Heart Failure...

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kwired

Electron manager
Location
NE Nebraska
It is extremely unlikely that the utility calculated the secondary faults for this specific installation. in most cases the utility uses lookup tables, for some utilities the value is a 'design level that will never be seen in real life (this is great for selecting the ratings of service equipment). There are some utliites that will provide L-G and L-L values for MV customers, although soemtimes we get them for 'secondary service' customers.

For equipment selection, based on SCA, there is no need to treat a center-tapped system any different than other delta. The big issue is open versus closed delta.

What happens in open vs closed delta? This is something I have similar issue as OP with myself. Need to know what fault current is on an open delta system for satisfying 110.24. I can't imagine the fault current is that high by the time the conductors hit the service disconnect that it is a problem for AIC rating, but not really sure how to figure out what difference is between open and closed delta, as well as what happens if the 120/240 pot is sized larger than the other(s).

I kind of figure a closed system should have higher level of fault current if all pots are same size, and if I maximum available current is acceptable with that then I am safe as far as not exceeding AIC rating of my equipment - but don't know what actual available current is.
 
T

T.M.Haja Sahib

Guest
One more suggestion:

A steel conduit requires bonding at both ends for not reducing the fault current in the EGC. If it were not bonded, the impedance of the EGC is increased.

So by putting flexible steel sleeve of suitable length over each over head service line in the customer premises, the fault level can be reduced.

Of course, there are problems of heat production in the steel sleeve, its suitable separation from service conductor, concurrence from the POCO etc., to be taken care of.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
One more suggestion:

A steel conduit requires bonding at both ends for not reducing the fault current in the EGC. If it were not bonded, the impedance of the EGC is increased. ...
The code does not require that for a raceway that contains an EGC. The bonding rule only applies to grounding electrode conductors installed in ferrous raceways. There is no real change in the impedance of the EGC when it is in a metallic raceway that is not bonded at both ends. The physics that increases the impedance of the GEC in an unbonded ferrous raceway does not apply to ferrous raceways that contain both the ECG and the ungrounded circuit conductors.
 
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T.M.Haja Sahib

Guest
The code does not require that for a raceway that contains an EGC. The bonding rule only applies to grounding electrode conductors installed in ferrous raceways. There is no real change in the impedance of the EGC when it is in a metallic raceway that is not bonded at both ends. The physics that increases the impedance of the GEC in an unbonded ferrous raceway does not apply to ferrous raceways that contain both the ECG and the ungrounded circuit conductors.
Well, then the abbreviation EGC may be changed to GEC in post# 23. Now I think the idea expressed in it is intact. :)
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
Would you provide us with the details of the utility installation?

I could have sworn this was a closed delta. When the heck did they take down a transformer?
What did they do with that 3rd primary line? Five more years...

Open delta with one 75 and one 50. Lateral to 3ph service entance is 350 Al in PVC, 4/0 neutral IIRC. 130 feet.

Lateral to 1ph service is 250 AL with 4/0 neutral in PVC, 130'.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Well, then the abbreviation EGC may be changed to GEC in post# 23. Now I think the idea expressed in it is intact. :)
It doesn't matter what you call it, if the grounding conductor is in the same raceway as the ungrounded conductors, the impedance of the grounding conductor is not increased when you install it in an unbonded ferrous raceway.
This increase in impedance only occurs when you have a single conductor of an AC circuit in a ferrous raceway and the single conductor is not bonded to the ferrous raceway at both ends.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... This increase in impedance only occurs when you have a single conductor of an AC circuit in a ferrous raceway and the single conductor is not bonded to the ferrous raceway at both ends.
Don -
Not questioning, just clarifying (this is my translation of the model you are using):

The short circuit current that is on the EGC is supplied by conductors in the conduit with the EGC. So summation I = 0 (in the conduit). The conductors are grouped close enough the magnetic field is all contained in the conduit. No interaction between the magnetic field and conduit, no increase in impedance.

Would this be the model your using?

ice
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Don -
Not questioning, just clarifying (this is my translation of the model you are using):

The short circuit current that is on the EGC is supplied by conductors in the conduit with the EGC. So summation I = 0 (in the conduit). The conductors are grouped close enough the magnetic field is all contained in the conduit. No interaction between the magnetic field and conduit, no increase in impedance.

Would this be the model your using?

ice
Yes it is. That being said, there is a small increase in the impedance of a circuit that is installed in a ferrous raceway as compared to the same circuit in a non-ferrous raceway. This is so small that a short length of ferrous raceway would not really reduce the available fault current. The increase in impedance when installed in the ferrous raceway is less than 0.03 ohms per 1000'. (based on the Effective Z columm for uncoated copper wires in Chapter 9, Table 9)
 
T

T.M.Haja Sahib

Guest
Don:

See below.

So by putting flexible steel sleeve of suitable length over each over head service line in the customer premises, the fault level can be reduced.

I suggested about putting steel sleeve over each overhead service line conductor whereas you are talking about putting the same over circuits. It is high time we reconciled. :)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Don:

See below.



I suggested about putting steel sleeve over each overhead service line conductor whereas you are talking about putting the same over circuits. It is high time we reconciled. :)
I did miss that point.

That would work, at least for awhile. If the service is fully loaded, I would expect the conductors to fail shortly after you turn the power on.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... So by putting flexible steel sleeve of suitable length over each over head service line in the customer premises, the fault level can be reduced.

Of course, there are problems of heat production in the steel sleeve, its suitable separation from service conductor, concurrence from the POCO etc., to be taken care of.


TM -
With all due respect, do you have a model to describe the behavior of these steel sleeves? What you are doing is adding series reactance. Can you calculate the reactance? If you can't, then the method is pretty useless.

I have never seen any IEEE papers, or any other accepted practice that uses this method (ferrous sleeves) to reduce Short Circuit Current. (Yes, ferrite beads are used on signal circuits. This is not that)

If you have a model, please share. If not, then consider doing the original research it takes to develop an excepted practice. If you do, you will have a peer reviewed paper published by the IEEE. We will be calling this the "TMHS SSC Reduction Method".

Otherwise this is a dead end.

Respectfully,

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
FWIW, the POCO informed me the FC was from L-N @ 120v.
pt -
I need a bit of clarification. Does the statement in post 1 still hold?

... Using MHs fault current spreadsheet, the single phase drops wll below the 10k when details of the secondary lateral are included. ...

Is there an SSC issue with the 240/120 3ph service?

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... There is no real way to take care of that heat if the load is over a couple of hundred amps. ...
Don - Again with all due respect: There is no model I know of. With no model, there is no calculation.

How would you know what the heat load is? How would you calculate it?

How would you know the heat gets excessive over "a couple of hundred amps."? Why not 50A?

By my thinking, if there is no model, the answer is, "unknown". If you have a model, please share.

ice
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Don - Again with all due respect: There is no model I know of. With no model, there is no calculation.

How would you know what the heat load is? How would you calculate it?

How would you know the heat gets excessive over "a couple of hundred amps."? Why not 50A?

By my thinking, if there is no model, the answer is, "unknown". If you have a model, please share.

ice
I don't have a model or any way of doing the calculations, but have seen the effects where there have been some type of isolated phase installation through ferrous materials. As far as the couple of hundred amp figure, that is based on the fact that the rule in the Canadian Electrical code equivalent of 300.20 only applies to circuits that exceed 200 amps.
 
T

T.M.Haja Sahib

Guest
....... the rule in the Canadian Electrical code equivalent of 300.20 only applies to circuits that exceed 200 amps.
Perhaps there would be no problem when a conductor carrying current not exceeding 200A is enclosed in steel conduit.
Also I think, instead of fully enclosing the conductor with steel sleeve, if the conductor is enclosed in parts with suitable openings along the sleeve, the temperature rise in the sleeve and in the conductor may not be excessive.
 
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