Right now I don't have the Code book, but I believe that NEC requires the equipment grounding conductor for resistance grounded systems also. What is the rationale behind this? For eg. for a LV system with 10A resistor, we would only be detecting GF current using CBCTs and only for detection, with no intentional tripping. In that event, why would we need a low impedance path for the fault current using a PE conductor? Even fault in milliamps can be detected using a CBCT and GF relay on each LV feeder.
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Need for Equipment grounding conductor in LV high Resistance grounded system 1
- Thread starter cherryg
- Start date
Dear Mr. cherryg (Electrical)(OP)19 Jul 22 18:51
"...#1. .... I don't have the Code book, .... I believe that NEC requires the equipment grounding conductor for resistance grounded systems also. #2. What is the rationale behind this?? eg. for a LV system with 10A resistor, we would only be detecting GF current using CBCTs and only for detection, with no intentional tripping. #3. In that event, why would we need a low impedance path for the fault current using a PE conductor? Even fault in milliamps can be detected using a CBCT and GF relay on each LV feeder..."
#0. Usually for LV installation works, the " Code " book refers to the NEC in US. Whereas PE conductor is an IEC terminology; which does NOT appear in the NEC.
#1. Yes. See NEC 250-36 titled High-impedance grounded neutral systems for detail.
#2. The equipment including enclosure is connected by PE conductor to the Ground bar. A bonding jumper conductor connects the Ground bar to the grounded side of the grounding impedance..
#3. The purpose is to ensure at all times that the equipment including enclosure is maintained at Ground (in US) or Earth (in IEC) potential; safe to touch.
Che Kuan Yau (Singapore)
"...#1. .... I don't have the Code book, .... I believe that NEC requires the equipment grounding conductor for resistance grounded systems also. #2. What is the rationale behind this?? eg. for a LV system with 10A resistor, we would only be detecting GF current using CBCTs and only for detection, with no intentional tripping. #3. In that event, why would we need a low impedance path for the fault current using a PE conductor? Even fault in milliamps can be detected using a CBCT and GF relay on each LV feeder..."
#0. Usually for LV installation works, the " Code " book refers to the NEC in US. Whereas PE conductor is an IEC terminology; which does NOT appear in the NEC.
#1. Yes. See NEC 250-36 titled High-impedance grounded neutral systems for detail.
#2. The equipment including enclosure is connected by PE conductor to the Ground bar. A bonding jumper conductor connects the Ground bar to the grounded side of the grounding impedance..
#3. The purpose is to ensure at all times that the equipment including enclosure is maintained at Ground (in US) or Earth (in IEC) potential; safe to touch.
Che Kuan Yau (Singapore)
@ Mr. Parchie (Electrical)20 Jul 22 01:20
"....#1. LV systems are usually grounded, with no resistors. #2. Unless your system dictates there are so many things that can happen if your system gets faults and the system needs to be ungrounded...."
#1. Yes. There are acceptations, for example that most electrical system on board of ships are without neutral. This is considered as essential that any one phase shorted to earth does NOT trip out the system !.
#2. Per NEC 250-36 titled High-impedance grounded neutral systems is NOT the same as ungrounded system for example widely practised in marine system. Each system has their pros and cons.
Che Kuan Yau (Singapore)
"....#1. LV systems are usually grounded, with no resistors. #2. Unless your system dictates there are so many things that can happen if your system gets faults and the system needs to be ungrounded...."
#1. Yes. There are acceptations, for example that most electrical system on board of ships are without neutral. This is considered as essential that any one phase shorted to earth does NOT trip out the system !.
#2. Per NEC 250-36 titled High-impedance grounded neutral systems is NOT the same as ungrounded system for example widely practised in marine system. Each system has their pros and cons.
Che Kuan Yau (Singapore)
- Thread starter
- #5
Che Kuan Yau.....I have used High impedance grounded systems in most of the process plants which need continuity. NEC is clear about the purpose for EGC. But I am questioning the need of return path through EGC where we are operating the load with ground fault. The modern day Core balance CT with ground fault relay will pick up the fault instantly and signal it. Isolation of the fault is responsibility of the AHJ. All we need is to flow some few amps or milliamps for operation of the relay. As such there is no need to run miles of EGC to achieve it.
- Thread starter
- #6
Parchie...the reference here is not normal domestic or industrial plants which can tolerate tripping the circuit on ground fault. The discussion here is for numerous process plants where no tripping for L-G fault is tolerated, without a decision taken by AHJ.
The NEC clearly requires the EGC even for HRG systems, unless something has changed recently. I suspect the concern is that it would be relatively easy to convert the system to a (much more common) solidly-grounded system at some point, and then the EGC is more important. I have seen this happen - system was designed as HRG, but after installation, it was immediately converted to solidly grounded.
Dear Mr. cherryg (Electrical)(OP)20 Jul 22 16:12
".....I have used High impedance grounded systems in most of the process plants which need continuity. NEC is clear about the purpose for EGC. But I am questioning the need of return path through EGC ........ The modern day Core balance CT with ground fault relay will... Isolation of the fault is responsibility of the AHJ. All we need is to flow some few amps ...... for operation of the relay. As such there is no need to run miles of EGC to achieve it."
1. Earthing of the equipment including enclosure is mandatory per IEC/NEC, irrespective of whether earthed/high impedance ground/isolated system.
2. To save cost on the long length of EGC conductor, I am " thinking " would it be acceptable? to earth one end of the impedance at the station and a separate connection of equipment + enclosure to earth at the load end (i.e. TT-system per IEC). The Earth (IEC)/Ground soil (NEC); serves the purpose of EGC.
Che Kuan Yau (Singapore)
".....I have used High impedance grounded systems in most of the process plants which need continuity. NEC is clear about the purpose for EGC. But I am questioning the need of return path through EGC ........ The modern day Core balance CT with ground fault relay will... Isolation of the fault is responsibility of the AHJ. All we need is to flow some few amps ...... for operation of the relay. As such there is no need to run miles of EGC to achieve it."
1. Earthing of the equipment including enclosure is mandatory per IEC/NEC, irrespective of whether earthed/high impedance ground/isolated system.
2. To save cost on the long length of EGC conductor, I am " thinking " would it be acceptable? to earth one end of the impedance at the station and a separate connection of equipment + enclosure to earth at the load end (i.e. TT-system per IEC). The Earth (IEC)/Ground soil (NEC); serves the purpose of EGC.
Che Kuan Yau (Singapore)
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1
- #9
It looks to me that you are talking about two different things.
1) Industrial installation: ECG is the bonding conductor between the equipment metallic enclosure and the ground terminal of 10A NGR so that during a GF the GF
current is limited to 10A. Without that set up how can you limit the current to 10A? You need to install a suitably sized CBCT (ZCT) to catch the current draining to the ground.
2) In a domestic installation there is no NGR installed. If the ground fault protection is provided by the branch circuit MCCB, then the EGC has to be bonded to the NEUTRAL
terminal at the panel board so that during a ground fault of the branch circuit adequate GF current is ensured to trip the MCCB.
3) In a domestic installation there is no NGR installed. If the ground fault protection is provided by a 10mA GFCI, then the EGC is NOT required because the ZCT inside the GFCI
is so sensitive that it will trip when there is a 10mA or 6mA current drain even to the wall.
4) If the equipment is double insulated (eg. electric iron) the it is provided with two wires only and no EGC. You may install a GFCI for GF protection. In fact, the ground fault
protection is provided by the insulation it self.
1) Industrial installation: ECG is the bonding conductor between the equipment metallic enclosure and the ground terminal of 10A NGR so that during a GF the GF
current is limited to 10A. Without that set up how can you limit the current to 10A? You need to install a suitably sized CBCT (ZCT) to catch the current draining to the ground.
2) In a domestic installation there is no NGR installed. If the ground fault protection is provided by the branch circuit MCCB, then the EGC has to be bonded to the NEUTRAL
terminal at the panel board so that during a ground fault of the branch circuit adequate GF current is ensured to trip the MCCB.
3) In a domestic installation there is no NGR installed. If the ground fault protection is provided by a 10mA GFCI, then the EGC is NOT required because the ZCT inside the GFCI
is so sensitive that it will trip when there is a 10mA or 6mA current drain even to the wall.
4) If the equipment is double insulated (eg. electric iron) the it is provided with two wires only and no EGC. You may install a GFCI for GF protection. In fact, the ground fault
protection is provided by the insulation it self.
- Thread starter
- #10
Dear che12345...I am thinking on similar lines. I am attaching the handbook excerpt of NEC Sec 250.36 which deals with high impedance system. The clear requirement is for ground fault detection (as earlier mentioned by me). The presence of equipment bonding jumper (with mention of equipment grounding conductor) in 250.36 (E) does obliqely mean that EGC needs to be installed. But the figure does not show the same. Unlike 250.32(B) Grounded system and 250.32(C) Ungrounded system, there is no direct reference to EGC in 250.36. I am almost certain that high impedance systems do not require EGC. The detection system using a CBCT and ground fault relay, which can be set pretty low, suffices the condictions. The equipment will ofocurse be locally grounded to the buried ground grid.
Dear Kiribanda: Only limiting our discussion to (1) for industrial setup, ofcourse the ground fault current will be limited to 10A or less since there is no other path for the return current. You can easily calculate the L-G fault current using the basic equation : V / {(Z1+Z2+Z0) + 3Zn}. Here the only difference is that the return path, instead of a lower impedance of EGC will now see the buried grounding system impedamce which wil be in the order of 1 ohm to 3 ohms. This will drive the current lower than 10A, but should be easily detectable by CBCT and senstive ground fault relay of the faulted feeder.
Dear Kiribanda: Only limiting our discussion to (1) for industrial setup, ofcourse the ground fault current will be limited to 10A or less since there is no other path for the return current. You can easily calculate the L-G fault current using the basic equation : V / {(Z1+Z2+Z0) + 3Zn}. Here the only difference is that the return path, instead of a lower impedance of EGC will now see the buried grounding system impedamce which wil be in the order of 1 ohm to 3 ohms. This will drive the current lower than 10A, but should be easily detectable by CBCT and senstive ground fault relay of the faulted feeder.
- Thread starter
- #11
Another point to be noticed, Sec 250.187 which deals with High impedance systems for greater than 1000V, clearly identifies Equipment Grounding conductor in Sec 250.187 (D). But any such EGC reference is not there in Sec 250.36 (A) to (G). I rest my case.
@cherryg,
The title of your post caught my eyes in the first place. LV systems are usually grounded without resistors. I did have jobs where LV systems were intentionally resistor grounded but there is no need whatsoever on having to compute ground conductor sizing when it is resistor grounded. That is because you will be choosing the ground amps when designing, either 5A or 10A. You'll only be choosing conductors sizes either for 5A or 10A.
The title of your post caught my eyes in the first place. LV systems are usually grounded without resistors. I did have jobs where LV systems were intentionally resistor grounded but there is no need whatsoever on having to compute ground conductor sizing when it is resistor grounded. That is because you will be choosing the ground amps when designing, either 5A or 10A. You'll only be choosing conductors sizes either for 5A or 10A.
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