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Richard3009

11kv/440-750kva 3 phase tx on 200A primary fuses, spotted this at work, whats the max current per phase you can draw on the secondary without rupturing the fuses.
 
Enough to set the transformer on fire, yes. Is this a trick question?
 
I'd hazard a guess that it probably won't be possible to draw enough current from the secondary to blow those fuses due to the windings melting first. This is based purely on the primary/secondary ratio being 1:25 so to see 200A on the primary you'd need 5000A on teh secondary which is going to equate to a couple of MVA.
Which is a ridiculously rough estimate as it makes no allowance for losses or efficiency or power factor or anything else I don't even know about.

But my knowledge of those kind of transformers doesn't extend past being able to point one out in a substation so I wouldn't trust my word on it!
 
I'd call that a fairly accurate rough estimate. You could probably blow the fuses if you were quick about it though.
 
Well I was assuming that the primary fuses wouldn't be the fastest acting fuses in the world, just using a bit of common sense (although I think that's been banned by the EU recently)
 
750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


And seen this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


Don’t interfere with things you don’t understand.
 
Unlike some of the Long and Crawford gear. That sometimes has been found to explode when switched live!
 
Last edited by a moderator:
Ooo, nasty!
We used Long & Crawford on 3.3 as isolators, never switched live.

That Yorkshire gear was OK once:
The oil leaks were fixed.
The trip bar pivots replaced.
The trip latch replaced.
There were other faults, I’ve still got the report somewhere.
 
There's an embargo on a few models of the long and Crawford gear. Almost exclusively to the models fitted with striker fuses.

Most of the gear they built was actually top notch.
 
750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


And seen this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


Don’t interfere with things you don’t understand.

I simply looked at the drawing of the circuit in the substation
 
750KVA @ 11KV = 39.3A.

Giving a bit of leeway for inrush and available fuse sizes 60A is what I would fit.

Now comes the BIG question, how do you know what size fuses are fitted? Have you looked in the switch?

I’m going to take a guess, you been looking at something like this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


And seen this

750kva on 200A primary fuses {filename} | ElectriciansForums.net


Don’t interfere with things you don’t understand.

Especially when there's a 'TYKE IIA' involved
 

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