AWA submain 400Amp install

[Rocboni]

hello

Just wondering at what level does circulating currents, short circuit current come in to play when installing single cables and how inportant the arrangement of cables on the ladder are, is it mainly high voltage installations or low voltage as well?

We are installing a 400A submain parallel runs of 120mm AWA around 90M long, it is to be arranged flat L1 L2 L3 L1 L2 L3 N N E and will be retained with cable ties (every second one being metal) both ends glanded into aluminium gland plates, a few things I disagree with

If it was me i think it should installed differently as below and was wondering if I’m correct?

Cables arranged in trefoil arrangement using aluminium trefoil cleats to minimise magnetic field and to avoid short circuit current forcing cables apart in the event of a fault snapping ties (is this an issue with low voltage or just where short circuit current is higher than achieved in low voltage installs)

Neutral beside each bunch earth to the side.

One side glanded into aluminium gland plate earthed at source, other end into Paxolin sheet to avoid circulating currents but leads to possible standing voltage on the opposite end?

 

[Rocboni]

No takers?

 

[Bobster]

You need to be designing your cable restraints to what your perceived fault rating is going to be. 400A isn't all that much in the face of things.

What sort of kA is your installation likely to see? Or is being designed to? Decent cable ties are rated to around 20kA.

Being in tri foil arrangement does reduce the overall magnetic force given out by a fault. I'll dig the equation out for you when I get a min. Baisicly the closer you can get the centre of each phase conductor to each other, the less magnetic force generated.

As for circulating currents. Anything over 200A starts causing a real problem. However you don't need to separate one end from earth like you are suggesting. Just install in such a way that Eddie currents aren't allowed to form in the armouring. Do this with non ferrous gland plates.

On a side note, you would normally seperate one end if you were feeding directly from a transformer housing.

 

[Rocboni]

I’m not sure What what the Pscc is at the moment I reckon it will be under 50KA submain is protected by a MCCB, that’s what I was wondering, how large does it have to be to need to be fixed with trefoil cleats looks like it's generally over 50Ka

Seem like ties were the correct choice as cleats are about £25 each and we’d need about 190 and the other issue is cable rating, as they cables have been laid with one cable spacing between, and the ties were much easier to install, every second one being metal, although I recon after a few years the plastic ones will be gone.

 

[Bobster]

Well if I were the designer of that job all of those ties would be ripped off and the cables installed in tri foil arrangement.

The designer should know the PSCC, or have calculated it from the transformer size and impedance.

Cleats could have been used every 2 meters maybe, and cost really doesn't come into play on installs like this, safety is key. I know a quarry and old member here worked at (Tony). It was before I started working there, but cables led loose in a cable duct experienced a fault. It was of such magnitude that it lifted the 1/4" steel checker plate coverings and unfortunately the plate hit a person and killed them.

This is the formula to work out the magnetic rappelling force generated under fault, it shows clearly why cables need to be installed in a tri foil arrangement:

Ft = (0.17 x Ip²)/S

Where
Ft = Maximum Force per unit length of cable (N)
Ip= Peak short circuit current (KA).
S = Center to center distance between neighboring conductors (m)

 

[Rocboni]

I expressed my concerns on the arrangement before I started to tie them under instruction from the journeyman who was instructed by the manager, however nobody listened. I have seen videos showing how the cables blow apart under fault conditions. Would probably have been quicker to cleat them.

For some reason they are slotting the aluminium gland plates which I view as a complete waste of time and unessary, just shows they may not fully understand.

They will know he PScc but I don't personally at the moment.

I can only do what I'm told but I did say it was wrong, at least I know to question it and how it should be done if I am to design a install like this, they dont teach you this at college!

 

[Rocboni]

*unnecessary

 

[Bobster]

I expressed my concerns on the arrangement before I started to tie them under instruction from the journeyman who was instructed by the manager, however nobody listened. I have seen videos showing how the cables blow apart under fault conditions. Would probably have been quicker to cleat them.

For some reason they are slotting the aluminium gland plates which I view as a complete waste of time and unessary, just shows they may not fully understand.

They will know he PScc but I don't personally at the moment.

I can only do what I'm told but I did say it was wrong, at least I know to question it and how it should be done if I am to design a install like this, they dont teach you this at college!

No they usually don't, they can't teach you anything. People designing installs like this usually have a few years under their belts, having worked and gained experience in this field.

Installs like this show up when inexperienced designers are thrown in deeper than their knowledge can cover them. Ignorance is bliss so they say.

 

[Rocboni]

The designer here is about 5 years from retirement !

 

[Bobster]

The designer here is about 5 years from retirement !

He was probably taught incorrectly himself then, old dogs new tricks spring to mind.

Unfortunately I see it quite a lot, but "it's how we've always done it!"