Proposed Connection for new build states prospective will be as high as 25.9KA

[John-]

Hi
A customer has a new farm shed going up supplied also from a new 100KVA transformer right outside. The supply spec states that PSCF could be as high a 25.9KA on the quote he has been given. The usual kit I use is rated at 10KA. There terms also state that the connection by DNO will not be carried out until the install is in.

Is this a normal reading for close proximity of Transformer straight off the HV?
Are these 'anticipated' readings likely to be reflected in real life, about 20m from transformer?
If i provide tails to an isolator and means of earth marshalling and connections, are they likely to connect - i can then at least test the actual PSCF current before purchasing distribution kit.
If PSCF current is that high, anyone any guidence on manufactures of 26KA rated devises we can use? I normally use MCG or Hager. Only looking at standard farming type equipment, ring mains corn dryer, lighting.

Thanks.

John

 

[Julie.]

Looks quite normal to me, but not for that transformer!

The flc on 100kVA is around 145A, if the transformer is 5% voltage impedance this would give a fault level of circ 2.9kA


I would suggest that the question is asked, 25.9kA (I assume that's what you mean ) would need a bigger transformer, but is quite normal for 1.5MVA and above.

Either the transformer size is incorrect or the fault current is incorrect, but either could be correct, the other is wrong.

 

[John-]

Okay maybe the transformer is more than 100KVA. 100KVA is what they said they were supplying him. In any case the questisn stil l remain: -

Is this a normal reading for close proximity of Transformer straight off the HV?
Are these 'anticipated' readings likely to be reflected in real life, about 20m from transformer?
If i provide tails to an isolator and means of earth marshalling and connections, are they likely to connect - i can then at least test the actual PSCF current before purchasing distribution kit.
If PSCF current is that high, anyone any guidence on manufactures of 26KA rated devises we can use? I normally use MCG or Hager. Only looking at standard farming type equipment, ring mains corn dryer, lighting.

Thanks

John

 

[Julie.]

A decent supply close (20m+) to a large transformer can easily exceed 30kA.

What are they supplying with, I assume 3 phase, say several hundred amps, or is it just a ~ 150A supply?

If it's just a couple or three hundred amps, they are going to fit fuses.

If it is at the higher end and you are responsible locally for the incoming protection then yes you would need to use ACBs or MCCBs rated for 26kA+, then design downstream accordingly.

 

[David Prosser]

Why not ask who ever is being paid for the transformer (I would guess DNO). It wil of cost a fare bit of money for the new supply and transformer so it's not unreasonable to check what size it is and why they have predicted the fault current at 25.9kA.

 

[John-]

A decent supply close (20m+) to a large transformer can easily exceed 30kA.

What are they supplying with, I assume 3 phase, say several hundred amps, or is it just a ~ 150A supply?

If it's just a couple or three hundred amps, they are going to fit fuses.

If it is at the higher end and you are responsible locally for the incoming protection then yes you would need to use ACBs or MCCBs rated for 26kA+, then design downstream accordingly.

Hi

It will be a TPN TNCS supply. Good question, i don't know - i assume 100A, it is one of my question to the customer

My concern is the type of protective devices that i will have to use for normal things like lighting and power - they will have to be huge moulded monsters just for standard circuits.

I am reaching out to some the manufactures i use to see what leeway they have through cascading or coordination, but most so far are saying what is that...

Any advise on manufactures that do support cascading right down to normal sized 10-20KA devices?

Thanks again.

John

PS what would you do for a normal house in this situation with many circuits and a much lower budget...

 

[David Prosser]

I think you can get ABB MCBs up to 25kA SCC.

2CDS282001R0635 S20B 63A - 15 KA | ABB System Pro M Compact S200P MCB, 2P, 63A Curve B | RS

uk.rs-online.com

 

[Julie.]

Hi

It will be a TPN TNCS supply. Good question, i don't know - i assume 100A, it is one of my question to the customer

My concern is the type of protective devices that i will have to use for normal things like lighting and power - they will have to be huge moulded monsters just for standard circuits.

I am reaching out to some the manufactures i use to see what leeway they have through cascading or coordination, but most so far are saying what is that...

Any advise on manufactures that do support cascading right down to normal sized 10-20KA devices?

Thanks again.

John

PS what would you do for a normal house in this situation with many circuits and a much lower budget...

If its 100A fuses, then I would expect that you would need 16kA devices , doubt you would get away with less than 10kA.

Crabtree and a few others do 16kA rcbos and mcbs.

I would really check with the dno, as I guess if it really is a smaller supply it won't need more than 6kA stuff.

Is it in or near large industrial estates, or is it a remote supply in a field somewhere?

 

[John-]

If its 100A fuses, then I would expect that you would need 16kA devices , doubt you would get away with less than 10kA.

Crabtree and a few others do 16kA rcbos and mcbs.

I would really check with the dno, as I guess if it really is a smaller supply it won't need more than 6kA stuff.

Is it in or near large industrial estates, or is it a remote supply in a field somewhere?

very close to heavy industry. Straight of HT stuff. Even if they fuse it at100A, i am still going to have a high PSCFC. Thanks though.

 

[Julie.]

very close to heavy industry. Straight of HT stuff. Even if they fuse it at100A, i am still going to have a high PSCFC. Thanks though.

I would check the figures from the dno, the fault level does sound realistic if it is close to a big solid supply from a large transformer.

I was hoping you would say it was remote, then it would be virtually guaranteed the fault level was wrong.

You need to get them to confirm the incoming fuses, as this will dictate the fault level you need to design for.

If it's 80A fuses, then 10kA kit would be ok, 100A fuses and you would need 12kA kit , 160A fuses - 16kA and so on.

 

[John-]

I would check the figures from the dno, the fault level does sound realistic if it is close to a big solid supply from a large transformer.

I was hoping you would say it was remote, then it would be virtually guaranteed the fault level was wrong.

You need to get them to confirm the incoming fuses, as this will dictate the fault level you need to design for.

If it's 80A fuses, then 10kA kit would be ok, 100A fuses and you would need 12kA kit , 160A fuses - 16kA and so on.

Where did you get the KA let through data from to know that the supply fuses would not allow >10KA for 80A, >12ka FOR 100A etc? I did wonder if I would see it in the regs book but no joy, manufacture of fuses i guess for a more detailed lower part of the current characteristics graph table?

 

[Julie.]

Where did you get the KA let through data from to know that the supply fuses would not allow >10KA for 80A, >12ka FOR 100A etc? I did wonder if I would see it in the regs book but no joy, manufacture of fuses i guess for a more detailed lower part of the current characteristics graph table?

Yes let through characteristics from a fuse data sheet. Example:

 

[John-]

Yes let through characteristics from a fuse data sheet. Example:

View attachment 95602

Thanks.

 

[pc1966]

@Julie. knows far more about this than me, but I can confirm the PFC sounds high for that as our local supply is about 20m from a 500kVA substation and SSE quoted a max fault current of just over 11kA and I measured it at 10kA (give or take quite a bit on my MFT).

Also to add if you look up "cascading values" for fuse and MCB combinations it gives you a more direct answer. For example, page 138 of the Hager commercial catalogue is to my hand, and it shows combinations of BS88 fuses and MCBs. With a 100A fuse and all NBN series MCBs above 10A the max PFC is the 80kA of the fuse. At 10A or 6A the max PFC is the 15kA of the MCB as that has total selectivity meaning the MCB has to break it all.

That 15kA is the once only break by the way!

However, once you get a few meters down a light circuit it would fall below 15kA quickly.

The other alternative if you have to feed small circuits from close to the supply is to consider a 2nd DB with a 63A fused-switch feeding it, that takes all the B-curve MCBs comfortably in to the 80kA fuse limit.

 

[Julie.]

@Julie. knows far more about this than me, but I can confirm the PFC sounds high for that as our local supply is about 20m from a 500kVA substation and SSE quoted a max fault current of just over 11kA and I measured it at 10kA (give or take quite a bit on my MFT).

Also to add if you look up "cascading values" for fuse and MCB combinations it gives you a more direct answer. For example, page 138 of the Hager commercial catalogue is to my hand, and it shows combinations of BS88 fuses and MCBs. With a 100A fuse and all NBN series MCBs above 10A the max PFC is the 80kA of the fuse. At 10A or 6A the max PFC is the 15kA of the MCB as that has total selectivity meaning the MCB has to break it all.

That 15kA is the once only break by the way!

However, once you get a few meters down a light circuit it would fall below 15kA quickly.

The other alternative if you have to feed small circuits from close to the supply is to consider a 2nd DB with a 63A fused-switch feeding it, that takes all the B-curve MCBs comfortably in to the 80kA fuse limit.

There is a simple way to calculate the maximum fault level on a single transformer, that is work out the flc (kVA/Vl-n x 3) so around 725A for 500kVA

Then divide by the voltage impedance of the transformer.

Typical 3 phase 11kV tx
50kVA - 4.5% - 5%
100kVA - 4.75% - 5%
500kVA - 5% - 6%
1MVA - 4.75 - 5%
1.5MVA - 5.5 - 6%

So a 500kVA would be somewhere between
725/5% = 14.5kA
725/6% = 12kA

Ignoring any cables.

 

[pc1966]

100kVA is 150A 3-phase, so I'm guessing 160A fuses. You won't find MCBs above 15kA in most cases, and MCCB options cost a lot.

Depending on what loads have to be supported you might want to look at two 80A fused-switches each feeding normal 125A or similar TPN boards with MCB/RCBO in them. The fuses will have total selectivity with the supply fuse, and for faults near the ends of final circuits probably selectivity MCB/fuse, but that would need checking if short cables, etc.

 

[John-]

Thanks for the support all, much appreciated.