Grounding v/s Sheilding

[Lancie1]

I have heard people saying that you only connect the shield on one side,

Technically, the shield performance depends on the frequency of the noise being shielded against. If it is 100 HZ or below, ground one end. If 300 Hz or higher, ground both ends. If in between, multiple-choices!

 

[seppoalanen]

All current are from VFD and all must came back to DC sourse directly from motor to VFD.
Small cables shield is enough for harmonic current emissions but for greater cable we need 3 symmetrical Ground wires.

 

[Bering C Sparky]

All current are from VFD and all must came back to DC sourse directly from motor to VFD.
Small cables shield is enough for harmonic current emissions but for greater cable we need 3 symmetrical Ground wires.

I hear ya, and I dont disagree with you one bit, but going to have to refer you back to post 14.
If you physically cant install it, then there is no since in purchasing it,

Just got the order ready and then started poking around with some voltage drop calculation and found that we may now have to increase the wire size even further, we will be over the 3% alowable VD at the current sizes.
Good thing this job is engineered right? (NOT)
It took a dumb country boy to catch it!

I have used cable similar to what you show there, but try and pull a 250kcmil, or better yet 500kcmil 3 cond with 3 - 3/0 ground wires wrapped in a shield + insulation rated to withstand being on the deck of a ship.
Your a better man than me if you can do it, not to mention all the obstacles you would face on here even if you could lift it.

So lets just say 1/4 of that amount to make it equal.
Pulling in 5000 feet of multi conductor like I just descibed, NO thanks,
I like the job but not that much.
It would not be possible anyway, you will just have to take my word on that.

I am ordering 20,000 feet of the stuff I posted the other day!
(Graybar or North Coast is going to be happy!
Sharpen your pencils guys, lets see how low the price can go) Big $$$$$
Bidding it out overseas also just to make it interesting.

Thanks for all the advice though guys, never know where you will pick up some new bit of information.

BCS

 

[Mark Snodgrass]

If I read your posts correctly, this is an installation on a ship? If this is so then it is not covered by the NEC, but it is referenced by the Federal Regulations. You need to reference those to determine if the NEC rules still apply. Being a Navy vet I know there are some differences between shipboard and land based wiring systems.

Here is the relevant portion of the NEC regarding scope:
https://www.inkling.com/read/nfpa-national-electrical-code-handbook-2011/article-90/90-2scope

 

[Mill_Control]

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.

NEC is along similar lines. The ground conductor doesn't need to be as heavy because it isn't 100% duty. It only needs to carry current long enough to trip a breaker or blow a fuse.

The OEMs "engineer" is likely thinking the VFD monitors ground current and will theoretically shut down way faster than any breaker or fuse would with the slightest ground fault. Here in the real world said fault could blow the sheild ground conductor before the drive faults.........................

Considering the application I think the OP is wise ignoring the "engineers" recommendation and pulling a real ground. Wet environment and no solid ground path thru' the structure is no place to trust a drives ground fault monitoring.

 

[iant]

@Mill_Control
"I think the OP is wise ignoring the "engineers" recommendation and pulling a real ground"

My thinking also

 

[Bering C Sparky]

If I read your posts correctly, this is an installation on a ship? If this is so then it is not covered by the NEC, but it is referenced by the Federal Regulations. You need to reference those to determine if the NEC rules still apply. Being a Navy vet I know there are some differences between shipboard and land based wiring systems.

What you say is true we are regulated by CFR (Code of Federal Regulation) but when your read that it is rather outdated like most of the Coast Guard literature you see when taking test for licensing. They are still referencing things from the 50's and 60's that hold no real relevance most times in the modern world.
I did find something in CFR 46 (111.30 I think it was) but it was refering to metalic braided sheath cable and armor clad cabling. But it did say in no uncertain terms that you are not allowed to use the metalic braiding or clad as a ground conductor.

Due to Federal Budget cut backs and such the Coast Guard no longer really inspects vessels for things like this. (Until there is an incident and then it is after the fact)
They have pushed most of this off to OSHA who do inspect us regularly but look more for things like Lock-Out Tag-Out logs, guarding of equipment, fall protection and saftey hazards.

About the only real regulatory institution we see regarding electrical installation on these vessels is from "Det Nos Veritas" (DNV) they due Annual Inspections but mostly look at structural things like tanks and also functionality of ships systems. (This is a regulatory institution that is paid for by our comany to inpect the vessel to satisfy our insurance company that the vessel is up to snuff, they are not a local, state or federal authority of the USA, actually they are based out of Norway, but have offices here in the states)
Most of there regulations regarding electrical just refer you to the Code of Nationality for which the vessel is Flagged . In our case USA.
OSHA's regulations for electrical are about the same, if you read them they are taken from NFPA Word for Word.

I have a conference call sceduled with the engineer tonight so looking forward to that.

Have a good one guys and someone please have a cold one for me, it will still be a few weeks before I get to have one myself.

BCS

 

[seppoalanen]

"His reply was the sheilding on the power cables is used as the ground wire. (!!!!WHAT!!!!)"

http://www.drakakeila.ee/public/product/MCMK 3 1-2-core.pdf

 

[iant]

#seppoalanen
reading those specs,I would say not bad depending on the Fault current protection for the application.

by the way it is the same cable configuration for High Voltage Underground. where the shied is the ground and mechanical protection

 

[Bering C Sparky]

PE conductor . . . . concentric copper wire layer and​

copper wire or copper tape binding

seppoalanen,​

I would have admit that the cable you show there does indeed have the GROUND wire laid around the outer conductors and is indeed used as the PE / Ground / Earth conductor.
Dont see how anyone could argue that fact. The cable assembly is rated and approved for that outer ring of copper conductors that are banded together with a copper wrap around them to be used as GROUND. It clearly says so several times in the spec sheet you supplied.​

However what I dont see in this speck sheet is anything that says this configurations purpose is or should be used as a sheild to limit the effects of EMI. Which is the intent and purpose of the cable I am speaking of.​

Just as I would not use the Shield of my cable for ground, I would not use the cable you show for say power cable from a VFD to a motor. It is not designed for that and does not have the proper configuration to be effective for that applicaion.​

I am not arguing the fact that my cable does have a tinned coper braid around it, or that it could actually direct a fault current back to the VFD.
But it is not listed for or intended for this purpose. It is there to suppress EMI and nothing more.
If it were ment to be a PE conductor the manufaturer would have gone though the trouble to have it tested and marked with approval ratings that say so, just as the conductor you show does.​

Its pretty much a mute issue for me now, I am over it and have moved on.
I have found nothing that says it can be used for that purpose nor have I found anything SPECIFICALY saying you cant.
But if you could use it as a ground conductor I think the manufacturer would be singing it from the rooftop, why wouldnt they, the braid is already part of cable assembly.
I highly doubt that they are just trying to save money on ink by not including that statement in the cable specifications.​

In the end you just have to do what you believe is correct, when you look in the mirror you want to feel good about who you see looking back at you.​

Have a good one and thanks for sharing.
BCS​

 

[Lancie1]

In the end you just have to do what you believe is correct, when you look in the mirror you want to feel good about who you see looking back at you.​

Well said, and I think you made the right choice.
​

 

[Bering C Sparky]

Thanks Lancie,
I think that is the first time I have smiled in week.

 

[TurpoUrpo]

Here's cable that has intended shield + pe

http://www.drakakeila.ee/public/product/MCCMK 4-core.pdf

 

[seppoalanen]

Here's cable that has intended shield + pe

http://www.drakakeila.ee/public/product/MCCMK 4-core.pdf

Type above is "MCCMK" for small cables. There is added copper band for emc protection becourse small gross section of concentric layer of copper "PE" wires can't give full EMC protection.

More than 16mm2 cable type is "MCMK". Concentric layer of copper wires "PE" is enough tight for "shielding" of EMC protection.
http://www.drakakeila.ee/public/product/MCMK 3 1-2-core.pdf

 

[TurpoUrpo]

Type above is "MCCMK" for small cables. There is added copper band for emc protection becourse small gross section of concentric layer of copper "PE" wires can't give full EMC protection.

More than 16mm2 cable type is "MCMK". Concentric layer of copper wires "PE" is enough tight for "shielding" of EMC protection.
http://www.drakakeila.ee/public/product/MCMK 3 1-2-core.pdf

Yeah, it was just headsup about the whole no spec for shield + pe together.