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Spudnik

Right chaps,

Due to a few unavoidable issues, we have lost the original thread and pics etc.

Sorry about that!!!

So, if you want to re-upload your pics then great, if not, give us something new!


Cheers!!
 
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t01474 said:
Good points made by everybody concerned. I'll bear them in mind for future works. Never even considered the tie wrap scenario
Would the same happen with 4-c SWA? Generally you see them just tie-wrapped to the ladder racking, tray, ect.
Click to expand...

Under extreme fault conditions a SWA can be blown apart. But that would really be worse case scenario.
When singles are used then magnetic repulsion is a real danger. I’ve actually watched bus-bars bending under the start up load of a squirrel cage motor. That came about because a MCC panel had been extended several times without due consideration to the overall effect on the bars. “It’s only a 100HP motor, it’ll be OK.” The trouble was several “it’s only” situations that had happened over the years. To the point where the bars gave up the ghost, bent and shorted out.
 
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Tony said:
Under extreme fault conditions a SWA can be blown apart. But that would really be worse case scenario.
When singles are used then magnetic repulsion is a real danger. I’ve actually watched bus-bars bending under the start up load of a squirrel cage motor. That came about because a MCC panel had been extended several times without due consideration to the overall effect on the bars. “It’s only a 100HP motor, it’ll be OK.” The trouble was several “it’s only” situations that had happened over the years. To the point where the bars gave up the ghost, bent and shorted out.
Click to expand...


I've only ever seen the results of a catastrophic fault on a dual supplied 3200A LV switchboard!! Where the bracing supplied by the manufacturer, was found to be, not up to the fault KA rating of the board. Which should have been 50KA but was in fact, barely reaching 32KA bracing levels. Two complete panel sections needed to be totally replaced and bracing upgraded throughout the entire switch board!! A very expensive exercise for the manufacture, which was only mitigated to a small degree by a wrong doing of the contractor who also ended up paying a pretty price!! lol!!

NOT one of my projects i should hasten to add!! lol!!
 
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Looks like we're dealing with large forces here...

A three-phase short circuit in trefoil formation, the maximum force on the
conductor as detailed in IEC 61914:2009 (Appendix B Equation B.6), is
described by the following:

F[SUB]t[/SUB] = (0.17 x i[SUB]p[/SUB][SUP]2[/SUP])/S

Where
F[SUB]t[/SUB] = Maximum Force per unit length of cable (N/m)
i[SUB]p[/SUB]= Peak short circuit current (kA)
S = Centre-to-centre distance between neighboring conductors (m)

when the currents are in 100s of kA!!!



Interesting stuff!
 
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Archy Styrigg said:
Looks like we're dealing with large forces here...

A three-phase short circuit in trefoil formation, the maximum force on the
conductor as detailed in IEC 61914:2009 (Appendix B Equation B.6), is
described by the following:

F[SUB]t[/SUB] = (0.17 x i[SUB]p[/SUB][SUP]2[/SUP])/S

Where
F[SUB]t[/SUB] = Maximum Force per unit length of cable (N/m)
i[SUB]p[/SUB]= Peak short circuit current (kA)
S = Centre-to-centre distance between neighboring conductors (m)

when the currents are in 100s of kA!!!



Interesting stuff!
Click to expand...

Fault currents are limited by what the transformer in question can deliver into a fault!! Obviously the larger the Transformers KVA/MVA rating, the more current it can deliver into a fault!! For a typical 240/415V 1000KVA transformer, is roughly about 20KA...
 
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So Ip is only 400,000,000 (20K²). That’s OK then :ack2:

It gets worse the more you look at it.

So if we allow 50mm conductor centers:

Ft = (0.17*(20K²))/20 = 3,400,000 N/m

A nylon strap’s going to hold that, isn’t it?
 
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Is this force only generated between conductors carrying different phases, or does it happen in single phase cables too? I'm wondering if this effect happens in, say, your domestic T&E to a lesser extent with the lower fault current.
 
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To be honest in final circuits, the fault current is very low so although there will of course be repulsion, it will not be anywhere near the levels of the forces generated at the origins of larger industrial installations. Use Archie's formula to try the numbers for yourself. You could transpose it to see what fault current you would need to cause a certain force.
 
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OK assume a fault current of 0.5KA on a 2.5 T+E, I get 127 N/m. It's not going to do it a lot of good.

To put this in to context under a load of 20A the 2.5MM T+E has got 0.2M/m of force acting within it.
 
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Tony said:
OK assume a fault current of 0.5KA on a 2.5 T+E, I get 127 N/m. It's not going to do it a lot of good.

To put this in to context under a load of 20A the 2.5MM T+E has got 0.2M/m of force acting within it.
Click to expand...

That would explain why cables blow themselves to pieces.
It's all starting to make sense now!
 
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The bars I watched bending were within 20m of the 1000KVA transformer. Don't know what current they were carrying at the time but couln't have been much above 1500A It was a 100HP DOL motor starting on an already loaded board. Normal running would be about 1100A The board was a Metro Vick open slate panel.
 
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Never seen bars actually bending, a sight to see that i bet!! lol!!


I have seen package sub-stations (MV/LV) that have come from the factory with next to no bracing on the 3 phase bars entering the LV panel. That was after a factory visit (GE) too, ...where the units that were up for delivery to site were inspected and witness tested by myself!! Those 3 units were fine, but the next 4 units were crap, (12 X 1MVA units in all on that project) and after opening the first unit, instructed the contractor to send them back for remedial work, which both the contractor and myself will inspect at the factory, before they are shipped back to site!! That was in Saudi, but the factory was run and operated by GE (USA) at the time!!
 
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The Metro Vic panel was from the late 40's, all open at the back. I was the wet behind the ears idiot that got talked in to watching what was happening.

After talking to Archy we’ve revised the figures for the 20KA calculation (I used A and not KA) the new value of F = 1360N/m. Not as bad as it was, but it’s going to throw things about violently if the cables get loose. The last transformer install I did I used 3.5T of cable on the secondary side. I wouldn’t fancy getting clobbered by that lot.
As Archy said the film of the multi-core cable exploding says it all.
 
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Archy Styrigg said:
That would explain why cables blow themselves to pieces.
It's all starting to make sense now!
Click to expand...


Many factors involved, depends on the cable size, (they also have a KA/s withhold rating too) length of cable etc. More chance on a short length of cable, say between TX and Main Switchboard, than on a long run (depending where the fault occurs), as the cable will act as resistance/impedance, limiting the fault current the transformer can deliver into the fault...

As i say too many factors involved that can/will influence each other, to pick-up on just one aspect and predict what will happen under a given fault condition.
Safe to say, any fault that involves the full fault current of a transformer, will always be catastrophic, ...and expensive!! lol!!
 
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onions1066 said:
Engineer 54 you are obviously very knowledgeable but I've never seen you post any pics of your work up, come on I think it's time to show us lol
Click to expand...


Should have asked a couple of weeks ago. Not that i have much in the way of past project photo's anyway!! What i do have are on the PC, and it's too late getting that out of mothballs now, as i'm off flying back tomorrow!! So you'll have to wait till the summer now!! lol!!
 
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53601.PNG
here he is
 
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Rough and missing terminal numbers, just rough copies before a proper computer drawing neat and standardised. Pretty basic circuit as you can see but I thought I'd share

This is just a control panel for a Plasma Nitrider. The complicated side of this machine is all in an Electronic Circuit (which I have the drawings for if anyone is interested) it's packed full of inverters and rectifiers. The more complicated stuff comes with the Arc Detection parameters. My company had a Romanian Physicist custom make it for us, he apparently works on the LHC now lol.
 

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Looking Good there!! Have you ever considering installing Slotted Panel Trunking to inside of the DB instead of cable ties around the cabling?? I always do this and think it looks neat, and a lot easier if need to trace cables out at a later date.
Not a critisism just some of my knowledge and learnings from over time.
Regards Darren
 
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Gage said:
Rough and missing terminal numbers, just rough copies before a proper computer drawing neat and standardised. Pretty basic circuit as you can see but I thought I'd share

This is just a control panel for a Plasma Nitrider. The complicated side of this machine is all in an Electronic Circuit (which I have the drawings for if anyone is interested) it's packed full of inverters and rectifiers. The more complicated stuff comes with the Arc Detection parameters. My company had a Romanian Physicist custom make it for us, he apparently works on the LHC now lol.
Click to expand...

IMG_1924.jpg

Just a quick update, getting there, just doing bits and bobs when I can. Not as pretty as I'd like lol but getting rushed to do things, have to show progress while dealing with other breakdowns and what not. Also I duno what's happening with my photos lol
 

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