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junmal (Electrical) (OP)
3 Jul 08 3:25
Hello All,
     I am an electrician working for a construction company here in the middle east constructing a big gas to liquid plant.  We have installed in the construction site several diesel engine generators ranging from 50 KVA to 900 KVA units as part of temporary electrical power supply.  We have also have several units of portable type diesel generators ranging from 5 KVA to 20 KVA.  Our client, a big name in the oil and gas industry, required us to install grounding/earthing system for each of the generators with measurement of no less than 4 ohms.  All activities here in the construction site are covered by our client's set of procedures and guidlelines.  For all electrical works the procedures are mostly based on the IEC and BS standards.  My question:  was the maximum 4 ohms earth resistance a standard from the IEC?  If yes, what particular IEC or BS standard is it?  When I was working in the deserts of Saudi Arabia our client would require us for our diesel generator an earth resistance of not  more that 25 ohms.  What about for the smaller units of portable gensets (less than 20KVA) which are always transferred from one area to another.  Is the 4 ohms requierment too stiff??  Can anybody, please, enlighten me on this.  Appreciate very much any response you could give.  Regards and more power to this site.   
Helpful Member!  ausgen (Electrical)
3 Jul 08 7:48
junmal,

Fluke publish a testing guide that includes a section on what a "good" ground resistance value should be. It makes reference to NEC and IEEE recommendations (though I'm unsure if these remain current).

http://support.fluke.com/find-sales/Download/Asset/2633834_6115_ENG_A_W.PDF

For the record, the relevant AS (1768) which is derived from IEC 62305 recommends 10 Ohms.  With your gens, bonding into the existing building earth is the usual method but if you have a transportable systems providing prime power for lighting rigs and the like you might struggle with the standard single stake that comes with these sets.
rbulsara (Electrical)
3 Jul 08 15:28
According to IEEE Green Book Std 142, the 25 ohms requirement in NEC is the maximum resistnace for a single ground rod. If you have multiple ground rods/electrodes it is not very difficult to achieve less than 5 ohms ground resistance. There is no implication that 25ohm per se is a satisfactory level for a grounding system.

IEEE 142 also says 2-5 ohm range is found suitbale for most industrial applications. 1 ohm may be obtained using multiple grounding electrodes connected together for larger systems

Plus a client may require to exceed codes or standards. If it was in a contract specification and you agreed to that you must strive to provide it, unless it is impossible for some reason.
jghrist (Electrical)
3 Jul 08 17:49
In the USA, the NEC doesn't require a ground rod at all for portable generators where the service is by cord and plug connection to receptacles mounted on the generator.  If you connect metallic parts of all equipment to the generator frame with an adequate equipment grounding conductor (EGC or safety ground), then resistance of the grounding electrode doesn't make any practical difference.  Essentially all of the fault current will flow in the EGC, not in the earth.

You still have a contractual obligation to get a 4 ohm ground resistance, however.  In desert areas, this may be next to impossible.
 
junmal (Electrical) (OP)
4 Jul 08 23:52
ausgen, rbulsara & jghrist,
    Thank you all for the valuable info you provided.  
    Jghrist, if not asking too much, could you explain more why it's not anymore necessary to provide ground rod for the portable genset units.  Because what we're doing here is that to drive a 600mm ground rod into the ground for all the small portable gnesets (less than 20KVA)as grounding system for the unit.  This is a part of safety precaution as per my superior.  By the way we have numerous portable units being used at the construction site which is a Robin brand 5KW unit single phase.  We also have Miller brand arc welding units deiesel engine driven.  These welding machines are also required to have a ground rod driven to ground.
edison123 (Electrical)
5 Jul 08 21:37
ausgen

LPS*
jghrist (Electrical)
5 Jul 08 23:46
It is not required by the USA National Electrical Code to drive a ground rod for portable or vehicle-mounted generators if the generator suppys only equipment mounted on the generator or cord- and plug-connected equipment through receptacles mounted on the generator.  Requirements may vary in other countries.  The basic idea is that any fault would return through the cord ground wire instead of earth regardless of whether or not there is a ground rod.  If the fault returns through the ground wire, then touch-voltages do not depend on ground resistance.

If the load is at a distance from the generator such that a significant part of the fault current would return to the generator through the earth, then there may be a touch-voltage consideration.  It would depend on the grounding both at the generator and at the fault point.  You could do a detailed grounding analysis to determine what the touch-voltages would be, but in general, I think with a higher ground resistance, more or the return current would be through the equipment grounding conductor.  I doubt that a detailed analysis would show that a 4 ohm ground resistance would offer any safety benefits over a higher resistance.

Extensive grounding electrodes provide higher safety where there is a remote source, and fault current flows through the earth to return to the source.  Where the source is local, the metallic paths back to the source keep voltages low.  Low ground resistance may be desirable for lightning protection, however, even where the power source is local.
 
Kiribanda (Electrical)
6 Jul 08 14:04
junmal,

If your wiring and earthing (mostly TT) is done per IEC/ BS then the earth resistance should be as low as possible because the earth fault protection is provided by ELCBs. In that case you have to satisfy the condition of "Maximum allowable touch voltage = 50V" because you taking the return earth fault current through the soil. Please refer IEC 60394 or BS7671.

If your wiring and grounding is done per NEC, then your ground fault current is taken through the neutral back to the source (equivalent to TNCS method of earthing in IEC 60394 or BS 7671). In that case your grounding electrode is just a safety ground and you can go up to 25 Ohms per NEC. Also ground fault protection can be provided by overcurrent devices supplementry to GFCI devices.

Also, if you install a ground grid inside a HV substation then 1 Ohm maximum for large generating stations and 5 Ohms maximum for medium size substations to achieve safe touch and step potentials during a ground fault inside the substation area per IEEE 80 or local code requirements.

So in my opinion the Ohm value depends on the application and the std/ code the client is using.

Kiri
ausgen (Electrical)
6 Jul 08 19:19
Edison - yes, our NEC equivalent (AS3000) deliberately avoids nominating a resistance value.  Although it's geared towards ensuring fault clearance under fully metallic return conditions I would prefer it nominate a value and then for the periodic verfication standard (AS3019) to enforce the same value over time.

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