Utility refuses to provide X/R ratio
Utility refuses to provide X/R ratio
(OP)
hello,
I'm working on performing a short circuit study in order to get the arc flash hazards. The utility servicing the customer will only provide a range of fault currents available but refuses to provide the X/R ratios.
This is what I received from the utility person I've been dealing with: (the utility's name has been replaced with XXXX)
___________________________________________________________
The following are directions I received from our corporate office:
The X/R ratios will not be provided
In order to assess the arc flash exposure both the fault current and clearing time are required to determine the level of protection needed. Instinctively it is easy to assume that the highest fault duty will provide the highest arc flash exposure but more often than not this is not true. For lower fault duties the clearing time can be quite large leading to a high calorie level. XXXX’s distribution system is very dynamic. The following factors can affect the fault duty:
1. Dependant on which circuit is supplying the transformer. XXXX’s system is dynamic and is continually being reconfigured. This can significantly change the fault duty level.
2. Transformer size (including any future replacement size)
3. Impedance of transformer (XXXX specifies a maximum and minimum impedance range)
4. Secondary impedance
The following factors can affect the fault clearing time:
1. Which protective device operates.
2. The transformer fuses may vary as there is more than one type and size that may be used.
3. The impedance of the fault.
Due to the many variables we are in the process of developing a generic table. Depending on the size and type of transformer a minimum and maximum fault current will be provided. The fault current will be calculated using the following assumptions:
1. For the minimum fault level:
Lowest system fault duty (i.e. 370A).
Use the maximum impedance for the smallest transformer able to be installed at that location.
2. For the maximum fault level:
100MVA system bus
Use the minimum impedance for the largest transformer able to be installed at that location.
Additionally it will be recommended that the customer use their own protective device to determine the fault clearing time.
____________________________________________________________
I believe I can not perform the required studies without the X/R ratio and the utility is preventing my client from providing a safe workplace in accordance with OSHA standards.
Any ideas on how to proceed from this point would be greatly appreciated.
I'm working on performing a short circuit study in order to get the arc flash hazards. The utility servicing the customer will only provide a range of fault currents available but refuses to provide the X/R ratios.
This is what I received from the utility person I've been dealing with: (the utility's name has been replaced with XXXX)
___________________________________________________________
The following are directions I received from our corporate office:
The X/R ratios will not be provided
In order to assess the arc flash exposure both the fault current and clearing time are required to determine the level of protection needed. Instinctively it is easy to assume that the highest fault duty will provide the highest arc flash exposure but more often than not this is not true. For lower fault duties the clearing time can be quite large leading to a high calorie level. XXXX’s distribution system is very dynamic. The following factors can affect the fault duty:
1. Dependant on which circuit is supplying the transformer. XXXX’s system is dynamic and is continually being reconfigured. This can significantly change the fault duty level.
2. Transformer size (including any future replacement size)
3. Impedance of transformer (XXXX specifies a maximum and minimum impedance range)
4. Secondary impedance
The following factors can affect the fault clearing time:
1. Which protective device operates.
2. The transformer fuses may vary as there is more than one type and size that may be used.
3. The impedance of the fault.
Due to the many variables we are in the process of developing a generic table. Depending on the size and type of transformer a minimum and maximum fault current will be provided. The fault current will be calculated using the following assumptions:
1. For the minimum fault level:
Lowest system fault duty (i.e. 370A).
Use the maximum impedance for the smallest transformer able to be installed at that location.
2. For the maximum fault level:
100MVA system bus
Use the minimum impedance for the largest transformer able to be installed at that location.
Additionally it will be recommended that the customer use their own protective device to determine the fault clearing time.
____________________________________________________________
I believe I can not perform the required studies without the X/R ratio and the utility is preventing my client from providing a safe workplace in accordance with OSHA standards.
Any ideas on how to proceed from this point would be greatly appreciated.






RE: Utility refuses to provide X/R ratio
RE: Utility refuses to provide X/R ratio
I don't believe that. You have all the information they are going to give you. The reason is that if they give you a fixed figure it may change the next day. What are you going to do with the X,R and transformer info and they change the
feed to a new sub further away? What happens when the transformer that is being used now is increased in size?
This is the problem in trying to make your caculations.
RE: Utility refuses to provide X/R ratio
I don't agree. How do you perform a short circuit calculation without the X/R ratio?
They have the min and max s/c's so they could give me the associated X/R ratios along with a disclaimer that this is only a range of possible values and would change if the circuit configuration changes.
RE: Utility refuses to provide X/R ratio
Arc flash calcs are not a fixed one time calcs, nor are any of short circuit calcs. You have to make 'reasonable' assumptions for may variable factors. You have to use best information available and qualify you analysis by assumptions you have made.
The electrical distribution systems are dynamic and it is not going to change. Arc flash calculations methods are even not perfect and you will not be able to wait till perfection so do the best you can. Even if the utility compnay gives you X/R ratio the arc flash hazard varies by many factors and configurations of your own system within a facility.
So no utility compnay is not preventing you from the making your or your clients workplace safer. You will make it safer than before just by applying best possible warnings but never bullet-proof.
RE: Utility refuses to provide X/R ratio
Will they give you the fault current at the primary terminals of the transformer? If so you can convert it to
an X quanity. R is small when compared to the other components.
RE: Utility refuses to provide X/R ratio
Just a rule of thumb that I have often used.
RE: Utility refuses to provide X/R ratio
I think the utility gives you enough informations. Even if the X/R ratio is kwown, in normal operation, the value of X during a fault goes up. So the X/R ratio during a fault is not known.
RE: Utility refuses to provide X/R ratio
RE: Utility refuses to provide X/R ratio
1.58 at end of line
15.7 at substation
RE: Utility refuses to provide X/R ratio
RE: Utility refuses to provide X/R ratio
RE: Utility refuses to provide X/R ratio
I have enough to worry about without having to notify countless customers every time that a distribution upgrade or feed change is made.
In addition to the X/R ratio, one needs to consider the L/E ratio of the area (look in the yellow pages to find the number of Lawyers vs the number of Engineers).
RE: Utility refuses to provide X/R ratio
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Utility refuses to provide X/R ratio
Additionally, the following statementfrom IEEE 1584 justifies the importance of obtaining the X?R ratio:
1. IEEE 1584, Section 4.2 states "Get the available fault MVA and power angle or X/R ratio from the utility. Most utilities will readily supply information on the available fault level and X/R ratio at point of service. When information is not provided, public utility commissions can be requested to require utilities to furnish this information. Available fault data must be realistic; not conservatively high."
Here is a document that is the basis for the arc flash calcultions, that has been developed by an internationally recognized group, been peer reviewed and adopted by the industry. I am going to be persuing this with the utilities public utility commission. This is the first utility that I have encountered that is unwilling to provide the required information.
RE: Utility refuses to provide X/R ratio
http:/
Mike
RE: Utility refuses to provide X/R ratio
As far as "peer reviewed and adopted", 1584 was sponsored by the Petroleum and Chemical Industry Committee of the Industry Applications Society. They'll get more buy in from utilities if they get the Power Engineering Society involved. The PUC statement sounds like they actually anticipated the problem you have. When utilities are forced into outages rather than reconfiguring (which may affect fault current) the PUC will be getting a lot more calls.
RE: Utility refuses to provide X/R ratio
RE: Utility refuses to provide X/R ratio
I agree with you but I'm also a utility engr and do some consulting on the side. In my utility job, I provide the fault levels with the X/R ratio. I add some disclaimers such as that this is for the normal system configuration and that it is advisable to check with the utility periodically (yearly) to see if there have been any changes.