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.
secondary fault current danger
- Thread starter carmie7
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Installing equipment for the appropriate fault current (see NEC 110.09 and 110.10) has been in effect for almost 1/2 century. People have ignored them for so long with such catastrophic results, the 2011 edition added additional wording for emphasis (and you wonder the code gets bigger every year) and to make enforcement easier for inspectors.
carmie7:
If the utility transformer fault increased to 65K, for example, its size should also have been increased, making it necessary to replace the existing secondary conductors etc.,. So the administrators of the subject medical center might have been, most probably made aware of it by the utility and might have taken some action. You may also check the available records, if possible.
If the utility transformer fault increased to 65K, for example, its size should also have been increased, making it necessary to replace the existing secondary conductors etc.,. So the administrators of the subject medical center might have been, most probably made aware of it by the utility and might have taken some action. You may also check the available records, if possible.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Utilities transformer sizing has nothing to do with customer controlled secondary conductors sizing.
Also the impedance of a replacement utility owned transformer could have been occurred without a change in kVA size.
Lets not forget about the utility upgrading its distribution capacity which would not have even caused a change in the transformer at all.
The important thing is to determine what the utility is currentl providing as the avaliable fault curren and compare it to the SCCR of the equipment.
Carmie
All these questions could be answered by asking the utility for the available fault current.
They probably would give you a figure based on an infinite source which would not be accurate.
Suppose the fault was greater than 25 ka. Would you require GE to remove the equipment already installed?
The medical center would likely not be too happy with that. So what will you do?
All these questions could be answered by asking the utility for the available fault current.
They probably would give you a figure based on an infinite source which would not be accurate.
Suppose the fault was greater than 25 ka. Would you require GE to remove the equipment already installed?
The medical center would likely not be too happy with that. So what will you do?
Yes. But I meant the secondary conductors of the utility transformer.
You are correct. But I wonder why the utility need to replace the existing transformer with another one of the same capacity but costlier due to decrease in its internal impedance.
This seems to be pedantic. An approximate calculation of fault level of a given transformer involves that transformer only:there is no condition about the up-gradation of its upstream transformers sizes in the calculation.
That is self-evident.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
In the US we have a saying: "You catch more flies with honey than with vinegar".
You have just lost one of the most educated responders for this topic. Actually, make that two.
You have just lost one of the most educated responders for this topic. Actually, make that two.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The secondary conductors are rarely replaced unless there is a service upgrade occuring. The utility may replace a failed transformer with a larger one depending on what they have in inventory when a replacement is needed. Many of our local POCOs require that the original pad and underground conduits be sized just for this situation.
The utility's purchasing procedures may have changed and they now order transformers with a lower impedance. While they care about operating costs and acquisition costs, they often make compromises between the two.
According to one of my local utilities some "Fault current calculations are based upon the distribution system configuration at the time of the calculations." I also have several POCOs that regularly report secondary fault currents that exceed the simple 'infinite bus method' of FLA/Z.
how do they do that?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Please do not be inhibited from responding to the OPs question based on the actions of another responder. I guess that the OPs question seems to have been addressed already, unless something was overlooked.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
They simply choose a value of fault current they are confident will never be exceeded even if they change the transformer. Effectively they are taking worst case possible and then adding a margin of safety. Many contractors have been burned when they install equipment based on an assumed transformer size and then got the value from the utility.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
My mistake. I thought he was the OP.
Admitted.
Admitted.
Not admitted. Please give an example calculation (theoretical or practical) that secondary fault currents that exceed the simple 'infinite bus method' of FLA/Z.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The statement was that the utility gave a number, not that they justified it with a calculation.
So the upshot is the safety of operation of even a properly designed consumer electrical installation is at the mercy of the Almighty POCO.
Please try looking at it from a different angle. Isn't it a good thing to have "the Almighty POCO" giving inputs to designers within its franchise area? Who's the person to call when you need a figure on available fault current at the POC in the area?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
A properly designed installation includes considering reasonably foreseeable events.So the upshot is the safety of operation of even a properly designed consumer electrical installation is at the mercy of the Almighty POCO.
The utility owns the transformer. They clearly state they reserve the right to replace a failed transformer with a larger unit. With all other things constant, the larger unit may result in fault currents beyond the rating of the customer's install equipment. In another case, say the utility purchase 300kVA 208Y/120 transformers with 3.5%Z, then they change their purchase specs or their supplier, so now their replacement units are 1.8%Z.
In the situation I presented, the utility has acted in a prudent manner by providing a fault current value that will never be reached, ensuring the customer's equipment will always be properly rated.
CTCOX
Member
- Location
- Bettendorf, IA
Fault Current
Fault Current
As a POCO engineering tech, I give fault current calculations quite often. In most configurations, there are more than one customer fed from a single transformer. Legal "suites" decided to give the calculation on infinite bus and also use the next size transfomer up or down (worst case is given) in case the transformer is changed due to emergency work (transformer availibility) or increased load from a neighboring customer. It might not seem fair to the customer buying the equipment, but it is important because it is very likely that the transformer will be changed without notice to the customer. 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. OK, ready for the POCO bashing now!
Fault Current
As a POCO engineering tech, I give fault current calculations quite often. In most configurations, there are more than one customer fed from a single transformer. Legal "suites" decided to give the calculation on infinite bus and also use the next size transfomer up or down (worst case is given) in case the transformer is changed due to emergency work (transformer availibility) or increased load from a neighboring customer. It might not seem fair to the customer buying the equipment, but it is important because it is very likely that the transformer will be changed without notice to the customer. 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. OK, ready for the POCO bashing now!
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
As a POCO engineering tech, I give fault current calculations quite often. In most configurations, there are more than one customer fed from a single transformer. Legal "suites" decided to give the calculation on infinite bus and also use the next size transfomer up or down (worst case is given) in case the transformer is changed due to emergency work (transformer availibility) or increased load from a neighboring customer. It might not seem fair to the customer buying the equipment, but it is important because it is very likely that the transformer will be changed without notice to the customer. 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. OK, ready for the POCO bashing now!
how would this help?
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