Article 250.4: useless in TN-S? 1

[Elie2]

Hi,

I have a 100kVA load 300m far from the substation and i had to run 3 cores for the 3 phases and 1/2 core for the neutral and 1/2 core for earth.

I did install an earth pit near the load but i put the 1/2core cable as well so that the earth will not be the only return path for the fault.

Now that everybody is happy and the money was paid, please someone correct me - isn't this a complere waste of money and duct space for the earthing cable? I mean as per 250.4 (B)4:

Electrical equipment ... shall be installed in a manner that creates a permanent ... to the electrical supply source to facilitate the operation of overcurrent devices should a second fault occur on the wiring system.

Ok. My 2 questions are:
1. If at the MDB i do NOT have RCDs or any other earth fault sensing equipment, then i do not need this cable - right?
2. What type of second fault can happen that will help trigger the breaker?

In my opinion: the resistance of the earthing cable over 300m is still much less than the 5 Ohms of the earth pit, so that the code does not want the short circuit current limited by the increased resistance - but still is is the magnetic unit around the faulty Phase in the circuit breaker that will interrupt this LE fault.

The purpose of the earth pit near the load reduces the touch voltage in case someone touches the metallic body when it is faulty.

Any clarifications is appreciated and thanks

 

[alehman]

It is absolutely not a waste of money and is indeed critical to the safety of the occupants. If the only return path is the earth, a fault to ground would have the earth as part of the circuit. That resistance would likely limit the fault current such that the overcurrent protection (whether GF or normal phase OC) may not operate. That would be a serious electrocution and fire hazard.

That is why the code requires an equipment grounding conductor (be that a cable or the raceway system). I agree that the earthing electrode may be a good idea for step potential, but it cannot ever replace the equipment ground.

I can't find your specific quote in my copy of the NEC. The bit about "should a second fault occur on the wiring system." makes me think you are reading in a section about high-resistance grounded or ungrounded systems. The same rule applies either way, though.

 

[Elie2]

Hi,

Thank you - so we agree it is only to prevent the s.c current be limited.

Ok, here's another: the earthing electrode near the load is connected to the earthing ring made of a bare 70mm2 copper cable buried @60cm depth all around the camp. This ring passes near the substation and the earthing bar in there is connected to it. So basically, i Do have a permanent path between All the distribution boards. So: if i size this conductor to be the maximum reuired, can i eliminate all the yellow and green cables?

For the code, i am refering to year 2002 edtion - i think they removed it.

Thanks again.

 

[rbulsara]

The answer to your last post is also a NO if you are saying you want to remove equipment grounding conductors.

The ground ring forms the grounding electrode and you still need the equipment grounding conductor in one form or another. A separate equipment ground conductor is best way of doing it. Other acceptable EGC could be metal conduit, armour etc, but they are not as reliable.

Please look up definitions in NEC and Read article 250 completely. Also refer to Green Book of IEEE for recommended practices for grounding/earthing.


Oh yes, the EGC is not to limit the SCC but infact to produce more SCC , to facilitate to trip a breaker or open a fuse. The EGC provides the path of least resitance back to the source for the fault currents. This allow quick isolation of a fault and ensures safety of the personnel.

If you would wait till you yourself get adequate or comparable experience and understanding of electrical systems, before you start challenging accepted good practices observed by other experienced people,especially related to safety, that will help a great deal to extend your career, help you succeed and earn respect.

 

[Elie2]

Thanks rbulsara.

As per my first post, i agree with the step voltage and the increment of s.c. value.

I have made a little drawing on this page to explain what i mean:

http://lndang.tripod.com/

I am installing all condictors as per NEC - also as per my first post. It is only after i do so i ask here to understand or maybe sometime i have a point.

So once again, the question is:

I have a permanent path between the MDB and the distribution panels made by the earthing ring conductor, sized not to 70mm2 as per major recommendations, rather its size is chosen to be equal to the highest would-be dedicated earthing conductor run from the MDB to any panel.

i.e. if i have 2 panels, one supposed to be fed by 4x185mm2 + T90, the other is supposed to fed by 4x240mm2 + T120, then the earthing ring around the facility would made of one bare copper 120mm2 conductor and the T90 conductor and the T120 are gone, the panels are fed only by 4x185 and 4x240mm2. The earthing is done by a link between the earthing bar in the panel and the earthing ring.

Why wouldn't this ring be useful as a return path for an LE fault?
What part of safety is at risk in this case?
In fact: all panels that needed an earthing cable less than 120 are now benefiting from increased safety.

Thank you for your help, it's just a question.

 

[rbulsara]

The ring is the "System Grounding Electrode" and not "EGC". Ring is just the extension of earth. You still need to install EGC.

EGC is the means that "earths" the equipment or connects to the System Groudning Electrode. System electrode and EGC are not substitue for each other, but both are necessary.

It is a different matter that the fault current will be shared by the EGC and the ring, but most current will flow thru the EGC.

In other words, just becasue a steel structure building have grounded steel available everywhere, does not mean that you do not install EGC and just connect each equipment to builing steel only. This does not meet the intent or requirement of the Code. If you want to get in the reasoning of Code requirement, this will be a looooobg thread!!!!!!!

 

[davidbeach]

Elie2,

Why are you mixing the NEC with IEC systems? TN-S has nothing to do with NEC systems, it is not defined, nor is the term even used. Your wire sizes and terminology also suggest an IEC origin. Then NEC represents the way things are done in one area; the IEC realm does things differently. I won't try to get into right or wrong, but there are many IEC things (from what I understand) that would make me extremely nervous, as I expect there are many NEC things that would make someone familiar with IEC requirements nervous.

I imagine that if I actually understood all of the relevant IEC requirements, they would make sense as a whole, just as the NEC requirements make sense as a whole. But for someone familiar with one system to try to implement a part of the other system without thoroughly understanding the whole concept is probably asking for trouble.

 

[Elie2]

davidbeach - tyvm

So what is the equivalent of IT, TT, TNC and TNS? NEC has only 1?

I follow BS and IEC - but whenver i can i try to value NEC as well. NFPA 70 is a little problematic for LV distribution when all products are to IEC and project speces in BS/CSI, but not for the rest of the code, like for ex. NFPA 72 is the one and only in fire alarm for us here.

Please, one more thing then: i need a quatity of floor boxes to install outdoors. I ran over EU catalogs for an IP65 box but could not find - and there are none. Finally i found one from Hubbell:

http://www.hubbellcatalog.com/raco/RACO_datasheet.asp?PN=5132-0&FAM=BellWeatherproof

but it only says Weather proof. IP codes are defined in IEC 60529, so how do you grade protection in the US system - is the word Weatherproof enough? The same goes for "Form" separation of enclosures, which in BS - for ex., "Form 4b Type 7" means busbars, breaker, input, output legs and lugs ... all are in separate compartments - does the same apply or you have something else? just a link will do - i will do reading.

Thanks again, nice tip.
Ty2 rbulsara and alehman.

 

[jghrist]

Using the earth ring instead of an equipment grounding conductor increases the impedance to the source for earth faults. This is because the distance between the phase and earth ring conductors creates a very large reactance.

 

[weh3]

I think you are going to get in difficulties if you try to mix grounding philosophies between IEC and NEC. BS=British Standard?

This may belong in another thread, regarding enclosure ratings. Enclosures in the US are rated according to NEMA 250, with a number. There is no equivalent conversion from NEMA to IP, but you can generally find an IP rating that meets NEMA 250:

NEMA 1: general purpose enclosure with no special protection
NEMA 3R: rainproof
NEMA 4: corrosion resistant
NEMA 4X: corrosion resistant, washdown (usually followed by a material specification such as stainless steel)
NEMA 6: various degrees of protection against submersion
NEMA 6P: prolonged submersion
NEMA 7: explosionproof
NEMA 9: explosionproof (one is indoor and one outdoor)
NEMA 12: dustproof
NEMA 13: oil resistant

For motor control centers, there is a Class and there is a Type, but check the catalogs (Square D, GE, Cutler-Hammer, and Allen-Bradley) for more details. You may also be interested to know for purposes of the original question that service equipment must be labeled as suitable for use as service equipment (it must have a main bonding jumper between the GCC and EGC buses).

Regards,
William

 

[davidbeach]

I imagine that I could do a web search and find out what the various IEC grounding systems are, there have even been discussions in this forum, but I don't know what they are. I do remember that at least some of them allow the use of one conductor to be both the neutral (grounded conductor, current carrying) and the equipment ground (grounding, bonding, not intended to carry current other than fault current) while the NEC does not allow this except for a very limited number of existing conditions but does not allow new installations.

There are a few basic grounding requirements, but in general most systems will be solidly grounded and the circuits include an equipment grounding conductor. There are limited conditions where high resistance grounded or ungrounded systems are permitted.

 

[EEIre]

Trying to create a hybrid NEC/IEC/BS electrial installation is a bad idea.

Take it from someone who has worked on both. When in an IEC country use IEC standards. Likewise in the US andother countries of American influence use NEC standards

 

[alehman]

jghrist provided the correct answer as to why the earth ring is not permited as an EGC under the NEC.

 

[Elie2]

davidbeach: the system you're talking about is TN-C: Terre et Neutre Confobdus or Combined EarthNeutral. It saves one conductor.

weh3: yes BS to me is British Standard - specifically BS7671 IEE Wiring Regulations 16th edition 2001 Amendment 2-2003 (which includes the new cable color code).

seanmx: i think so, nect thing you know they want RCDs

alehman & jghirst: a design software i use (not yet ETAPS but soon) allows the option for the earthing conductor as either withing the cable, outside the cable but within the same duct, or in another duct. But i do not have the option of using the earth ring as EGC So i cannot simulate the value of this impedance. Normally i take the reactance Lw=0.08, what is a good figure in case the earth ring was used?

 

[mc5w]

For systems that are outside of buildings and ahead of the service switch, NEC does allow the neutral to be grounded at multiple points and to also serve as that equipment ground. Multiple grounding of over 600 volt primary wiring is also permitted similar to utility practice.

NEC and IEC grounding practices are slightly different but the laws of physics that dictate how to do grounding will always be the same. You can have some differences about how large and equipment grounding conductor needs to be but that is really about it.

 

[jghrist]

To find the impedance of the circuit with a earth ring return path, you'll have to get back to basics. An accurate calculation would be very complex because of the multiple return paths (at least two wires and earth). A conservative approximation could be made by assuming return in only one side of the earth ring. If you can find tables that show self-reactance and a separate spacing factor, you could use an average spacing between the phase and earth ring conductors to determine the spacing factor. Note that complications arise because the return path is a different conductor and length than the phase conductor.

I'm afraid that except to get a rough idea of the impedance, you will have to get out your old college textbooks on power system analysis and calculate the inductance with

L = 2·10^-7·ln(D/D_s) H/m

Tables of self-reactance and spacing factors just break this basic equation into two parts:

L[self] = 2·10^-7·ln(D_base/D_s) H/m

and

L[spacing] = 2·10^-7·ln(D/D_base) H/m

All this won't help if you are required to meet the provisions of the NEC. You'll have to install an equipment grounding conductor anyway.

 

[alehman]

I think another reason for the NEC rule may be enforcability. In many situations, I could envision an electrician arguing that there is a good return path without any way to prove it. Showing an earth ring on an as-built drawing is one thing. It may not be practical in many cases for an inspector to require the earth ring be execavated to prove it exists, particularly in old installations.

 

[mc5w]

I have also seen what happens when electricity flows through the soil. What you get are nasty tingle voltages. 1 instance was a dying underground wiring run that was leaking 120 volts into the soil. The other was an open neutral condition also at a residence.