Posted by MRB


The standard voltage ratios at no-load shall be as specified in  the Bill Of Qualities and/or Drawings and shall in generalbe as
follows :-
(a) 11000/433-250 V, so as to deliver load at the declared voltage of 415/240 volts, 3 phase, 4 wire system with the neutral solidly earthed.
(b) 11000/3300 V
(c) 6600/433-250 V

Unless otherwise specified, the winding of all 3 phase step-down transformers shall be connected as follows :-

Higher Voltage – Delta
Lower Voltage – Star
Vector Group – Dyn 11


Completed transformers arranged for service but with arcing horns removed shall be capable of withstanding the following negative polarity 1.2/50 microseconds impulse voltage on the higher voltage windings :-

Nominal System Voltage Impulse Withstand Voltage
( Volt rms) ( Kv peak )
———————- ————————-
3300                  45
6600                  60
11000                75

All impulse shall be carried out in accordance with the provisions of BS 923 BS 171 or IEC 76. Type test certificates may be accepted in lieu of separateimpulse tests at the discretion of the S.O.


For the purpose of protection against short circuit stresses,the minimum value of impedance voltage at 75 degrees Celsius rated current shall be as follows :-

4.75% for transformer up to 1000 KVA
5.5% for transformer between 1000 – 1500 KVA
6.0% for transformer between 1500 – 2000 KVA


The maximum flux density at any point in the magnetic circuit when the transformer is connected on the principal tapping and operating at normal voltage and frequency shall normally be inthe region of 1.55 to 1.65 Tesla, but at the time of tendering alternative designs employing higher flux densities may bemay be submitted. However with such higher flux densities for consideration the magnetic circuit shall not unduly saturate during system over voltage conditions.


The no-load and load losses shall be separately quoted in the Schedule of Particulars And Guarantees at the time of tendering. The losses shall be low commensurate with the economic use of materials such that the total capitalised cost is a minimum. The no- load and full load losses shall not exceed the values as follows :-

Size No Load Loss Full Load Loss
(KVA)              (Watt)                (Watt)
—– ———— ————–
100                     220                     1950
250                     650                     2100
315                      700                    3800
500                     970                     5400
800                    1450                    9500
1000                   1650                  10500
1250                    1900                 13000
1600                    2450                17000


Tappings shall be provided on the higher voltage winding for a variation of the no-load voltage, as specified in clause 5.0 of + 5.0%, + 2.5%, 0%, – 2.5% and – 5.0%. All tap changing shall be carried out with the transformers off circuit by means of an externally operated tapping switch with tap
position indications. The operation handle shall be mounted on the tank side and provision shall be made for padlocking in any tap position. Padlocks of any approved typeshallbe supplied by the Electrical Contractor.


The transformers shall be designed for continuous operation at their rated power without exceeding the temperature rise limits as follows :-
In Top Oil : 50 degrees Celsius In winding : 55 degrees Celsius


The class of winding insulation shall be Class A to BS 2757.


Due care shall be taken to ensure that the design and manufacture of all transformers shall be such as to reduce noise and vibration
to a level normally obtained with good modern practice.


  1. N0UL9d Way cool! Some very valid points! I appreciate you penning this post and the rest of the website is also very good.

  2. Normally I do not learn post on blogs, however I wish to say that this write-up very pressured me to take a look at and do it! Your writing style has been surprised me. Thanks, very great article.

  3. My website is in the very same area of interest as yours and my users would truly benefit from

  4. Really informative post.Thanks Again. Really Cool.

  5. go there now says:

    You ave made some good points there. I looked on the web to find out more about the issue and found most individuals will go along with your views on this site.

  6. Very interesting information!Perfect just what I was searching for! Being rich is having money being wealthy is having time. by Margaret Bonnano.

  7. The Silent Shard This may likely be quite useful for some of your positions I decide to you should not only with my website but

  8. I really liked your blog post. Will read on

  9. Really informative article post.Thanks Again. Much obliged.

  10. Remarkable! Its actually remarkable post, I have got much clear idea on the topic of from this post.

  11. Major thankies for the article post. Much obliged.

  12. Wow! Thank you! I constantly wanted to write on my site something like that. Can I include a fragment of your post to my blog?

  13. Thanks-a-mundo for the blog post.Really looking forward to read more. Much obliged.

  14. Regards for all your efforts that you have put in this. very interesting information.

  15. These people work together with leap close to they will combined with the boots or shoes nevertheless search great. I truly do think they may be well worth the charge.

  16. This page definitely has all of the information and facts I wanted about this subject and didn at know who to ask.

  17. need, and just what the gaming trade can supply. Today, these kinds of types

  18. payday loan online no fax quick and easy payday loan

  19. Im thankful for the blog article.Really looking forward to read more. Great.

  20. Say, you got a nice blog article.Much thanks again. Will read on

  21. Yeah, it is clear now ! Just can not figure out how often do you update your blog?!

  22. Thanks-a-mundo for the article post.Really thank you! Fantastic.

  23. You have made some good points there. I looked on the internet to learn more about the issue and found most people will go along with your views on this website.

  24. Muchos Gracias for your post.Thanks Again. Awesome.

  25. weeks of hard work due to no back up. Do you have any solutions to stop hackers?

  26. Say, you got a nice article post.Much thanks again.

  27. speedtest says:

    pals ans additionally sharing in delicious. And of

  28. Major thankies for the blog post.Really looking forward to read more. Will read on…

  29. Im obliged for the blog article.Really looking forward to read more. Keep writing.

  30. with this. Additionally, the blog loads super quick for

  31. You made some really good points there. I checked on the net for more information about the issue and found most individuals will go along with your views on this website.

  32. you ave an incredible blog right here! would you like to make some invite posts on my blog?

  33. I think this is a real great article post.Really looking forward to read more. Much obliged.

  34. In general, the earlier (or higher ranked on the search results page)

  35. Very nice post. I just stumbled upon your blog and wanted to say that I have really enjoyed browsing your blog posts. After all I will be subscribing to your feed and I hope you write again soon!

  36. This unique blog is really awesome and also factual. I have found helluva interesting tips out of this source. I ad love to return over and over again. Cheers!

  37. That as some inspirational stuff. Never knew that opinions might be this varied. Thanks for all the enthusiasm to supply such helpful information here.

Leave a Reply