Ground wire (PE conductor) within power cable? 2

[JuanBC]

Hi,

Imagine a big (O&G/power plant) facility (composed of many buildings and equipment) that has a large grounding mesh beneath it. This mesh is completely interconnected with each other.

Is it mandatory that every power cable feeding different boards/motors in this installation has a bare/isolated grounding (PE-Protective earth) wire within it? Or can I only distribute 3 phases and neutral and bond each board's ground bus to this mesh?

I am looking for standards under IEC or NEC world to justify or not this practice

Thank you in advance!

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[cuky2000]

Yes, equipment-grounding conductor (EGC) is needed. Check the NEC art 250. The plant ground grid is not recognized as EGC per any recognized code that I know. Below are a few remarks associated with equipment grounding conductors:

a) EGC is used to ground the noncurrent-carrying metal parts of equipment. Its function is to keep your equipment as close as possible to ground potential and provide a safe path for ground-fault current to flow back to the source.

b) An effective grounded-fault current path instead of a random return path to via the earth, guaranty a low-impedance path designed and intended to carry current under ground-fault.

c) The EGC is used to connect non-current-carrying metal parts of equipment, raceways, or other enclosures to the system neutral (grounded conductor) and/or grounding electrode conductor at the service with a direct low impedance path to allow the highest current flow back to the source with the best chance to operate the overcurrent protective devices and shut down the system.

d) The EGC under normal operating conditions does not carry current. EGC also provides a different function than the neutral conductor.

e) The EGC is not an option. If you are in the USA, beware that power plant is under the building code and the NEC jurisdiction and must comply with OSHA safety standard.

 

[dpc]

cuky2000 is totally correct. There is no option. Article 250 in the NEC requires an EGC and for good reason. In addition to cuky2000's points, here's another - the EGC is intended to provide a LOW IMPEDANCE path for ground fault current to flow, allowing sufficient ground fault current to trip the upstream protective device. Any ground fault path outside the conduit containing the phase conductors will have a much higher inductive reactance, regardless of the cross-section of the conductor. Ground conductors outside the conduit are markedly inferior to the EGC inside the conduit at providing a fault return path.

Test data clearly shows this. Do a Google search for "Let's Be More Specific About Equipment Grounding". This is an old paper by Kaufmann of GE. It's a good starting point if you want to understand the physics involved.

 

[waross]

I mostly agree with cuky and dpc, but;
The grounding conductor provided two functions;
1.

the EGC is intended to provide a LOW IMPEDANCE path for ground fault current to flow, allowing sufficient ground fault current to trip the upstream protective device.

The EGC does an adequate job of doing this.
BUT;

a) EGC is used to ground the noncurrent-carrying metal parts of equipment. Its function is to keep your equipment as close as possible to ground potential and provide a safe path for ground-fault current to flow back to the source.

The Equipment Grounding Condustor does a very poor job of doing this in many installations.
On a 480 Volt circuit, there is 277 Volts to ground.
In the event of a ground fault in a motor, the faulted phase conductor and the EGC form a voltage divider across the 277 Volts.
If the EGC is sized to code, it will be smaller than the circuit conductors.
Hence more than 1/2 of 277 Volts or greater than 139 Volts of touch potential may appear on the surface of a motor with a fault until the protective device clears.
In many petro-chemical plants, As well as the code compliant EGC with the supply conductors, there is also installed a grounding conductor directly to the local ground grid.
The NEC is a minimum standard and in some instances the NEC cannot be depended upon to limit touch potentials to safe levels during a ground fault.
I don't have the motor grounding tables to hand but from memory, a motor fed with #12 AWG phase conductors may require a connection to the local grid by a conductor of about #4 AWG to limit touch potentials and possible ignition sources.
If any one has access to the API grounding tables, please correct my memory of a #4 AWG conductor.
Thanks
Short version:
NEC requires an EGC installed with the circuit conductors.
Engineering standards and other codes (American Petroleum Institute?) may require an additional grounding conductor directly to the local ground grid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JuanBC]

Thank you all for your reply

I am reading that cable trays and metallic conduits (as RMC or IMT) can be used as EGC as long as they meet some requirements.

In that case, an "extra" EGC within the power cable is not necessary, am I right?

Is this still valid if the entry to the boards/motors is made using cable glands? In this case, the conduit and the metallic parts of the boards/motors won't be bonded because it's always exist a gap between the conduit end and the equipment terminal box

JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[wroggent]

There are a couple of old plants in our fleet that were apparently built without wire type EGCs for some voltage levels. In some cases the raceway system may serve as an EGC but this is more by coincidence than by design. And in those cases bonding between raceway and enclosure or raceway to raceway is definitely sub par with respect to today's requirements. For the electricians that work at those plants and that's all they've ever known, they believe a connection to the plant's buried/encased grid to be an excellent means of grounding. I need to find a way to teach them that an EGC is both necessary and superior. I'm kind of curious when and EGC with or around the circuit conductors became an NEC requirement. Anyone know? The oldest NEC version available for viewing on NFPA's website is 1968 and back then it was a requirement. Although, in both the 1968 and 1971 versions it says that equipment connected to structural members are considered grounded. The 1978 version clarified that a connection to structural members is not to serve as the required equipment grounding conductor - perhaps that was a point of confusion back then. It wasn't until the 1999 revision that the statement about a connection to the structure went away.


cuky, I think applicability of certain codes is dependent on your state, county and city. If you know of something that says the NEC is applicable to power plants at a federal level I'd like to learn about this. Definitely OSHA is applicable though. Note the exception for installations prior to 1981.

 

[dpc]

The NEC still allows metal conduit to be used as an EGC due to the steel conduit lobby, but it's a horrible idea. The continuity of the conduit path deteriorates with time and becomes close to worthless after a few years. Don't even think about doing this, at least IMO.

 

[waross]

The old coupling were bad. The couplings had a straight thread that allowed the ends of the conduit to touch together. Over time corrosion of the zinc coating compromised the continuity.
The newer style couplings with tapered threads are much better for long term continuity. There are still concerns with transitions to flexible conduit and loose locknuts.
Electrical Metallic Tubing with die cast. set screw couplings and connectors is good if it is not disturbed in any way.
That is almost impossible. Any bending movement stretches the couplings or connectors and leaves then loose with poor continuity.
Given the numerous conditions that may compromise bonding when rigid conduit, EMT, and/or flexible conduit are used, good practice is to run an equipment grounding conductor with the power conductors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[resqcapt19]

Conduit couplings used in the United States are straight thread. The only tapered threads on our conduit systems is the male threads on the conduit and some of the female threads on fittings. As far as I know the US has never used tapered threads for conduit couplings. I do know that up until the late 70's or early 80s the conduit couplings used in Canada had tapered threads. UL 6, Electrical Rigid Conduit--Steel, says: "5.7.3 A coupling shall be straight tapped."

 

[waross]

Canada used to use straight threads. Years ago they changed over to a special coupling; tapered threads but shorter than a plumbers plain coupling. The couplings were stamped EC.
I haven't done or even been close to rigid conduit work for years now. I don't know what is the standard coupling in Canada now.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

A kind of EGC has to be provided.
If a grounding grid it is considered earth then EGC has to be connected to this grounding grid in no matter what manner:
the grounded conductor in a multiconductor cable, a grounded conductor close to the ungrounded conductors a cable tray or a metallic conduit.
If a cable run in a cable tray EGC will be cable tray. In an underground run the grounding conductor will run along the cable ducts and will be grounded through grounding electrodes in each manhole and at the ends.
In IEC World EN 50552 or IEC 60364-1,or -4-41 BS 7671/2008 Regulation 312.2 Type of system earthing. For type TN -for instance[fig 3.8]:
The protective conductor [PE] is the metallic covering of the cable supplying the installations or a separate conductor
metallic covering=suitable to withstand the prospective short-circuit current, as cable tray, conduit, duct or else.

By the way, the grounding grid has to comply with IEEE 80/2013 or EN 50552/2016 [BS 7430/2011?]