Tools are Grounded to Send the surge of electricty to earth

[user 100]

........ But still the problem is

And the 250 section ( another suitable section would be the definition of "effective ground fault current path" in art. 100) posted by K8MHZ isn't a problem- it's there for a reason. The earth not being suitable for clearing faults in lv systems is a difficult pill to swallow for many in other countries, but facts are facts:
earth is a poor path for fault current and a ground rod/earth path is not a viable, or even theoretical, replacement for an egc connected to the source.

 

[don_resqcapt19]

The bulk of ground resistance is at the contact of ground rod with the ground and it can be reduced and so the parallel ground path could be made effective. But still the problem is

For a under 600 volt system it is very unlikely you could get a ground rod to make any significant difference in the impedance of the fault return path. The resistance of the path via the ground rod would be 100s to 1000s of times greater than that of the EGC.

 

[kwired]

For a under 600 volt system it is very unlikely you could get a ground rod to make any significant difference in the impedance of the fault return path. The resistance of the path via the ground rod would be 100s to 1000s of times greater than that of the EGC.

And if you try to use a GFCI to cause trip at lower fault current levels you will have nuisance trips from capacitive current on long runs. If you used 30-100 mA GFP you would get a response on ground faults where a standard OCPD wouldn't trip, but shock hazards for a user are a higher risk.

 

[Saturn_Europa]

Tom, can you elaborate a little on this. I have read about this in the NYT and other newspapers but they never really got into the technical side of what was causing the deaths. I always assumed it was copper piping carry current.

I was in Iraq in 2003. It was amazing the people Haliburton would hire. People from Bangladesh that were so poorly paid and treated that they would refuse to work. We used to have armed guards follow them around to make sure they worked. I thought that was pretty darn unethical. I did meet an EE from Bosnia that was pretty sharp.

 

[Sahib]

In an electrical installation an electrician working on an overhead lines attachment post got a shock and fell down. The electrical installation also included living quarters. The foreman did not switch off the incoming supply (to keep the living quarters powered) but, by mistake, removed the neutral link. He also removed the jumper connections of R,Y,B phases to the overhead lines but again failed to remove the jumper connection of the neutral. He thought the shorting link of all the overhead lines R,Y,B & N without grounding it would make it a safe installation for the man working on the post. Unfortunately, when an occupant in the living quarter switched on a light switch, the electrician got a shock, as the circuit was completed through his body, and fell down and injured.

The foreman should have avoided the above mistakes so that the accident would not have happened in the first place.

Also, the grounding of shorting link and/or grounding of neutral at the entry to the quarter (per Code) would have avoided the shock to the electrician and made the installation safe in spite of the above mistakes by the foreman.

 

[Sahib]

The illustrative drawing is attached herewith and sorry for the poor quality.

 

[Sahib]

Another advantage of having a ground rod with low ground resistance is that it reduces the possibility of flash over when a high voltage line accidentally touches a low voltage line and subsequent fire/life hazard is also reduced.

 

[user 100]

Another advantage of having a ground rod with low ground resistance is that it reduces the possibility of flash over when a high voltage line accidentally touches a low voltage line and subsequent fire/life hazard is also reduced.

A rod might help if a primary drops on a secondary or in the case of abnormally high voltages associated w/lightning. But how would you guarantee a permanent low resistance earth path in a way the is cost effective and/or practical? Besides, both of those scenarios are quite a bit different from a less than 600v fault that was the subject of the op, where a rod wouldn't be effective at all.

 

[Sahib]

A rod might help if a primary drops on a secondary or in the case of abnormally high voltages associated w/lightning. But how would you guarantee a permanent low resistance earth path in a way the is cost effective and/or practical?

MGN.

both of those scenarios are quite a bit different from a less than 600v fault that was the subject of the op, where a rod wouldn't be effective at all.

Consider 'phantom' voltages which can be quite dangerous. But if grounded, they disappear.

 

[kwired]

Another advantage of having a ground rod with low ground resistance is that it reduces the possibility of flash over when a high voltage line accidentally touches a low voltage line and subsequent fire/life hazard is also reduced.

Few years ago in a nearby town a 34.5kV conductor happened to fall on a local distribution conductor of a 4160/2400 Y system. Though I'm sure the grounding electrodes/MGN system helped facilitate quick operation of protective devices and did lessen damages - there was still a lot of damage all around town. Appeared as though if your service were supplied from the phase that saw that fault - your risk of having something damaged was going to be higher.

 

[don_resqcapt19]

MGN.

Consider 'phantom' voltages which can be quite dangerous. But if grounded, they disappear.

Can you define "phantom" voltages? The phantom voltages I am aware of have almost no current behind them and are not a danger.

 

[Sahib]

Consider a 14.5Kv single phase primary with neutral grounded and 240/120V secondary with neutral not grounded POCO transformer. Also suppose no grounding downstream with insulated EGC. Under these circumstances hazardous voltage may be present on the secondary load side depending on the capacitances between primary and secondary, secondary and ground. But if secondary side is grounded, such hazardous voltage disappears.

 

[mgookin]

Few years ago in a nearby town a 34.5kV conductor happened to fall on a local distribution conductor of a 4160/2400 Y system. Though I'm sure the grounding electrodes/MGN system helped facilitate quick operation of protective devices and did lessen damages - there was still a lot of damage all around town. Appeared as though if your service were supplied from the phase that saw that fault - your risk of having something damaged was going to be higher.

That happened in our downtown a little over a month ago. 35kV fell on a law firm (perfect place!). Caught the building on fire in several places. Plenty of damage and drama. Zero injuries.
Earlier this year POCO had R&R'd those very conductors while replacing a tranny across the street. This event happened on a clear blue day - no weather - no wind - no nothing. A crimp failed.
POCO did not even argue about liability. They just said send us the bill. They are good about that around here.

 

[iwire]

Consider 'phantom' voltages which can be quite dangerous. But if grounded, they disappear.

Can you define "phantom" voltages? The phantom voltages I am aware of have almost no current behind them and are not a danger.

Consider a 14.5Kv single phase primary with neutral grounded and 240/120V secondary with neutral not grounded POCO transformer. Also suppose no grounding downstream with insulated EGC. Under these circumstances hazardous voltage may be present on the secondary load side depending on the capacitances between primary and secondary, secondary and ground. But if secondary side is grounded, such hazardous voltage disappears.

What you are describing here is not a 'phantom voltage' it is a wiring fault. A better plan than adding an electrode is to fix the wiring faults.

 

[Sahib]

How would you correct this 'wiring fault' without connecting to a ground rod?

 

[kwired]

Consider a 14.5Kv single phase primary with neutral grounded and 240/120V secondary with neutral not grounded POCO transformer. Also suppose no grounding downstream with insulated EGC. Under these circumstances hazardous voltage may be present on the secondary load side depending on the capacitances between primary and secondary, secondary and ground. But if secondary side is grounded, such hazardous voltage disappears.

Most of the transformers used by utility companies I believe you would have to intentionally remove the bonding jumper from the secondary neutral for that to happen. I think they usually are shipped out with the bond installed, some maybe even are permanently bonded. So your scenario though possible, does not have very good chance of ever happening.

 

[Sahib]

kwired: Just a what if to stress the importance of grounding.

 

[user 100]

kwired: Just a what if to stress the importance of grounding.

And we're trying to stress that in a less than 600 v fault earth grounding is voodoo and almost never beneficial, and only might help with higher voltages.

 

[K8MHZ]

All of our HV and MV lines have a well grounded static line on every run. About 5 years ago, a gust of wind caused a live 138kv line get blown out of the control of a linesman crew working on it. It landed on a 63kv line. The result was 100's of acres of burnt lowlands. Since the fire was in the low lands, a gravel road about 3 miles long had to be built just to get the equipment in to repair the damage.

Multiple ground rods, one at every pole that has a transformer, plus extras down the line to keep the impedance low, did nothing to stop the static line from going 'poof' when the big 138 came down on it.

http://photos.mlive.com/photogallery/2010/04/muskegon_river_flats_fire_apri_3.html

http://www.mlive.com/news/muskegon/index.ssf/2010/04/downed_wire_brush_fire_leads_t.html

 

[K8MHZ]

Take note that despite dropping on a grounded system, the power stayed on and had to be turned off hours later by the POCO. There is no mention of the 138 dropping on the 63 in the news. I found that out from a linesman. There was a big union vs. non-union stink that followed, as the crew the dropped the 138 was working in a typically union area. The 138 lines had been sold to a line company, and they had a non-union crew. The 63's were still owned by the POCO, which was 100% union. Both lines ran down the same right of way.

I could see the smoke from the fire nine miles away. It was a big deal.