**Show Us Your Installs!!!**

[pennywise]

I had no idea cables jumped with fault current. Does the impulse come from the large magnetic field generated, or something else?

This illustrates it quite nicely and why the correct type of cleats are needed.

Short Circuit Testing new - YouTube

 

[Stroppy]

Great video, most informative.

For non-mobile users: Short Circuit Testing new - YouTube

 

[Knobhead]

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.

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.

 

[High Tower]

Tony and E54 thanks for that I must say I'm taking away some knowledge from this!

 

[Engineer54]

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.

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!!

 

[Silly Sausage]

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!

 

[Knobhead]

Excel went in to E numbers :fuk2:

 

[Engineer54]

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!

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...

 

[Silly Sausage]

Deleted, note to self, 'Don't attempt any maths after a few beers!'

 

[Knobhead]

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?

 

[Stroppy]

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.

 

[johnboy6083]

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.

 

[Knobhead]

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.

 

[Silly Sausage]

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.

That would explain why cables blow themselves to pieces.
It's all starting to make sense now!

 

[Knobhead]

Have a look at Home - Scatco Europa Ltd. I installed quite a few of these. Look for the fault withstand current. It's not the CCC of the bus-bars, it's the current the bars will carry without distortion.

 

[Engineer54]

Have a look at Home - Scatco Europa Ltd. I installed quite a few of these. Look for the fault withstand current. It's not the CCC of the bus-bars, it's the current the bars will carry without distortion.

Along with the bracing to hold those bars in place during a high magnitude fault!! 50KA for 3 secs, takes a fair bit of bracing to be in place!! lol!!

 

[Knobhead]

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.

 

[Engineer54]

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!!

 

[Knobhead]

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.

 

[Silly Sausage]

You'll notice I've edited a few of my recent posts!!!