Armoured cable - radius of bends?

[Spectric]

now a question, why cant you just put your foot on a bit of armoured cable and bend it as tight as you can,

Because it does not like it, the outside will be in tension whilst the inside will be under compression and there is a limit to how far the armour can accomodate these forces. It is not a major problem with SWA's upto 25 to 35mm CSA but once the cable uses shaped conductors then it becomes an issue for everyone.

 

[Stevekane]

Many thanks Roy, 25mm to 35mm csa (individual?) cables,,I obviously mjssed the bit about the OP recreating Dr Frankensteins lab in his shed!
But seriously, I looked up shaped conductors, never seen them before and a clever idea, and I can see that they wouldn't take kindly to being tightly bent. We learn something every day,,
Steve.

 

[Spectric]

16mm or 25mm are the typical supply cables to a domestic property, 60 / 100 amp DNO fuse. Once you get very large csa cables, having them round makes for a very large diameter cable, shaping them like the segments of a pie makes for a more compact cable. Also the armour can be aluminium because it is non magnetic and also the cores can be aluminium, this is the heavy end of electrical engineering and best left to the younger guys.

 

[nickds1]

6943X is 2.5mm three cored armoured, it is not twin and earth. 6943X2.5MM BASEC Approved 6943X Black 3 Core Steel Wire Armoured Cable 2.5mm (priced per metre) - Shop4 Electrical

Anyone in the game would know that, so is this qualified electrician called Joe by any chance.

If you measure the OD it will be around 15mm, the minimum bend radius will be 90mm, it will probably not notice and is not really worth the worry but if you want to hide the corners then garden ordaments or flower baskets. Start running 185mm 4 core and then bend radius can be a real headache!

I redid a small data centre in Dubai where we had a 400KVA 3-phase Volvo generator on the roof and 7 floors below, the ATS, UPSs and machine room...

The armoured cables between the ATS and genset were humungous, not least because of having to travel 7 floors with minimum loss. The cable was one piece, made to order and pulling it up through the building risers required a 20tonne capacity crane.

I totally hate big cables. To get even a start at bending it we had to use high temperature heating jackets to get the cable and cores to move at all...

When you get into the world of high voltage transmission lines, the world of cable design gets very complicated - there's a lot of elements of the cable design related to minimising losses, particularly resistive, electromagnetic and from corona discharge.

 

[morqthana]

The problem with normal cables is that if you bend them too tight you'll damage the insultion - maybe not immediately, but when the conductors get warm they can push their way to the outside, and end up insufficiently protected.

But it is probably impossible to bend armoured too tight by hand.

 

[morqthana]

This is all great fun and I cannot resist jumping in. First a sensible bit,,,are you talking about 3 core plus earth or as you said “twin and earth”?

He's talking about 3-core SWA, using the cores as L/N/E. He just referred to that as armoured T&E, which I don't think is as heinous or indicative of incompetence as some would like to make out.

The reason I ask is that if you go for the “4 core stuff” you can for instance use that 4th wire to switch on say an outside light on the shed from inside the house which can be very handy,,

Good idea, but the cable choice ship has sailed. He could use just the armour as the earth (or cpc if you prefer) - and free up a core that way, if he wanted.

 

[morqthana]

I redid a small data centre in Dubai where we had a 400KVA 3-phase Volvo generator on the roof

Why the <whatever> did they drag it all the way up there?

 

[Spectric]

That is like referring to a horse as a unicorn, one exist whilst the other is imaginary except everyone knows that.

 

[Spectric]

Why the <whatever> did they drag it all the way up there

Up there or on the ground, either way you would still need the risers but roof space may be available whilst ground space had other uses. I bet they had some issues with storing the fuel for it when it came to fire safety!

 

[OldGreyDog]

Hands up… I confess that I have a 2.5mm armoured leading to my shed thats bent a little tight around a bit of brickwork although its been in place almost ten years and has given no trouble. I bent it by hand and at the time was unaware of a specific minimum radius. this bit is in the shade, below a decked area (but secured to a brick wall not touching any timber). This spur has its own 16a circuit breaker in a DP switched box, so I think i’ll leave it be for now. I’ll know better next time!

 

[Stevekane]

He's talking about 3-core SWA, using the cores as L/N/E. He just referred to that as armoured T&E, which I don't think is as heinous or indicative of incompetence as some would like to make out.



Good idea, but the cable choice ship has sailed. He could use just the armour as the earth (or cpc if you prefer) - and free up a core that way, if he wanted.

Quite, and I certainly wouldn't argue with that, but just as a matter of intrest, I had a vague notion that now using the armour as an earth conductor even though its earthed was verboten,,handy to know if its still okay to do?
Steve.

 

[morqthana]

It's absolutely fine, apart from some large cables (far larger than you'd ever ecounter domestically) where the armour isn't big enough. Basically steel is a much poorer conductor than copper, so the cross-sectional area of the armour needs to be greater than the csa of a copper earth conductor, and over a certain cable size it isn't OK.

 

[nickds1]

Up there or on the ground, either way you would still need the risers but roof space may be available whilst ground space had other uses. I bet they had some issues with storing the fuel for it when it came to fire safety!

Indeed there was nowhere inside the building or outside.

The fuel (diesel) is stored as part of the genset container which was craned onto the roof (which was built to take it).

The main problems are that we then had to build a sunshade for it as in summer we hit 45C, plus the fuel has to be changed every 12 months (because it rots) and there is no lift access to the roof, which means that a team of poor sods has to empty the tank (24 hour running qty) into Jerry cans then carry it down the access ladder to the stairs to the top floor, then take it down in the lift and to a waiting truck.

Then the reverse - carrying 1000 litres or so back up the building...

Did I mention that it gets quite warm there?