Service neutral protection

[LISHAJI]

That's interesting. If it's single phase i'd use a 3 pole breaker? Would 1 of the poles of that breaker get the neutral? Would i then use a splice through breaker style connection to feed the bus bar? If not, how would i make the connections? Is this totally NEC and AHJ acceptable?

The problem with this is that if there is a ground fault on the service entrance feeder, at the service disconnect, it would be a high impedance fault and may not clear, because it would be using earth as the fault return path and this is a violation of NEC.

 

[T.M.Haja Sahib]

The problem with this is that if there is a ground fault on the service entrance feeder, at the service disconnect, it would be a high impedance fault and may not clear, because it would be using earth as the fault return path and this is a violation of NEC.

Neutral connection at the neutral bus bar in the service entrance equipment is not broken.Nor is the connection of EGC.

You may wonder the high voltage fault current would enter into the building through the EGC.Let it enter.Nothing would happen.........

Because the EGC is bonded to ground at multiple points inside the home.

 

[dala]

The problem with this is that if there is a ground fault on the service entrance feeder, at the service disconnect, it would be a high impedance fault and may not clear, because it would be using earth as the fault return path and this is a violation of NEC.

Would a non fused arm switch disconnect on the neutral be an NEC violation. I guess you'd need a 3 phase switch so you could disconnect the lines coming to the house at the same time. This would at least disconnect the neutral for the 8 months of the year that the homeowners are not in the house and give them a little more "peace of mind" knowing their house is more safe during the time they are not there.

 

[LISHAJI]

Would a non fused arm switch disconnect on the neutral be an NEC violation. I guess you'd need a 3 phase switch so you could disconnect the lines coming to the house at the same time.

As I understand, as soon as one break all grounded connection between the transformer and service disconnect, its a violation. For a service with a neutral, it is bonded at the transformer and thus consequently, there need to be at least one grounded connection between the transformer and service disconnect, which is what you don't want and want to break in the first place. For any ungrounded 3 phase 3W system, there is no neutral bonding at the transformer, and so one can disconnect all the three phase safely.

 

[kwired]

Neutral connection at the neutral bus bar in the service entrance equipment is not broken.Nor is the connection of EGC.

You may wonder the high voltage fault current would enter into the building through the EGC.Let it enter.Nothing would happen.........

Because the EGC is bonded to ground at multiple points inside the home.

The main bonding jumper at the service equipment is the only low resistance connection and only intentional earthing connection (normally) is any grounding electrodes but they are almost always high enough resistance they will not carry enough current to open overcurrent devices fast enough to prevent any damage especially when voltage is in the above 600 volts categories.

 

[Little Bill]

Just have the POCO pull the meter.

 

[T.M.Haja Sahib]

......... any damage especially when voltage is in the above 600 volts categories.

The OP did not mention whether the service drop wires or any part of it up to the energy meter terminals were also damaged or not.........

 

[kwired]

The OP did not mention whether the service drop wires or any part of it up to the energy meter terminals were also damaged or not.........

do they have to be? Outside of a service supplying a single load, the service conductors are usually larger than any branch circuit conductors because they usually are designed to carry more current.

 

[T.M.Haja Sahib]

do they have to be? Outside of a service supplying a single load, the service conductors are usually larger than any branch circuit conductors because they usually are designed to carry more current.

As you expressed concern over the voltage rating of cables to withstand the high voltage fault, I intended to clear it.........

 

[kwired]

As you expressed concern over the voltage rating of cables to withstand the high voltage fault, I intended to clear it.........

I was not as concerned about voltage I was mostly concerned about current - that is what is going to heat things up.

You mentioned "EGC is bonded to ground at multiple points inside the home". Other than grounding electrodes where is it bonded besides coincidental contact through equipment that is likely fairly high resistance to ground?

With a low impedance path current will be high but for very short time before overcurrent device cuts out. With high impedance through ground rods or other electrodes current may be very low at under 600 volts but for POCO distribution voltages it is going to find several paths at higher voltages than those paths normally operate and will cook some equipment in the process.

If the ground rod is as good as you have claimed they are in the past then there is nothing to worry about, it should carry enough current to operate the overcurrent device almost instantly, reality is they are not that low of an impedance so the overcurrent device takes more time to operate in meantime current may be flowing through all kinds of paths that can't handle it.

 

[T.M.Haja Sahib]

I was not as concerned about voltage.........

If the service drop wires up to the energy meter terminals are intact,there would be no need to worry about insulation breakdown.

I was mostly concerned about current - that is what is going to heat things up.

You mentioned "EGC is bonded to ground at multiple points inside the home". Other than grounding electrodes where is it bonded besides coincidental contact through equipment that is likely fairly high resistance to ground?

With a low impedance path current will be high but for very short time before overcurrent device cuts out. With high impedance through ground rods or other electrodes current may be very low at under 600 volts but for POCO distribution voltages it is going to find several paths at higher voltages than those paths normally operate and will cook some equipment in the process.

If the ground rod is as good as you have claimed they are in the past then there is nothing to worry about, it should carry enough current to operate the overcurrent device almost instantly, reality is they are not that low of an impedance so the overcurrent device takes more time to operate in meantime current may be flowing through all kinds of paths that can't handle it.

Consider a bare EGC.It would have both intentional and unintentional bonding with ground throughout its length so that any fault current through it would diffuse away from it with a little heating to it.

As for the fault current through the neutral,please note that for such current flow there should be a ground fault of neutral inside the building.In that case,if the fault current is high,the breaker would trip.If the current is low,there would be no damage to equipments (remember no damage to service wires in the first place!)But there may exist electrocution hazard until the fault is cleared by high voltage over current devices upstream.

 

[iwire]

Just have the POCO pull the meter.

That won't do it either.

 

[kwired]

If the service drop wires up to the energy meter terminals are intact,there would be no need to worry about insulation breakdown.




Consider a bare EGC.It would have both intentional and unintentional bonding with ground throughout its length so that any fault current through it would diffuse away from it with a little heating to it.

As for the fault current through the neutral,please note that for such current flow there should be a ground fault of neutral inside the building.In that case,if the fault current is high,the breaker would trip.If the current is low,there would be no damage to equipments (remember no damage to service wires in the first place!)But there may exist electrocution hazard until the fault is cleared by high voltage over current devices upstream.

Lets go back to what was said in the OP. He was concerned about damage that occurre when POCO primary conductor fell on a grounded conductor.

My original diagnosis was that there must be something wrong that the utility is not saying in the first place. As this short circuit condition should have had low enough impedance to open overcurrent protection very quickly. This is all speculation, there is a lot of information that we could be missing that will change things. First we do not know how long the fault condition existed, OP indicates that it was at least 20 minutes, if that is so current is flowing in all kinds of paths, things have had plenty of time for insulation breakdown, some paths probably burned clear and others developed. If the grounding electrodes at these effected homes were as low of resistance as you have tried to claim in past threads the overcurrent protection should have operated within a second or two at most.

Now lets figure that there was a high impedance and the overcurrent protection (on POCO primary conductor) did not operate - now you are getting into discussion we have in last few posts.

A few conditions that can't be ignored:

1. The service neutral although is connected to a grounding electrode which itself likely has a resistance of at least 10 ohms or more is the "hot conductor" during this incident.

2. The service neutral has no overcurrent device in it. The only overcurrent device that will interrupt the conditions is the one protecting the POCO primary phase that has faulted to this conductor.

3. Once this conductor energized to a voltage well above its normal design enters a building wiring system there will be breakdown of insulation, individual premesis wiring conductor insulation may hold up to some extent, but there will be connected equipment that likely will not.

 

[T.M.Haja Sahib]

Lets go back to what was said in the OP. He was concerned about damage that occurre when POCO primary conductor fell on a grounded conductor.

My original diagnosis was that there must be something wrong that the utility is not saying in the first place. As this short circuit condition should have had low enough impedance to open overcurrent protection very quickly. This is all speculation, there is a lot of information that we could be missing that will change things. First we do not know how long the fault condition existed, OP indicates that it was at least 20 minutes, if that is so current is flowing in all kinds of paths, things have had plenty of time for insulation breakdown, some paths probably burned clear and others developed. If the grounding electrodes at these effected homes were as low of resistance as you have tried to claim in past threads the overcurrent protection should have operated within a second or two at most.

Now lets figure that there was a high impedance and the overcurrent protection (on POCO primary conductor) did not operate - now you are getting into discussion we have in last few posts.

A few conditions that can't be ignored:

1. The service neutral although is connected to a grounding electrode which itself likely has a resistance of at least 10 ohms or more is the "hot conductor" during this incident.

2. The service neutral has no overcurrent device in it. The only overcurrent device that will interrupt the conditions is the one protecting the POCO primary phase that has faulted to this conductor.

3. Once this conductor energized to a voltage well above its normal design enters a building wiring system there will be breakdown of insulation, individual premesis wiring conductor insulation may hold up to some extent, but there will be connected equipment that likely will not.

In the final analysis,do you recommend a 4 pole main breaker or not?

 

[kwired]

In the final analysis,do you recommend a 4 pole main breaker or not?

No. The neutral is still required to be bonded to the service equipment. This would make no sense as it would bypass the breaker. If you opened the neutral before the bonding jumper and had bonded meter socket or other enclosures on supply side you still have similar problems. My final analysis (so far, more details could change that) is the POCO is withholding information about some other problem on their system that allowed this condition to last as long as it did.

We had a situation nearby a couple years ago where a phase from a transmission line dropped onto a distribution line.

It did cause damage in a lot of homes and businesses from the surge. The surge was not 20 minutes long, more like a second or two max, 20 minutes could have been a disaster.

 

[George Stolz]

I was wondering if anyone knows of a product (NEC legal) like a blocking diode or in-line fuse/breaker that can protect a house/service from getting accidentally powered through the service neutral. The reason i ask is because where i live and work, this has happened and houses have burned down. Most of the homes are unoccupied for 8 months of the year and the power is shut off. In these cases, the high voltage primary wire broke and came down to feed the secondary neutral which in turn fed all the homes connected to that neutral. Without any protection at all on the homes' neutral, ALL wires connected got fed and burned. Is there any way to protect a home from this?

I too am having a hard time visualizing and believing this. Where did this happen? Do you have a link to news accounts of these events?

 

[T.M.Haja Sahib]

No. The neutral is still required to be bonded to the service equipment. This would make no sense as it would bypass the breaker. If you opened the neutral before the bonding jumper and had bonded meter socket or other enclosures on supply side you still have similar problems.

Even when the EGC is bare and it is intentionally or unintentionally bonded to ground through out its length so that the fault current will diffuse away from it to ground with little heating to it and POCO side over current device operates quickly on this fault?

 

[WIMaster]

OK back to the OPs scenario. If we somehow manage to get ANY of the secondary conductors energized to the primary voltage (there are videos of this on the web) we have a very serious problem that the secondary side is not really designed to deal with. As NONE of the equipment or insulated conductors are rated for the primary voltage (12KV-25KV on the pole in my area). Opening the neutral with a multipole breaker (usually rated at 240V residential) will do only give us another place to arc over.

The only real world solution I can think of is to make sure there is a COMPLETE and thorough grounding electrode SYSTEM and hope and pray for the best. If the customer is willing to pay for it add more electrodes than the minimum. At minimum this will CYA you for liability, at its best it will help to quickly open the OC device on the primary.
Again none of the secondary equipment etc. is designed for primary voltages.

 

[kwired]

Even when the EGC is bare and it is intentionally or unintentionally bonded to ground through out its length so that the fault current will diffuse away from it to ground with little heating to it and POCO side over current device operates quickly on this fault?

The grounded conductor is common to both systems. If there is open circuit on conductor someplace along the way back to the primary source then the higher impedance paths is where all current has to flow. If connections are good all the way to primary source then that is the lower impedance path and the majority of current follows that path.

There obviously was a high enough impedance in the OP's situation that it allowed current to follow alternate paths for an extended period of time. You can describe as many situations as you want with bare, insulated, whatever the return through earth is not as low of resistance as a good conductor.

Someone said it before on this forum. Earth is a good conductor, but it is hard to make a low impedance connection to it.

 

[Little Bill]

That won't do it either.

Not entirely, but it could minimise the damage to the inside. Better yet, have the POCO disconnect at the pole/transformer.