IDMT Tripping Time

And the question is???????????????????

The question is for ANSI IEEE curves, there are two mathematical formula. One formula is picked up from relay manual and other one from Reference book by ALSTOM

When you calculate the tripping time based on the above formulae, I get two different answers. Which formula is applicable.

That was my question

Who do you trust more (it seems to me a question of trust).

But it might be different:

Are you sure both formulas refer to the same item, context or situation?

How far away one answer from the other? Do the units match up?

Gosh not being an electrician I better hide under the cloak of Anonymity!

Sir, Can you repost the calculation sheet so that we can read clearly. I am not sure about others but i am not able to read the formula's clearly on my system. Thanks.

Both the images are reproduced here for clarity.

My query is for ANSI/IEEE curves the manufacturer of relays specify one formula.(right side)

whereas the NPAG reference book gives similar formula with variation.(left side)

In practice which one to be taken for calculation.

I am developing suitable APP for Iphones/IPads and Android devices.Hence this query.

My other APPS are

Short circuit Calculator

BUSBARS for SwitchBoards & Busducts

CEC Ampacity Calculator

CEC Transformer Protection

CEC Motor Protection

I suggest that you read this document which goes into great detail about this question, then perhaps you can decide for yourself:

"...In most cases, use of the standard SI curve proves satisfactory, but if satisfactory grading cannot be achieved, use of the VI or EI curves may help to resolve the problem. When digital or numeric relays are used,other characteristics may be provided, including the possibility of user-definable curves. More details are provided in the following sections..."

Schneider electric in their document
www.schneider-electric.com/documents/electrical.../page_103.pdf‎
recomends use of "Customized tripping curve with the help of software SFT2841 which may be used to solve all special cases involving protection coordination or revamping using all the three curves.
IEC curves (SIT, VIT/LTI, EIT), IEEE curves (MI, VI, EI), usual curves (UIT, RI, IAC).
The details of calculation is explained there with customised tripping current curve.The details are explained in the document where a suitable solution is use of Schneider Electric software.

Dear Friends

Thanks for your replies and valuable suggestions

Actually ,as I informed, I am developingan APP to be operated with IPHONE/IPAD/Android devices to calculate the tripping time given the set current, fault current ,CT ratios, curve selected. In my opinion this APP will be handy for commissioning Engineers when they visit site. This will also useful for Protection Engineers.

When I ventured into this I have noticed the discrepancy.

There is no issue with regard to IEC curves. Only problem is with respect to IEEE curves. These APPs will be available to the entire world and I do not want to give wrong calculations. This is the reason I am requesting for the correctness. Since NPAG from Alstom is good reference book, I am planning to follow the formula given in the NPAG reference book.

Any suggestions welcome please.

Further on this matter, I have collected more information like different Relay manufacturers use different values in the formula for calculating the tripping time .

I am giving the details below. This information may be useful for Commissioning Engineers and Protection Engineers.

Formula used for IEC and ANSI/IEEE curves

Where,

The following table will give the values for Constants,

For the RI curve following formula is used by all the relay manufacturer except Merlin Gerin Sepam relay

and for Sepam relays, RI curve formula would be,

Dear Friends

As I informed earlier App for" IDMT TCC Calculator " for IPhones and android phones has been developed and posted in iTunes/App Stores.

This APP will be handy for Commissioning and Protection Engineers.

The relevant link is here.

Ramvinod, unfortunately my monitor lacks resolution needed to view your chart, it is a blur to me. maybe you can just attach you chart as a link from engineering.com or something? Thank you for your help, and we look forward to seeing what you post!

I can send the relevant files to you, if you could give your email id.

ramesh