Electromagnetic effect in 4C Swa cable with high kA

[Leyland30]

Hi All, hope someone can provide some clarity;
We have a number of 400mm^2 4C Swa sub-mains subject to potential 43kA fault currents. Since the cables have TP&N contained within a shared SWA sheathing we were of the understanding that the electromagnetic response resulting in a violent and destructive effect on the conductors would be largely neutralised and contained.
The cabling is being fixed to a HD ladder tray with metallic cable ties, there is some confusion as to whether these are sufficently robust.
Any one got any guidance.
Thanks

 

[Andy-1960]

A few questions

Are these parallel feeders?
What is the protective device?
Do you have any supplementary Earth?
What is the length of the run?
Is Earth loop reading at load end ok?

 

[Leesparkykent]

I would of thought munzing rings would be more approprite.. This is the calculation you need and have a read of page 48 of this beama guide. http://www.beama.org.uk/asset/317F58BB-6204-4B5C-80B5F76C80824BD7/

 

[Andy-1960]

I don't believe the electromagnetic interaction between cables is an issue as you have installed multicore cables. What I would be concerned about is the fault rating of the cable itself. The armour will have a significantly lower fault rating than the cores and I doubt the armour would cope with such a high fault level.

 

[Leyland30]

Thanks lee, We have reviewed the calc previously and it returns a value of over 100kN/m which is equivalent to a small battleship (1000 ton /m) This doesn't seem feasible and the cable cleats only appear to go upto 15kN/m.

Andy, I'm assuming the design software hasn't flagged anything regards any insufficient conductor sizes. I'll review calcs tomorrow as these have been undertaken by a third party.

 

[davesparks]

I would of thought munzing rings would be more approprite.. This is the calculation you need and have a read of page 48 of this beama guide. http://www.beama.org.uk/asset/317F58BB-6204-4B5C-80B5F76C80824BD7/

View attachment 36949

That's for single core cables, the OP is using 4 core so the restraint is already there in the cable.

 

[Leyland30]

Dave, this is our understanding but is there something in text that qualifies this. Thanks

 

[davesparks]

Dave, this is our understanding but is there something in text that qualifies this. Thanks

Speak to the manufacturer of the cable or get a copy of the standard that the cable is manufactured to

 

[marconi]

Hi All, hope someone can provide some clarity;
We have a number of 400mm^2 4C Swa sub-mains subject to potential 43kA fault currents. Since the cables have TP&N contained within a shared SWA sheathing we were of the understanding that the electromagnetic response resulting in a violent and destructive effect on the conductors would be largely neutralised and contained.
The cabling is being fixed to a HD ladder tray with metallic cable ties, there is some confusion as to whether these are sufficently robust.
Any one got any guidance.
Thanks

You might want to take a gander at this:

http://www.deltastarconnectorcompany.com/catalogx/pdf/TECHDATA.pdf

a paper on forces on conductors under short circuit conditions.

 

[davesparks]

You might want to take a gander at this:

http://www.deltastarconnectorcompany.com/catalogx/pdf/TECHDATA.pdf

a paper on forces on conductors under short circuit conditions.

I haven't read through it properly, but at first glance it appears to be concerned with single core cables rather than multicore armoured cable as is being discussed here

 

[marconi]

Leyland30, I have been thinking on your query some more. If you don't obtain a satisfactory answer from the cable manufacturer (see Davesparks post#8) then you might want to contact Cooper Industries for some advice - see;

http://www.cooperindustries.com/con...ble_tray_systems/all_products/CableFixing.pdf

I also turned up this on the IET Forum - see;

IET Forums - SWA cleats faut current ratings - http://www.------.org/forums/forum/messageview.cfm?catid=205&threadid=103119

The question I think is not so much will the cleats hold the cable(s) in situ in the cable tray but whether the swa and cleats around the cable will be able to handle the lateral and longitudinal forces and vibrations before the 0ver current protection clears the fault.

I do observe on the London Underground long runs of thick high voltage(red) and low voltage(black) swa cables clamped about every metre (- I will take a close look on the spacing next time I take the tube) to linear metal supports, often side by side (- not red and black mixed though). I have no idea what the prospective fault currents are but if they comparable to your application then perhaps here might be avenue of enquiry.

I understand the physics of electromagnetic forces acting on the current carrying conductors and the electrical theory for fault current analysis. As Davesparks points out, your question is qualified by having conductors surrounded by swa. If there is no current flowing in the swa then there can be no electromagnetic force on the strands of the swa - it will only be subjected to mechanical forces from the conductors it contains. If the fault does cause very large current to flow in the swa then this further complicates matters - we cannot assume there is no net electromagnetic field within or without the swa because the conductor physical arrangement means there will be high leakage flux during high kA faults; so the swa will be subject to electromagnetic forces like the conductors within it.

I expect the solution will be to pick suitable strong clamps and to space them quite close together to thereby reduce the longitudinal tensile stress in the swa during fault conditions.

Anyway, not an answer but maybe some food for thought and avenues to research.

 

[Leyland30]

Marconi, thanks for the links; the iet one we have already reviewed. The cooper one is interesting but, like most information we have found so far; refers to single core and /or single core arrangements- very frustrating. However it does offer an alternative avenue to explore via the cable cleat manufacturers.

I find it odd that their appears to be a lack of cleat manufacturer promotional videos on the likes of YouTube relating to multicore SWA s under fault when there are a vast array of single core SWA trashing around.

 

[Andy-1960]

I ran this through our Amtec software yesterday and it did not throw up any issues. I take it someone has flagged up a potential issue.. what exactly was said?

 

[Leyland30]

The type of cable restraints used to fix the multi cores to the ladder containment have been questioned given the high potential fault currents. We are happy with electrical performance of the cables but the electromagnetic effect has been suggested as a reason to utilise cable cleats. This has huge cost implications but more importantly we are not convinced they are required.

We await technical responses from both the cable and cleat manufacturers.

 

[Leyland30]

Had the following response from prysmian:

Under normal operating conditions in a balanced loaded multi-core cable, the magnetic field created due to current flow in one conductor is negated by the current flow in the other conductors, i.e. it is balanced.

However during fault conditions, the current flow and hence the magnetic field generated is not always balanced. Under these conditions there is a repulse force between the conductors.

Many years ago tests were carried by ERA which indicated that the mechanical strength of the covering over the cores was important with regard any damage that the cable might sustain due to these repulsive forces. It was found that if the cable was armoured then the full short circuit rating of the cable could be used. However if the cable was non armoured then a limit of 25 K.A for 1 second should apply.

So there you go ...