Objectionable Current

[pete m.]

Heres one...... If you have a current transformer metered service ahead of the service disconnect and you use the neutral(or grounded conductor) to bond the metallic cabinet enclosing the current transformers, and you have a rigid nipple between the CT cabinet and the service disconnect which is bonded correctly and you have your main bonding jumper installed in your service disconnect as you should, would the neutral current thatwillflow on the rigid nipple be considered objectionable current by the NEC?

Pete

p.s. also think of the rules for a separately derived system as it applys to objectionable current.

 

[bennie]

Re: Objectionable Current

Pete: All common mode current flow is classified as objectionable. The CT can is on the supply side of the main. There is no overcurrent device to see a LG fault in the can. A fault would likely burn clear before the drop down fuse activates.

CT cans should be non-conductive for safety.

I am interested in your perception of common mode current created by the popular interpretation of a transformer meeting the definition of a separately derived system.

 

[charlie]

Re: Objectionable Current

Pete, more current will flow in the grounded conductor than in the metallic conduit because of the much higher resistance of the metal conduit. This is not considered objectionable; however, I prefer to see non-metallic conduit or SE cable used so as to eliminate that current altogether.

Bennie, non-metallic meter fittings would not fly, in my opinion, since we can't put solid locks on them to prevent further tampering. By the way, how are you getting along?

 

[bennie]

Re: Objectionable Current

Charlie: I agree the current may not be objectionable in that it will not create any system noise at that location.

My non-conductive proposal is due to a line side fault, in a meter base, usually sets the building on fire.

The Doctor says I am not going to improve and that he can only prolong the problem as long as possible.

However I am feeling better, and breathing a lot more comfortable.

Thanks for asking.

Bennie

 

[russ]

Re: Objectionable Current

Why is the grounded conductor used to bond the current transformer panel, when with a rigid nipple between it, and the bonded main panel. I don't think the C/T panel should be tied to the G.C. directly. These panels come with the neutral isolated from ground. I do not allow the grounded conductor to be bond in the C/T.

Russ

[ April 12, 2003, 02:42 AM: Message edited by: russ ]

 

[bennie]

Re: Objectionable Current

I agree Russ. The parallel path would not exist.
I don't think load current is a problem anyway.

The burn down problem remains.

I know of some apartments that have a meter base caused fire at least once every year. The top lug terminals break away, from the meter socket, and hit the side of the housing.

 

[don_resqcapt19]

Re: Objectionable Current

Russ,

I do not allow the grounded conductor to be bond in the C/T.

What code section do you use to do this? Do you require an equipment grounding conductor to be run back from the service to bond the CT can?
Don

 

[charlie]

Re: Objectionable Current

I do not allow the grounded conductor to be bond in the C/T

Russ, we (Indpls Pwr & Lgt) require all equipment be bonded to the grounded conductor on the line side of the service equipment and the bonding jumpers sized in accordance with 250.66. Since we will not give a customer service if he doesn't comply, how would you respond to us here?

 

[bennie]

Re: Objectionable Current

Bonding is to complete a low impedance path to facilitate the overcurrent device to function.

There is no overcurrent device on the supply side. The MGN landing only in the service panel, and with an equipment ground conductor back to the CT can, creates a higher impedance path.
This conductor will carry a high current in the event of a LG fault. This conductor should be capable of withstanding enough current to blow a hole in the CT can.

I have seen drop down fuses activate only after a 500 MCM cable was melted to an open circuit, and the lid blown from the tank.

The MGN should be connected first in the CT can and then to the service buss.

 

[hurk27]

Re: Objectionable Current

This is a dangerous problem with any type of service equipment located on the outside of a wooden sided structure as I have seen many fires caused by shorts in meter cans CT cans and even riser pipes from tree limbs falling and hitting the drop wires. or in one case a homeowner decided to use rock salt to remove a ice-dam that was leaking water down into his meter can. guess where the salt ended up. But the main problem is there is no protection of the service drop wires to a building. I think there should be if a wooden sided structure is involved. but I dont think my two cents will change anything.

At least there could be a closer fuseing of the primary side of the transformenr

 

[charlie]

Re: Objectionable Current

At least there could be a closer fusing of the primary side of the transformer

Until we get public service commissions (in Indiana, the IURC) to back off on their desire to have extremely low minutes of outage and the customers be willing to pay for additional service problems and minutes of outage, that will never happen.

This could be an very long article but I will try to make it short. Basically, the transformer fuses are there to protect the primary line, not the transformer. If an animal gets into the line at the transformer (load side of fuse), we would rather it be burned free than racking up minutes of outage and paying for a service call. We do not charge for a service call but the trouble man's wages are rolled into the rate base.

Right now, IPL is in trouble for having too many minutes of outage under certain conditions. Do you think we are going to make our system less or more stiff? If we were to back off, our minutes of outage would go up and the IURC would fine us.

 

[hurk27]

Re: Objectionable Current

Charlie I can understand this, but there is a few things that could prevent long down times and outages that will also protect the house.

1. Install a fusable link in the ungrounded drop conductors to protect the wires from shorts.
2. Install break-away joints that in the event of a tree limb or somthing falling on the drop conductors. It will not pull down the service and leave the hot wires laying on the ground for someone to get a hold of. of course the neutral will have to have a little more pull pressure to pull apart but this will prevent the neutral from seperateing first or a connector will have to be designed to do this, and it must be placed at the pole, so there will be no hots on the ground.

We had a fire in which a tree fell and pulled the neutral wires apart. this could be done to increse the safty and lower the repair time.

 

[bennie]

Re: Objectionable Current

Hurk: Outside wiring is a new ball game. The outside is designed to not shut down, the inside is designed to shut down.

Fuses are only for dumping a faulted system, they do not protect anything except the rest of the system.

Continuity of service is the goal for all utility companies. Many overcurrent devices are set a bit below the melt point of some conductors.

 

[hurk27]

Re: Objectionable Current

Bennie I understand this but I cant see any reason that the single drop going to one house can not be fused with fuse links if this house has a short in the service and it catches the house on fire it wont matter anyway as far as the outtage goes. but the fuse links will prevent maybe looseing the primary drop-down fuse and keep any other houses that might be tied to this same transformer from loosing power then just the house that had the problem will loose the power and the outtage will be minimized to one house.

The other prooblem is that we replace aprox. 40 to 60 services a year from falling limbs or trees.
With a break-away installed you solve two dangerous problems. Hot wires laying on the ground, and the service from being ripped off the house in which involves allot of work to reinstall. and that is if the local AHJ dont try to make you bring the whole house up to code. all for a part that might cost $100.00 to $150.00 that could prevent this. and again if it is not installed you will loose this house any way and probley more.

 

[charlie]

Re: Objectionable Current

Hurk, that sounds good but do you have any concept about how much cost would be involved? Who is going to pay for the installation of these cable limiters? If we went back to retrofit all of our existing service drops, the cost would be extremely high and the ratepayers would scream. We install almost all of our services underground now and don't have the problems that we do with service drops.

As far as the break away links; what company makes one that is rated to hold a service drop under icing conditions (1/2" radial ice and 4 pounds of transverse wind loading per square foot in Indiana) while allowing the drop to break if a limb fell on it? That animal doesn't exist. Additionally, as I said before, most of our services ire installed underground.

The bottom line is that what seems like a great idea has already been looked at by most of the electric utilities already and rejected for the reasons stated above.

 

[hurk27]

Re: Objectionable Current

Ya got me on that one as I didnt think about the iceing problem. guess the Florida in me keeps getting in the way lol. and yes the man hours to install a fuse-link on every service drop would be cost prohibiting. and since most power co's are phaseing out over head wires it wouldn't make sense to install them now. but it was a good thought while it lasted. Here in Portage,The city is in the progress of renovating the downtown area and nipsco, and telco. is putting all the wires in the ground. of course this is costing the tax payers alot too but when it's done it will look alot better.

 

[wocolt]

Re: Objectionable Current

Bennie, and et al
Could someone please explain to me what is "common mode current" ??????

Thank You

Wm.Colt

 

[dereckbc]

Re: Objectionable Current

WOCOLT, Common Mode Noise: The noise voltage that appears equally and in phase from each current-carrying conductor to ground.

Most common mode problems occur from parasitic capacitance leaking current into EGC's from current-carrying conductors or outside sources such as RFI, EMF, etc. This produces a small voltage drop to occur along the length of the EGC.

So when you have to different pieces of equipment, sharing a grounded metallic communication link like RS-232 or unbalanced audio, and plugged into an AC distribution system from different points, the ground references are at a different voltage potentials. The communication recievers port sees this potential difference as an error voltage or noise.

One example would be a stereo system that has multiple components. The common mode noise is heard as a hum in the speakers. Another example is RS-232, the voltage difference can cause data bit errors, or not being able to tell a logic 1 from a zero.

Here is a good link that will walk you through it http://www.pulseeng.com/pdf/4019.pdf

[ April 17, 2003, 10:53 AM: Message edited by: dereckbc ]

 

[bennie]

Re: Objectionable Current

Dereck does a better job than I.

I refer to common mode voltage, and current, as unwanted current flow, created by ground potential difference.

This current is always objectionable even if it does not interfere with intel communication signals.

Common mode current does not have a nearby opposite polarity magnetic field. This fact creates EMF.

Normal (differential) mode current is actual load current. Common mode is a negative side effect.

[ April 17, 2003, 11:15 AM: Message edited by: bennie ]

 

[pete m.]

Re: Objectionable Current

I find it interesting that the NEC gives specific prescriptive methods for avoiding parallel paths on the secondary side of a transformer after the service disconnect, in this avoiding "objectionable current". Although it is a common (and allowable) practice to use the grounded, or neutral, conductor for the purposes of bonding on the line side of the service disconnecting means it would seem that the scenario I described in the initial post would be a big no-no if we were on the secondary side of a transformer after the service disconnecting means. What would be the difference in the fact that we would have a parallel path either way you cut the cake. Why is one method spelled out as an incorrect method and the other, it would seem, is not a concern?
Pete