Indian electrical code.

[kwired]

As for line surges, I got to wire a $450,000 log home because its predecessor was burnt to the ground by a line surge.

What happened was (this from POCO engineers) a couple lines got crossed in a windstorm, fully saturating a transformer. When the lines came back apart, the transformer 'outrushed', surging the voltage on the lines for a few cycles. Enough to turn a night light into a fire. From there the fire spread to the curtains and shortly thereafter the entire log home was engulfed and destroyed.

The ground rods, obviously, were of no help. The new home is now outfitted with whole house surge protection.

As for Americans not having this grounding thing figured out yet, I have to agree. I get to see grounding from a different perspective than most, being a ham radio operator and all.

I can tell you that there are many myths about grounding that refuse to die. I can also tell you that most people can't explain the difference between bonding and grounding.

It's funny you bring up 'limit the voltage to ground'. I just had a conversation about that. If you read the entire passage, the reason for grounding is to limit the voltage between the ground and normally non-current carrying materials, such as metal enclosures and raceways.

While a rod or two will provide some limitation, they are poor choices for grounding electrodes. You have to realize that the NEC is a bare minimum. It's perfectly fine to use a more effective means of 'limiting', which is actually an attempt at equipotentiality.

Note the part of quote in bold. You are not going to eliminate surges from lightning you can help minimize them.

If the surge is on an ungrounded conductor grounding electrode does not directly protect the ungrounded conductor, surge protective devices will perform better however if there is a low impedance to shunt the surge to. Remember a ground rod of 25 ohms is a high impedance for a 120, or 277 volt source, but for lightning that is thousands of volts it is a low impedance.

There can be surges on POCO lines when there is switching occuring or like you said when there are faults on the line there can be resulting surges. They typically are not as destructive of a surge as lightning but is still a surge - the type that surge protective equiment silently protects you from. Another surge that happens at times is when POCO has transmission voltage line fail and fall on distribution lines. Seen the results of that a couple of times. Incident is quick like lightning but can cause damage in several homes and businesses if they are connected to the phase that had a rise in voltage there is a good chance they suffered some damage someplace.

 

[iceworm]

As for line surges, I got to wire a $450,000 log home because its predecessor was burnt to the ground by a line surge.

What happened was (this from POCO engineers) a couple lines got crossed in a windstorm, fully saturating a transformer. When the lines came back apart, the transformer 'outrushed', surging the voltage on the lines for a few cycles. Enough to turn a night light into a fire. From there the fire spread to the curtains and shortly thereafter the entire log home was engulfed and destroyed.

The ground rods, obviously, were of no help. The new home is now outfitted with whole house surge protection.

As for Americans not having this grounding thing figured out yet, I have to agree. I get to see grounding from a different perspective than most, being a ham radio operator and all.

I can tell you that there are many myths about grounding that refuse to die. I can also tell you that most people can't explain the difference between bonding and grounding.

It's funny you bring up 'limit the voltage to ground'. I just had a conversation about that. If you read the entire passage, the reason for grounding is to limit the voltage between the ground and normally non-current carrying materials, such as metal enclosures and raceways.

While a rod or two will provide some limitation, they are poor choices for grounding electrodes. You have to realize that the NEC is a bare minimum. It's perfectly fine to use a more effective means of 'limiting', which is actually an attempt at equipotentiality.

Note the part of quote in bold. You are not going to eliminate surges from lightning you can help minimize them.If the surge is on an ungrounded conductor grounding electrode does not directly protect the ungrounded conductor, surge protective devices will perform better however if there is a low impedance to shunt the surge to. Remember a ground rod of 25 ohms is a high impedance for a 120, or 277 volt source, but for lightning that is thousands of volts it is a low impedance.

There can be surges on POCO lines when there is switching occuring or like you said when there are faults on the line there can be resulting surges. They typically are not as destructive of a surge as lightning but is still a surge - the type that surge protective equiment silently protects you from. Another surge that happens at times is when POCO has transmission voltage line fail and fall on distribution lines. Seen the results of that a couple of times. Incident is quick like lightning but can cause damage in several homes and businesses if they are connected to the phase that had a rise in voltage there is a good chance they suffered some damage someplace.

First, I know you guys are pretty familiar with this stuff. So I know I'm not teaching here - just discussing and opining (is that a word?)

Let's discuss this surge thing definition:
The NEC is clear that it differentiates between "line surges", "lightning", "unintentional contact with higher voltage lines" - and I think also "switching transients", although they are not listed in .4.A.1.

Since the NEC differentiates, then they must think that "surge" is not lightning, or unintentional contact. I can define and describe the effects of lightning strikes, unintentional contact, and switching transients. I can define sag and swell. "Surge" does not show up in IEEE 100. I don't think one should group all overvoltage events together and call them surge. If not, what is it?

Lets look at surge arresters:
With one exception, this surge energy does not get disipated into earth. Here is one example. When there is an overvoltage condition between the hot and neutral, the arrester breaks down (impedance drops) and conducts. While the overvoltage exists, there is a very large current flow between the hot and neutral. The large current flow pulls the voltge down - essentially voltage drop across the transformer impedance. So where did the energy go. It was disapated in the transformer impedance and in the arrestor impedance.

Carry the example further. Say there is transient and the hot to neutral rises to 160V. The arrester conducts and draws 1000A to pull the voltage down to 130V. So, there is 30V across the xfm impedance and 130V across the arrester. The xfm is disapating 30,000W and the arrester is disapating 130,000W. Kind of makes you hope the event down not last too long.

This scenerio covers switching transients, unintentional contact, swell. I still can't tell you if it covers "surge" - still can't define it.

None of this has anything to do with the impedance to earth. One rod or 1000, it is the same.

Yes, lightning is different

As for "transformer outrush" - the most polite thing I can say is "That's nice". I have not seen any models that discuss "transformer outrush"

ice

 

[mike1061]

After 40+ years in the business I still don't know what a "line surge" is. I also don't know how driving a ground rod will "limit the voltage to ground".

I don't think us Americans have this grounding thing figured out yet.

ice

Thanks
Mike

 

[kwired]

First, I know you guys are pretty familiar with this stuff. So I know I'm not teaching here - just discussing and opining (is that a word?)

Let's discuss this surge thing definition:
The NEC is clear that it differentiates between "line surges", "lightning", "unintentional contact with higher voltage lines" - and I think also "switching transients", although they are not listed in .4.A.1.

Since the NEC differentiates, then they must think that "surge" is not lightning, or unintentional contact. I can define and describe the effects of lightning strikes, unintentional contact, and switching transients. I can define sag and swell. "Surge" does not show up in IEEE 100. I don't think one should group all overvoltage events together and call them surge. If not, what is it?

Lets look at surge arresters:
With one exception, this surge energy does not get disipated into earth. Here is one example. When there is an overvoltage condition between the hot and neutral, the arrester breaks down (impedance drops) and conducts. While the overvoltage exists, there is a very large current flow between the hot and neutral. The large current flow pulls the voltge down - essentially voltage drop across the transformer impedance. So where did the energy go. It was disapated in the transformer impedance and in the arrestor impedance.

Carry the example further. Say there is transient and the hot to neutral rises to 160V. The arrester conducts and draws 1000A to pull the voltage down to 130V. So, there is 30V across the xfm impedance and 130V across the arrester. The xfm is disapating 30,000W and the arrester is disapating 130,000W. Kind of makes you hope the event down not last too long.

This scenerio covers switching transients, unintentional contact, swell. I still can't tell you if it covers "surge" - still can't define it.

None of this has anything to do with the impedance to earth. One rod or 1000, it is the same.

Yes, lightning is different

As for "transformer outrush" - the most polite thing I can say is "That's nice". I have not seen any models that discuss "transformer outrush"

ice

I call a surge any transient rise in voltage above normal operational voltage, maybe this is incorrect but is the case most of the time.

You are correct about so called surges between two conductors and the overvoltage condition will need to be dissipated via items connected between those two conductors - I will add that connected loads will also dissipate some of the surge but if the connected loads can not take the increased voltage they may be damaged which is why we use surge protection to hopefully keep the overvoltage low enough to protect the intended load.

Lightning is pretty much the only incident in which an earth connection is going to provide much useful protection because earth is the second conductor of the transient voltage in this instance.
You are correct.

 

[K8MHZ]

As for "transformer outrush" - the most polite thing I can say is "That's nice". I have not seen any models that discuss "transformer outrush"

ice

That was one of those 'for lack of a better term' things, that's why the quotation marks.

What is the proper term for when a transformer gets saturated by a short, the short abruptly opens and for a few cycles the transformer is way over voltage?

Just a surge?

 

[iceworm]

That was one of those 'for lack of a better term' things, that's why the quotation marks.

What is the proper term for when a transformer gets saturated by a short, the short abruptly opens and for a few cycles the transformer is way over voltage?

Just a surge?

I don't know. Even worse, I didn't even know that a transformer would do that. Here's what I think I know (Electrical Machinery Fundamentals, Chapman). My simplistic view:

Saturation has to do with the primary voltage, the magnetic field in the core and magnetizing current required to sustain the core magnetic field.

The core flux is set by the primary voltage. Flux = (1/N) Integral(Vp)

The primary coil will draw sufficient magnetizing curent to provide the magnetomotive force (ampere-turns) it takes to provide the core flux. The relation between the flux and the magnetomotive force is the transformer magnetization curve of the core. Saturation is at the ends of the curve, where a large increase in the Ampere-turns gives only a small increase in the flux. This happens if the primary voltage is above design.

So high primary voltage causes saturation - not high current.

... What happened was (this from POCO engineers) a couple lines got crossed in a windstorm, fully saturating a transformer. When the lines came back apart, the transformer 'outrushed', surging the voltage on the lines for a few cycles. ...

So what are they trying to describe?
Are the crossed lines on the secondary? That won't saturate the xfm. It is short circuited, output voltage goes low. If the line uncross before the OCP operates, the secondary voltage goes back to normal (assuming the primary voltage did not sag) - but there s nothing to drive a secondary voltage to overshoot. This doesn't fit.

Crossed lines on the primary? That won't saturate the xfm. Output voltage goes low. If the line uncross before the OCP operates, the secondary voltage goes back to normal. Again, nothing to drive a secondary voltage to overshoot. This doesn't fit.

Let's try dropping a 138kV line on a 12,460V distribution feeder out to a residential area. That puts about 80kV to ground/neutral on a 7200V line. That will put the house side of the residential xfm ~1200V to ground/neutral. Now that could cause an incancescant bulb to get overly excited. The OV event will last until the substation protective devices operate, the house transformer cutouts open, the house main CB opens. The substation xfm didn't saturate. The residential transformer probably did. But this satuation did not contribute to the OV event. And there is no stored up energy in any of the transformers that will release as an OV event. This fits. It's called "unintentional contact ..." in 250.4.A.1

I don't know what the POCO engineers were trying to describe. I've never seen any models that characterize the events described by transformer saturation followed by voltage overshoot caused by the transformer coming out of saturation.

As for using "surge" - I'd like to see a definition somewhere. The NEC is not consistent between 280, 285, 250.

Just my thoughts

ice

 

[K8MHZ]

I don't know. Even worse, I didn't even know that a transformer would do that. Here's what I think I know (Electrical Machinery Fundamentals, Chapman). My simplistic view:

Saturation has to do with the primary voltage, the magnetic field in the core and magnetizing current required to sustain the core magnetic field.

The core flux is set by the primary voltage. Flux = (1/N) Integral(Vp)

The primary coil will draw sufficient magnetizing curent to provide the magnetomotive force (ampere-turns) it takes to provide the core flux. The relation between the flux and the magnetomotive force is the transformer magnetization curve of the core. Saturation is at the ends of the curve, where a large increase in the Ampere-turns gives only a small increase in the flux. This happens if the primary voltage is above design.

So high primary voltage causes saturation - not high current.



So what are they trying to describe?
Are the crossed lines on the secondary? That won't saturate the xfm. It is short circuited, output voltage goes low. If the line uncross before the OCP operates, the secondary voltage goes back to normal (assuming the primary voltage did not sag) - but there s nothing to drive a secondary voltage to overshoot. This doesn't fit.

Crossed lines on the primary? That won't saturate the xfm. Output voltage goes low. If the line uncross before the OCP operates, the secondary voltage goes back to normal. Again, nothing to drive a secondary voltage to overshoot. This doesn't fit.

Let's try dropping a 138kV line on a 12,460V distribution feeder out to a residential area. That puts about 80kV to ground/neutral on a 7200V line. That will put the house side of the residential xfm ~1200V to ground/neutral. Now that could cause an incancescant bulb to get overly excited. The OV event will last until the substation protective devices operate, the house transformer cutouts open, the house main CB opens. The substation xfm didn't saturate. The residential transformer probably did. But this satuation did not contribute to the OV event. And there is no stored up energy in any of the transformers that will release as an OV event. This fits. It's called "unintentional contact ..." in 250.4.A.1

I don't know what the POCO engineers were trying to describe. I've never seen any models that characterize the events described by transformer saturation followed by voltage overshoot caused by the transformer coming out of saturation.

As for using "surge" - I'd like to see a definition somewhere. The NEC is not consistent between 280, 285, 250.

Just my thoughts

ice

It was about 10 years ago so I don't have anyone to cross examine. Also, I kind of got the info third hand. I wasn't looking to pick apart the explanation the POCO gave as they (surprisingly) took the blame. I was just happy to get the work.

 

[iceworm]

It was about 10 years ago so I don't have anyone to cross examine. Also, I kind of got the info third hand. I wasn't looking to pick apart the explanation the POCO gave as they (surprisingly) took the blame. ....

I wasn't wondering about your part. I was just trying to figure out what they meant.

Where I am headed is most use the term "surge" to mean "system over voltage of unspecified origin, duration, magnitude". It's like saying, "The problem is: harmonics, dirty grounds, ground loop, surge." It doesn't mean anything. And that's okay, it is not necessary to be technically accurate in all conversations. Generally one can translate by the context.

However, NEC 250.4.A.1 uses the term "line surge" as though it has a meaning. For this case, I'd like to see them either define it or admit the phrase is useless and ditch it. I expect to see this about the same time the CMP admits the mythical 25 ohms is useless. (insert ancient, Father Time emoticon)

ice