MVA to Ka - What device is acceptable?

[frm2005]

Hello, 

I am currently working on an installation and a issue has come up in regards to distribution. Below is a ruff break down of the installation the install consists of four apartment blocks:- 

270Kva supply quoted 25MVA!! fault current 

Main LV 800amp MCCB panel 
leaving this panel are the supplies feeding the other blocks (A/B/C/D) 

Looking at block A 

From the main panel a supply is feeding a secondary 400amp MCCB panel within block, from that then feeds a 250amp MCB TP&N distribution board feeding RCBO populated consumer units within the apartments via a 63amp mcb rated at 15Ka. 

My issue is the consultant has pointed out that the apartment distribution board needs to be a MCCB panel board due to the high symmetrical fault level quoted by the DNO. He has pointed out that the fault level quoted is 25Ka, however, on inspection of the supply documentation today the actual fault level quoted is 25MVA not KA?? 

My question is how do I convert MVA to KA to select the correct device? From some digging around in some old text books I have come across this formula - MVA/1.73 * KV L-L, so on doing this I get the following 25/1.73*0.4 (kilovolt line to line voltage) = 5.7KA. 

Just to point out the transformer is located onsite so I would expect a high Ics value. I have also carried out discrimination calculations and board calculations according to heavacomp no final circuit on this job even touches 2ka. I have based these calculation on using a 270kva tarnsformer with a 25ka fault current. 

Has the consultant misunderstood the 25MVA as 25KA or am I cultching at straws lol. 

Any help would be appreciated 

Cheers 

Daz

 

[Kurt]

25MVA looks wrong, it is rather high even if the tx is onsite. The tx has an impedance which drags down the maximum fault current, If you have a look at it, it may tell you on the rating plate. He's probably right at 25kva and its probable that your 15K breaker could be outside its limits when you do the ZDB.

 

[Tony S]

An 800A ACB incomer would indicate a 11/0.433kV 500kVA 4.5% reactance transformer.

Worse case scenario based on infinite source would be 25kA Ph→Ph or 11MVA

Ask the DNO for confirmation because they are out by miles.

 

[frm2005]

An 800A ACB incomer would indicate a 11/0.433kV 500kVA 4.5% reactance transformer.

Worse case scenario based on infinite source would be 25kA Ph→Ph or 11MVA

Ask the DNO for confirmation because they are out by miles.

The incoming supply is a 270kva (400amp per phase) from the transformer, we intend to fit a panel with the option to upgrade the supply to 800amp.

The documentation/quote from the DNO states 25MVA however the spec states 25Ka little confusing TBH  

 

[Sidewinder]

There is definitely some confusion somewhere.

Are you sure that you don't have a 25MVA tx, with a 25kA fault level.

25kA is about right for a "normal" 400A DNO supply.

Thus you need MCCB's or fuses until your fault level decreases below that which can be coped with by MCB's, else you will be relying on backup protection, and you should be changing any breaker that trips, every time it trips.

 

[Kurt]

To convert VA to A , I generally divide by 700, although if using Tony's line voltage at 433 it would be 750. So 25MVA gives a fault current of 33.33KA.

You can get MCCBs at that level, I think Merlins would be 36KA standard, however although I have measured PSSC higher than 25KA with on site transformers its not very often.

As mentioned 25MVA is a little high and it may simply be a DNO fixed number for all installations rather than specifically this one.

 

[Tony S]

Do you know the transformers % reactance? The smaller transformers generally have a lower reactance.