Shielded 15kV cable fault current capacity

[Isaiah]

We have 15kV shielded cable (13.8kV system) from an outdoor substation with low resistance ground on the upstream transformer neutral. The associated CT interfaces with a protective relay that opens the line side CB.
Engineering says the circuit needs an EGC (#2/0) for fault current however NEC 250.190(C)(2) states the shield can be used as the EGC if a fault can be cleared without damaging the shield.
I’m just wondering where the relay needs to be set before the fault causes said damage.
Does anyone have experience with this?


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[petersonra]

We have 15kV shielded cable (13.8kV system) from an outdoor substation with low resistance ground on the upstream transformer neutral. The associated CT interfaces with a protective relay that opens the line side CB.
Engineering says the circuit needs an EGC (#2/0) for fault current however NEC 250.190(C)(2) states the shield can be used as the EGC if a fault can be cleared without damaging the shield.
I’m just wondering where the relay needs to be set before the fault causes said damage.
Does anyone have experience with this?


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It seems to me the engineer that designed the system would be the guy to ask.

 

[David Castor]

The cable manufacturer can give you data on the short-time capability of the shield during faults. But I'd advise pulling in a ground wire, regardless of the grounding method.

 

[Isaiah]

The cable manufacturer can give you data on the short-time capability of the shield during faults. But I'd advise pulling in a ground wire, regardless of the grounding method.

Thanks Dave. But is the additional EGC actually required or just “advisable”…


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[don_resqcapt19]

Thanks Dave. But is the additional EGC actually required or just “advisable”…


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That takes some engineering design to answer...you have to know the short circuit capacity of the shield, the available fault current, and the OCPD clearing time to determine if the shield is adequate for fault clearing purposes.

 

[petersonra]

That takes some engineering design to answer...you have to know the short circuit capacity of the shield, the available fault current, and the OCPD clearing time to determine if the shield is adequate for fault clearing purposes.

Which kind of takes you back to asking the guy that designed the system

 

[David Castor]

But is the additional EGC actually required or just “advisable”…

If you are comfortable you meet the restrictions in the NEC regarding the thermal capacity of the shield then it would not be required by the NEC. (But if I were doing the design it would be required. )

Again, the NEC is not a design guide.

 

[petersonra]

If you are comfortable you meet the restrictions in the NEC regarding the thermal capacity of the shield then it would not be required by the NEC. (But if I were doing the design it would be required. )

Again, the NEC is not a design guide.

If the shield is adequate why require a wire type egc?

 

[Isaiah]

If you are comfortable you meet the restrictions in the NEC regarding the thermal capacity of the shield then it would not be required by the NEC. (But if I were doing the design it would be required. )

Again, the NEC is not a design guide.

Construction doesn’t want to spend the extra $$$ for the additional EGC if the shield is sufficient to carry the fault until the CB opens (via the relaying). I really can’t force them to add it if they are correct.
I just have to do my homework to see if they are right.


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[petersonra]

The way this is supposed to work.

Engineer creates design and drawings. Puts PE seal on drawings.

Drawings get transmitted to contractor.

Contractor bids and builds according to drawing.

If contractor does not want to follow drawings, there is usually a path for him to take to request some kind of change.

 

[David Castor]

If the shield is adequate why require a wire type egc?

Cable shields are often not dressed and terminated correctly to permit high current flow. Relays might end up not set correctly or not function properly or the breaker might fail to trip and the rating of shield could be exceeded and the shield damaged, requiring a lengthy outage to repair. Grounding resistors can fail and can easily be bypassed in the future.

The NEC says that steel conduit can be used as a ground but we still always run a copper grounding conductor inside the conduit. Why use copper conductor if the NEC allows aluminum? Why use Steel conduit if the NEC allows PVC? Why use circuit breakers if fused switches are cheaper?

These design decisions depend on the type of facility, expected lifespan, cost of downtime, and obviously the cost of construction. In the fast-track design-build world, the field construction forces have a lot more influence over what is installed, no question.

 

[electrofelon]

Cable shields are often not dressed and terminated correctly to permit high current flow. Relays might end up not set correctly or not function properly or the breaker might fail to trip and the rating of shield could be exceeded and the shield damaged, requiring a lengthy outage to repair.

Personally I don't design around assumptions of poor or improper work by others.

The NEC says that steel conduit can be used as a ground but we still always run a copper grounding conductor inside the conduit.

I pretty much never waste my clients money on a wire EGC when they already have a perfectly good one

Why use copper conductor if the NEC allows aluminum?

Personally I use AL most of the time. It's cheaper and much easier to work with

Why use circuit breakers if fused switches are cheaper?

I have absolutely no preference, which ever one has the properties i want such as coordination and series ratings.

 

[petersonra]

Cable shields are often not dressed and terminated correctly to permit high current flow. Relays might end up not set correctly or not function properly or the breaker might fail to trip and the rating of shield could be exceeded and the shield damaged, requiring a lengthy outage to repair. Grounding resistors can fail and can easily be bypassed in the future.

The NEC says that steel conduit can be used as a ground but we still always run a copper grounding conductor inside the conduit. Why use copper conductor if the NEC allows aluminum? Why use Steel conduit if the NEC allows PVC? Why use circuit breakers if fused switches are cheaper?

These design decisions depend on the type of facility, expected lifespan, cost of downtime, and obviously the cost of construction. In the fast-track design-build world, the field construction forces have a lot more influence over what is installed, no question.

The egc might get hit by forklift and fail. Maybe you should have two of them in case one fails.

There is no benefit to using copper conductors over aluminum. A bazillion miles of aluminum utility wires strung all across the world shows that.

People deliberately sabotaging things by messing with the grounding resistor or the relay settings is not something that anything you have proposed would stop.

Generally speaking, steel conduit is a lower impedance egc than a wire.

Usually, fuses will give you better protection than a cb. The only benefit to a cb over a fused switch is you can reset the cb without having to hunt down spare fuses at 3 am on Sunday morning.

 

[Isaiah]

Cable shields are often not dressed and terminated correctly to permit high current flow. Relays might end up not set correctly or not function properly or the breaker might fail to trip and the rating of shield could be exceeded and the shield damaged, requiring a lengthy outage to repair. Grounding resistors can fail and can easily be bypassed in the future.

The NEC says that steel conduit can be used as a ground but we still always run a copper grounding conductor inside the conduit. Why use copper conductor if the NEC allows aluminum? Why use Steel conduit if the NEC allows PVC? Why use circuit breakers if fused switches are cheaper?

These design decisions depend on the type of facility, expected lifespan, cost of downtime, and obviously the cost of construction. In the fast-track design-build world, the field construction forces have a lot more influence over what is installed, no question.

I agree with you - there are too many variables to rely completely upon the shielding.
But.
When it comes to construction “cheap” usually wins.


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[David Castor]

The responses are a good reminder of why well-designed plans and specifications are a good investment for any owner. If you come up with your cheapest possible installation that meets the NEC, I guarantee that someone else can find a still cheaper way to do it.

 

[masterinbama]

We have 15kV shielded cable (13.8kV system) from an outdoor substation with low resistance ground on the upstream transformer neutral. The associated CT interfaces with a protective relay that opens the line side CB.
Engineering says the circuit needs an EGC (#2/0) for fault current however NEC 250.190(C)(2) states the shield can be used as the EGC if a fault can be cleared without damaging the shield.
I’m just wondering where the relay needs to be set before the fault causes said damage.
Does anyone have experience with this?


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My question is. What's a shield (and cable or cables) damaged by a fault going to cost to repair, later down the road?
Probably a lot less than a 3/0 EGC.

 

[Isaiah]

My question is. What's a shield (and cable or cables) damaged by a fault going to cost to repair, later down the road?
Probably a lot less than a 3/0 EGC.

This argument is viable and has already been presented.
They still said no


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[Julius Right]

NEC Art.250.190 Grounding of Equipment.
(C) Equipment Grounding Conductor
(2) Shielded Cables. The metallic insulation shield encircling
the current-carrying conductors shall be permitted to be used
as an equipment grounding conductor, if it is rated for clearing time of ground-fault current protective device operation without damaging the metallic shield.
(3) Sizing. Equipment grounding conductors shall be sized in
accordance with Table 250.122 based on the current rating of
the fuse or the overcurrent setting of the protective relay.
If the ampacity of cable is 500 A and the breaker is rated for 500 A 2 awg has to be the minimum conductor area.
If calculated according to ICEA P-45-482-1994 an usual 15 kV single-phase cable shield of 5 mils thick copper tape and 1.2 inch shield diameter presents only 18143 cmils.
From ICEA P-45-482-1994:3-Helically applied flat tape ,overlapped.[new cable, good contact ] area=4*b*dm*sqrt(100/2/(100-L)) b=5;dm=1200;L=12.5% A=18142.3 cmils 2awg=133000 cmils

 

[JoeStillman]

This is what concentric neutral cables are for.