Direct Buried Teck Cable Grounding--Sheath Question

[ghostbuster]

Not familiar with electrical code in your country.

Site has 480 volt 350 mcm 3 conductor (3 phase) plus bare wire ground in bundle.Having induced ground current issues at this site.

Is it possible to ground the bare wire ground at the source end (leave load end bare wire ground floating) and use the sheath grounded at both ends as the load equipment ground bond?--in our country this is not allowed

Thanks for any guidance

 

[Rampage_Rick]

FYI Teck is roughly equivalent to MC-HL cable, or ACWU with the addition of an inner jacket around the conductors.

 

[MasterTheNEC]

Not familiar with electrical code in your country.

Site has 480 volt 350 mcm 3 conductor (3 phase) plus bare wire ground in bundle.Having induced ground current issues at this site.

Is it possible to ground the bare wire ground at the source end (leave load end bare wire ground floating) and use the sheath grounded at both ends as the load equipment ground bond?--in our country this is not allowed

Thanks for any guidance

Ok, I will assume when you say "Your" country that you mean America......since you appear to be Canadian.

Typically Teck 90 Cable is a "Canadian" thing but that does not mean we can't construct it here in the US under UL 1569. Typically that added construction (inner cover and outer cover) is reserved where higher requirements like "HL" would be deemed...but not the only way to achieve it.

The cable armor itself, as I am sure you know, has to be bonded and It is required to be bonded which takes place at the termination ends in listed fittings for example (for brevity) but the EGC that is within the product (bare or insulated) must serve it's purpose. The integrity of the EGC to aid in the operation of an overcurrent protective device is paramount and to facilitate a low impedance fault current path.

This brings us back to the golden rule of Section 250.4(A)(5) in a "grounded" system, which I will assume you are referring too.

So no I would not leave it "Floating"...good luck on your installation and I am sure others will opine.

 

[MasterTheNEC]

I should also add that one difference between Teck 90 and MC-HL as stated would be the use for Class I, Div I and Class II, Div I. The Type MC for those locations would have to be continuous corrugated style where as Teck 90 could be interlocked.

So while it may look like MC-HC....it is not.

 

[ghostbuster]

Thanks for all the info

Enclosed is a cross-sectional picture displaying the non-symmetrical position of the bare wire ground in this teck bundle.This is creating the high induced ground currents at this site.

Conversations yesterday with a Canadian cable manufacture application specialist confirm this cable conductor arrangement produces induced current in this ground cable and anything this cable is attached to.

Our client is now forced to spend very large $$$ at this site to install an additional grounding system.

 

[ptonsparky]

Thanks for all the info

Enclosed is a cross-sectional picture displaying the non-symmetrical position of the bare wire ground in this teck bundle.This is creating the high induced ground currents at this site.

Conversations yesterday with a Canadian cable manufacture application specialist confirm this cable conductor arrangement produces induced current in this ground cable and anything this cable is attached to.

Our client is now forced to spend very large $$$ at this site to install an additional grounding system.

What would an additional grounding system be?

 

[Phil Corso]

GhostBuster...

What is the "load"?

Regards, Phi lCorso

 

[ghostbuster]

Phil

The load is overhead lighting controlled through standard contactors with 5th and 7 th current harmonics less then 5 %.(approx. 5 to 10 mva. of lighting load)

Photon:

The new proposed grounding system would be custom designed and pre-approved by code authorities prior to installation.It would be similar to the specialized grounding systems we have had to design for large audio and video studios.

 

[Phil Corso]

GhostBustor...

1) Would you say that most of the hi-harmonic load is at 277V, i.e., Ph-neutral?

2) Have you measured phase-conductor(s) currents, neutral-conductor current, and shield-conductor current, including harmonic-order?

3) Have you measured Ph-Ph, Ph-neutral, and Ph-ground voltages at each end?

4) Have you measured Neutral-ground voltage?

5) Ar you sure that Neutral conductor is not in contact with shield?

6) Have you measured Ph-Ph capacitance, Ph-shield capacitance, Ph-Neutral capacitance, and neutral-shield capacitance?

Phil

 

[Phil Corso]

GhostBuster...

7) How is the problem, if any, manifested? Spurious tripping? Excessive heating of cable or controls?

Phil

 

[ghostbuster]

GhostBustor...

1) Would you say that most of the hi-harmonic load is at 277V, i.e., Ph-neutral?

2) Have you measured phase-conductor(s) currents, neutral-conductor current, and shield-conductor current, including harmonic-order?

3) Have you measured Ph-Ph, Ph-neutral, and Ph-ground voltages at each end?

4) Have you measured Neutral-ground voltage?

5) Ar you sure that Neutral conductor is not in contact with shield?

6) Have you measured Ph-Ph capacitance, Ph-shield capacitance, Ph-Neutral capacitance, and neutral-shield capacitance?

Phil

Phil

The lighting loads are all single phase 480 volts phase to phase + ground--there is no neutral--refer to picture cross sectional picture posted

Load on each teck cable phase in each bundle is 100 to 150 amps-- 3 phase balanced.There are approx. 90 to 100 buried teck cable bundles at this site.Each teck cable bare wire ground has 10 to 15 amps induced current flowing.

Conducting electrical load simulation testing on a surplus separate teck cable produced similar ground induced currents.

This is a 100% induced ground current and has been confirmed by a cable manufacturer.

This induced ground current is entering sensitive equipment within this site and "wreaking havoc"

 

[GoldDigger]

I have a hard time seeing 15 amps of current flowing in an EGC in this situation simply because of asymmetric magnetic coupling.
But the cable manufacturer may know something I don't.
I do expect a pretty substantial induced phantom voltage because the EGC is adjacent to only two of the three phase conductors. But that electrostatic coupling should not support much current.
The net current in the cable should be zero or close to that unless there is a matching current flowing over some other ground path.
How long are the wires in question?
If the distance is thousands of feet, and the alternate path is a very low impedance earth path maybe I could be convinced.
Solving the problem could certainly be interesting given that the NEC requires that the EGC either be a raceway or be included in the cable. Just pulling a separate EGC wire which is not forced to match the twist pitch of the hot wires wires would be a physical solution, but not a code solution.

 

[Phil Corso]

GhostBuster...

8) Is this a new phenomenon to the site?

9) During testing, when you confirmed similar current values were measured in the un-insulated-conductor (presumably 'floating'), what was the current-frequency?

10) What was the voltage-to-ground at each end?

11) What material is used as sheathing for the Phase conductors?

12) Were any measurements done with an Oscilliscope?

13) If currents were measured with a Clamp-on, where was the measurement taken, if the cable is "floating"?

Phil

 

[ghostbuster]

Goldigger

First:

The exisiting EGC will be left connected at the source end.If the Teck cable electrically fails intenally the upstream breaker will trip.The load panels will be fed with a separate ground cable configuration (sized large enough to handle any fault current and low enough impedance so as not to delay the upstream breaker trip times-co-ordination report will be supplied) and will be pre-approved by special code examination prior to installation,as mentioned earlier it will be similar to some audio /video installation perfomance grounded sites.

Each underground cable run to each load centre is 500 to 700 feet in length.Also do not forget we have 90 to 100 cable runs (that is over 50,000 feet of buried teck cable) at this site ---each of them contributing to this excessive ground current condition ( because each of them is tied to the same (low impedance)building steel structure at each grounded cable end).These are acting all like single turn transformers connected all in parallel feeding this induced ground current loop into this building metal structure.

Electrical load tests conducted on a single (surplus)Teck cable at site confirmed these induced current levels on this ground cable.We deliberately perfomed this additional (witnessed)simulation load test--to convince all the non-believers that "pop-up" that this could happen at this site.

This is not the first time we have run across this situation and it will not be the last (until the cable configuations/codes etc. are changed )

On a single 50 foot long teck install this is not a problem.This is a rather unique site and all the math simply added up at this site to create this problem.

 

[ghostbuster]

GhostBuster...

8) Is this a new phenomenon to the site?

This is a brand new site
9) During testing, when you confirmed similar current values were measured in the un-insulated-conductor (presumably 'floating'), what was the current-frequency?

Frequency was 60 hz.
10) What was the voltage-to-ground at each end?

11) What material is used as sheathing for the Phase conductors?

The bare wire ground is in direct contact with the phase conductor insulation material-mfr would have to be contacted for insulating material composition

12) Were any measurements done with an Oscilliscope?

Used various fluke analyzers
13) If currents were measured with a Clamp-on, where was the measurement taken, if the cable is "floating"?

The ground from the cable at each end was tied together and the clamp-on was placed over this bare wire ground cable
Phil

Refer to enclosed pictures for additional info and responses in bold

 

[Phil Corso]

GhostBuster...

14) how about a "guess-timate" for length?

Phil

 

[Phil Corso]

GhostBuster,

15) During the test, with the EGC connected in a loop, what were the phase currents?

16) Did the 12-15A ground-current measurement vary with phase-current?

Phil

 

[ghostbuster]

GhostBuster,

15) During the test, with the EGC connected in a loop, what were the phase currents?

16) Did the 12-15A ground-current measurement vary with phase-current?

Phil

Phil

Yes, the EGC induced current in this loop was proportional to the phase current and varied:ie. as phase current increased--- the induced EGC current increased accordingly

I believe legal action maybe taken on this project therefore exact test values etc. are confidential to our client.

 

[Phil Corso]

GhostBuster...
Ahh...litigation! The thing that keeps "Arragantors" rich!

14) You can't estmate lengths of the conductors used for the test?

17) When you tested was phase-current source free of Harmonics?

Phil

 

[ghostbuster]

GhostBuster...
Ahh...litigation! The thing that keeps "Arragantors" rich!

14) You can't estmate lengths of the conductors used for the test?

17) When you tested was phase-current source free of Harmonics?

Phil

Phil

We used a pure resistive load bank--therefore there was only 60 hz. current components.

These test results corresponded 100% to the actual site measurements at each lighting load

panel.