secondary fault current danger

[jim dungar]

As a side note, I have worked with customers that installed a fused disconnect after the meter Ahead of the original main panel to de-rate the fault current.

Too bad this is not a very good solution.
I have seen it do no good in more situations than it has helped.
Fuses do not actually reduce fault current until they melt and clear. So for medium and low level faults, large fuses may be no better than bus bars.
There is a reason the NEC requires protective devices to be tested in series, or to be engineered by a licensed professional (but only for existing installations).

 

[petersonra]

I was inspecting a medical center. GE was installing a new MRI machine re-using existing circuits. I checked main 150 amp 3 phase 480 disconnect outside located on building. I then inspected new factory service panel/ control panel located in examining room. Everything looked good, I noticed on inside panel cover the panel was rated at 25,000 max fault current, I checked disconnect main breaker outside it was rated at 25,000. Good so far right ? My question is this is a 10 year old building should I ask for a fault current letter from the local utility, I have no idea if the original plan review or inspector asked for it in 94. I have no idea if the local utility changed the transformer. I do think this is being addressed in 2011 code book we are still using the 2008 in Florida. If for say the utility transformer had a 65 K fault current would this created a potential hazard.

suppose you had 1/0 wire on this 150A service.

At 115V L-G fault would only need about 0.005 Ohms of resistance in the wires to keep the current below 25kA. I think that is about 50 feet of 1/0.

 

[topgone]

suppose you had 1/0 wire on this 150A service.

At 115V L-G fault would only need about 0.005 Ohms of resistance in the wires to keep the current below 25kA. I think that is about 50 feet of 1/0.

That is assuming your line to ground resistance is nil. But I guess that seldom happens. 50 feet of 1/0 should be more than enough to limit the fault current below 25kA, IMO.

 

[mivey]

For grins here is a sample with ranges of transformer impedances giving a minimum, average and maximum 3-ph fault (also including source impedance):

Start with 500 kVA unit: Min= 9,857 A, Avg= 11,787 A, Max= 14,653 A, Infinite bus for avg. transf.= 15,035 A

Make some source improvements but keep 500 kVA unit: Min= 10,255 A, Avg= 12,363 A, Max= 15,562 A, Infinite bus for avg. transf.= 15,035 A

Change to a 750 kVA unit: Min= 12,073 A, Avg= 12,720 A, Max= 13,643 A, Infinite bus for avg. transf.= 15,554 A

Change to a 1000 kVA unit: Min= 15,226 A, Avg= 15,785 A, Max= 17,085 A, Infinite bus for avg. transf.= 20,387 A

Change to a 1500 kVA unit: Min= 20,587 A, Avg= 21,519 A, Max= 22,825 A, Infinite bus for avg. transf.= 31,107 A

Non-uniformity is because the impedance change is not always linear with kVA change.

 

[petersonra]

That is assuming your line to ground resistance is nil. But I guess that seldom happens. 50 feet of 1/0 should be more than enough to limit the fault current below 25kA, IMO.

My point was that it does not take all that much to limit the SCC to an acceptable value, not that this is the answer to the OP's problem. In any case, how would you determine the impedance of say RGS that it faulted to? You almost would have to assume something like zero. There might well be all kinds of things in parallel with whatever EGC you had.

 

[Sahib]

A properly designed installation includes considering reasonably foreseeable events.

What a burden to the facility Engineer: he not only has to foresee the load growth of his own facility but also that of the POCO's. hmy:

 

[jim dungar]

What a burden to the facility Engineer: he not only has to foresee the load growth of his own facility but also that of the POCO's. hmy:

Where did you get that idea?
It appears you have decided to depart from a discussion based on facts and entered into one of just general disagreement.

I have provided statements from POCO service manuals that say they may supply you with a larger transformer. This has nothing to do with their load growth, it has to do with their equipment availability. They are providing the design engnieer with a foreseeable condition.

I have seen many plans, where the designer never even considered the actions of the providing utility. They designed only on their local experience or on their personal preferences.

A poor designer does not recognize that a power system planned for Berlin, Wisconsin, will be need to be implemented differently than one intended for Berlin, Germany.

 

[Sahib]

Where did you get that idea?

From our discussion only.

It appears you have decided to depart from a discussion based on facts and entered into one of just general disagreement.

Sorry, jim, if I so appear. I just try to make things clear.

I have provided statements from POCO service manuals that say they may supply you with a larger transformer. This has nothing to do with their load growth, it has to do with their equipment availability. They are providing the design engnieer with a foreseeable condition.

I do not know what kind of foreseeable condition POCO is providing to a designer because the designer may not know beforehand how big the replacement transformer of the POCO would be to make provision in his design.

I have seen many plans, where the designer never even considered the actions of the providing utility. They designed only on their local experience or on their personal preferences.

A poor designer does not recognize that a power system planned for Berlin, Wisconsin, will be need to be implemented differently than one intended for Berlin, Germany.

Please provide a case study in this regard or a web link to it.

 

[jim dungar]

I do not know what kind of foreseeable condition POCO is providing to a designer because the designer may not know beforehand how big the replacement transformer of the POCO would be to make provision in his design.

A good designer would contact the POCO to find out what they are supplying and design around those requirements.
A mediocre designer would make it the responsibility of the installer to determine the requirements and then modify the design as needed.
A poor designer would not recognize that different localities have different requirements, and assume what ever they design can be installed.

I'll let you figure out some of the possible differences between Berlin, Wisconsin (which is served by Alliant Energy http://alliantenergy.com/index.htm) and that of Berlin, Germany (served by RWE AG http://www.rwe.com/web/cms/en/8/rwe/)

 

[Sahib]

Jim, my objections still hold, because the OP does not seem to have so far discovered the designer of his installation is good, mediocre or poor with respect to your criterion.