

Took me a moment to find a Siemens data sheet for VFD's but here is a Quote from page 20 of the link below.

"The shielding must be connected through the largest possible surface area to the grounded drive inverter housing and the grounded motor frame."

https://www.automatyka.siemens.pl/docs/MM4_Wsk_proj.pdf

We have always followed this and have not had any problems, if I am incorrect please let me know, I am always willing to learn something new.

I reference Siemens because they have always been the Lions Share of the Drives we use and the ones I am most accustom too, but on most new projects we install, A.B. and ABB have been becoming more and more prevalent.

Thanks again though for your input, I have seen many of your post and do respect your opinion and your knowlageable advice.


Regards,
BCS
 
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Copy and paste

Cable shield connections and groundloops There are sometimes lots of talk about how the cable shields should connected whn the equipments are interconnected. Some arguments say that to avoid ground loops is a good idea to connect the cable shield only at one end to avoid large ground currents on cable shield. On the other hand EMC books and text talk about how important is to connect the cable shield on both ends to get good shielding against interference trying to get out from the cable and into the signal in the cable from external sources. Cable shield connections are still controversial. There are strong, valid arguments both ways. For cable shield connections there are four options, with advantages and disadvantages:

1. Shield grounded at both ends: Good r.f. shielding but susceptible to ground loop currents that can be VERY large - up to at least 100 A in bad cases.

2. Shield grounded at both end, with large-area parallel bonding wire: Good at r.f. and now the ground current flows mainly through the bonding wire, but the intense magnetic field that may result is not good news. The bonding wire size needs to be, for example, 10 mm^2.

3. Shield grounded at one end only: No ground current but not good at radio frequencies for which the cable is more than 1/8 wavelength long. R.F. interference may actually be worse than for an unshielded cable.

4. Shield grounded at the sending end and grounded through a capacitor (may be two in parallel, for effectiveness from about 100 kHz up to 1 GHz and beyond) at the receiving end: Good at r.f. if the capacitor type and positioning are correctly-designed, and no low-frequency ground current. One capacitor needs to be of the type with an integral spark- gap so that it is not damaged by transient high-voltage spikes that may be induced on to the shield. What wiring practices are used in practice The professional lighting and sound industries generally favour solution 4, based on practical experience of what works and is reliable. There are also 'ground both ends' people in sound industries who remain unconvinced of this and use option 1. Video industry generally favours option 1.

Read more at: http://www.epanorama.net/documents/groundloop/cable_shielding.html
 
WOW, for a minute there I was thinking to myself, this guys English has come a long way in a couple of days. LOL
JUST KIDDING,:ROFLMAO:

But seriously, thats a good read, interesting stuff.

Thanks for posting it osmanmom I appriciate it and have added it to my private stash.

Regards,
BCS
 
...and has the fault carrying capacity...

IMO, this is the most important point. What is the ampacity of the shield? We don't know. I would never trust the shield in place of a dedicated grounding conductor.

Somewhat off-topic, but an interesting Mike Holt discussion about grounding myths: http://youtu.be/qNZC782SzAQ (especially the talk about CNC machine grounding, around 12 minutes in)

🍻

-Eric
 
IMO, this is the most important point. What is the ampacity of the shield? We don't know.

Exactly, there is no UL or any other listing for Ampacity of the shield, only the conductor. (at least that I am aware of)

I threw this out on the Mike Holt Forum earlier also, but no bites on that hook yet.

I was hoping Mike would shine some light on the subject.
I took his class back in 1989 when I got my Journeymans ticket, and used his video course to study for my Masters in the mid 90's, he was a busy man even back then.
Hopefully someone on his staff will chime in with a code artical to confirm or deny this one. I would love to have something in writing that I could toss back to the engineer.
If not I guess it will just remain one of those Urban Myths.

No matter though, Im going to do what I think is correct and pull the ground cables anyway.

Cant look at your link. NO U-TUBE for us.
I.T. has us locked down with Websense. 🔨
Its a miracle that I can even access this forum.

BCS
 
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you can calculate the mm2 of the shield
(too many years have past for the imperial)

(Dia of strand in mm divided by 2 then squared x pi x number of conductors)
then look up the table for the closest match (round down)
 
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If you open the PDF of the cable you quoted it gives a cross for the protective conductor. This means that the shield is not suitable as a PE for the motor. It also says max. run of 10 mtrs.
AS Iant says , 4 core & shield .
Paul
 
Hello Paul,
Thanks for the Input.

4 Core + Shield or (250mcm)150MM^2 4 conductor Shielded Cable, is just simply too large, heavy, bulky, requires too large a bending radius, etc, etc, to be fesable to use for this application. This is a ship not a building and we have to work with and overcome the obstacles that go along with that.

We are using 4 cond shielded cables for all winches that require smaller cables but for the really big ones this is not possibe.

We have done these winch conversions in the past on other vessels and due to the confined and limited spaces and multitude of bulkheads that must be penetrated, using 4 conductor in one shielded cable assembly for the really large winches just does not work.

Going to put this one to bed and move on.
I have already ordered the ground cables and have had a few days to get over my inital frustration with the engineer, So all is good.
It is nice to have a place like the Forum to vent frustrations and get advice, feedback and perspective from others though.

Thanks to you all, and Best Regards,
BCS
 
I haven't looked at the shield size.
Two things to note
1. By using three shielded cables. The total size is x 3

2. I note that a ground conductors current carrying capacity is considerably higher than the equivalent line conductor.
Not certain about NEC. But definitely in our regulations.
 

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