HT VCB vs. LBS

Its better to use a VCB in the main incomer as LBS cannot handle fault currents.

As length of the cable is 400m, it is advisable to use VCB as incomer. You are mentioning that you hv used 200A LBS with ABB140SP + OC & EF functions. Will your isolator trip in case of short circuit fault on a HT feeder cable?

Yes,i want to isolate in case of cable earth fault and short circuit isolation if possible.

Thanks and Regards

That is why u need a VCB as Isolator cannot open on a fault current Got it?

Rajendra,

Isolator (usually cannot be trip) should never be tripped.Isolator is for isolating a section of the ckt after the line or section is free from flowing current,i.e.after switching "off" up or down stream of the isolator.It is used with fixed type of switchgears,overhead lines,inside SF6 immersed/insulated CB for busbar/ckt side isolation.Isolators' operating mechanism is simple and slow.Its contact points do not cater to the the "drawing" and breaking of arc of normal current.If you trip or open an isolator switch when short circuit fault current is flowing through it (to shorted cable );you can only do it once as the isolator and you will not survive to do it the second time.The length of the cable will contribute higher impedences and lower the magnitute of the fault current.However to send supply from such a distance without a local "isolating arrangement" near the transformer is a bit worrysome.Very strict switching procedures requiring authorised/knowlegeable operator, ckt open and isolate with electrical and mechanical locking facility is necessay before you touch the TX terminals.Remember to switch off the LV ACB too.

Regards. Khor

Hi, aikhh!

In addition to the above, your answer also depends partly upon potential overload causes and partly on what's involved in the application you are feeding.

For more help, you might have a look at:

http://www2.electronicproducts.com/Overvoltage_protection_technologies-article-farr_littelfuse_jun2008-html.aspx

Mark

The suggestion and replies I read were incomplete.I feel the need to answer this question which I was first asked about 30 years ago.The fundamentle differences are LBS can BREAK (switch or open and close circuit) on full load (200A) and can switch onto fault and withstand fault current (to pass through it,such as 15 to 20 KA for a period of 1 second only) BUT it cannot break a fualt ( not even once),i.e.break a circuit when that circuit is experiencing fault current 20 to 25 KA.VCB are designed to break a circuit with fault currents(breaking capacity,25KA and making capacity,50KA) besides the normal load current.Japanese VCB has 10,000 times of electrical switching life cycle at normal current ratings 630A,800,1250,2000A for 22KV and 20 times at fault currents of 20 to 25ka, (100A 22KV VCB is not commercially available;DO NOT confuse it with Vacuum Contactor which is available but with very low KA ratings and not suitable here).LBS has very much lower capacity,thus cheaper.If you switch on the 22kv supply to the transformer without knowing that the 400M cable is cut,for example by an excavator,and if the ABB SPAJ140 IDMT relay with 3OC/1EF protection and is set to trip very quickly,very likely you will have an explosion at the LBS.In your case use 3 withdrawable (for isolation and lockout after opening ckt) VCB truck/panels with IDMT relays(the best and more expensive design) or one incoming VCB with 150/5A CTs and o/c e/f relays couple to two LBS with 120A HRC fuses with tripping pin for 2.5MVA transformer short ckt/over load protection (what about TX high temp and high oil pressure alarm and trip).It is always better to place LBS/VCB near to transformers.

Hi Guest,

Thank you very much of your very Good Answer.

Regards

Hi aikhh,

You are welcome.

I am very new to this or any forum.This answer is the very first I have ever writen to someone I do not know.I did not know about registering myself with CR4 before sending out the answer.I did this out of concern for technical people and new engineers to do it right and safe.I believe this is one of the driving force behind so many who are contributing their knowledge,experience and time here.

I am an experience Electrical Engineer with a B.Sc.Engineering degree.Power and high voltage engineering are my knowledge and experience area.LV is not a problem for me.If you need, I can easily design ( in detail c/w protection and technical specifications/dimension)for you the scheme that you mentioned which is nearly enough imformation for me to start with.I cannot draw with computer.22kv seems to be in a commonwealth country.

Best regards,

Khor